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GII TCH (C S)
Phn 1. Khng gian metric1. Metric trn mt tp hp. S hi t.
Khng gian y
Phin bn chnh sa
PGS TS Nguyn Bch Huy(Typing by thuantd)
Ngy 10 thng 11 nm 2004
A. Tm tt l thuyt
1. Khng gian metric
nh ngha 1. Cho tp X 6= . Mt nh x d t X X vo R c gi l mt metric trnX nu cc iu kin sau c tha mn x, y, z X:
i. d(x, y) 0d(x, y) = 0 x = y
ii. d(x, y) = d(y, x)
iii. d(x, y) d(x, z) + d(z, y) (bt ng thc tam gic)Nu d l metric trn X th cp (X, d) gi l mt khng gian metric.
Nu d l metric trn X th n cng tha mn tnh cht sau
|d(x, y) d(u, v)| d(x, u) + d(y, v) (bt ng thc t gic)
V d 1. nh x d : Rm Rm R, nh bi
d(x, y) =
[mi=1
(xi yi)2]1/2
, x = (x1, . . . , xm), y = (y1, . . . , ym)
1
l mt metric trn Rm, gi l metric thng thng ca Rm.Khi m = 1, ta c d(x, y) = |x y|Trn Rm ta cng c cc metric khc nh
d1(x, y) =mi=1
|xi yi|
d2(x, y) = max1im
|xi yi|
V d 2. K hiu C[a,b] l tp hp cc hm thc x = x(t) lin tc trn [a, b]. nh x
d(x, y) = supatb
|x(t) y(t)|, x, y C[a,b]
l metric trn C[a,b], gi l metric hi t u.
2. S hi t
nh ngha 2. Cho khng gian metric (X, d). Ta ni dy phn t {xn} X hi t (hi ttheo metric d, nu cn lm r) v phn t x X nu lim
nd(xn, x) = 0.
Khi ta vit
limn
xn = x trong (X, d)
xnd x
xn xlimxn = x
Nh vy, limn
xn = x trong (X, d) c ngha
> 0,n0 : n N, n n0 d(xn, x) < Ta ch rng, cc metric khc nhau trn cng tp X s sinh ra cc s hi t khc nhau.
Tnh cht
1. Gii hn ca mt dy hi t l duy nht.
2. Nu dy {xn} hi t v x th mi dy con ca n cng hi t v x.3. Nu lim
nxn = x, lim
nyn = y th lim
nd(xn, yn) = d(x, y)
V d 3. Trong Rm ta xt metric thng thng. Xt phn t a = (a1, . . . , am) v dy {xn} vixn = (xn1 , . . . , x
nm). Ta c
d(xn, a) =
mi=1
(xni ai)2 |xni ai|, i = 1, . . . ,m
2
T y suy ra:
limn
xn = a trong (Rm, d) limn
xni = ai trong R, i = 1, . . . , n
V d 4. Trong C[a,b] ta xt "metric hi t u". Ta c
xnd x ( > 0,n0 : n n0 sup
atb|xn(t) x(t)| < )
dy hm {xn(t)} hi t u trn [a, b] v hm x(t)= lim
nxn(t) = x(t), t [a, b]
Nh vy, limn
xn(t) = x(t), t [a, b] l iu kin cn limxn = x trong C[a,b] vi metrichi t u.
Ch ny gip ta d on phn t gii hn.
3. Khng gian metric y
nh ngha 3. Cho khng gian metric (X, d). Dy {xn} X c gi l dy Cauchy (dy cbn) nu lim
n,md(xn, xm) = 0
hay > 0,n0 : n,m n0 d(xn, xm) <
Tnh cht
1. Nu {xn} hi t th n l dy Cauchy.2. Nu dy {xn} l dy Cauchy v c dy con hi t v x th {xn} cng hi t v x.
nh ngha 4. Khng gian metric (X, d) gi l y nu mi dy Cauchy trong n u ldy hi t.
V d 5. Khng gian Rm vi metric d thng thng l y .Tht vy, xt ty dy Cauchy {xn}, xn = (xn1 , . . . , xnm). V
{d(xn, xk) |xni xki | (i = 1, . . . ,m)lim
n,kd(xn, xk) = 0 limn,k |x
ni xki | = 0,
nn ta suy ra cc dy {xni }n (i = 1, . . . ,m) l dy Cauchy trong R, do chng hi t v Ry .
t ai = limn
xni (i = 1,m) v xt phn t a = (a1, . . . , am), ta c limn
xn = a trong
(Rm, d).
V d 6. Khng gian C[a,b] vi metric hi t u d l y .Gi s {xn} l dy Cauchy trong (C[a,b], d).
3
Vi mi t [a, b], ta c |xn(t) xm(t)| d(xn, xm). T gi thit limn,m
d(xn, xm) = 0 ta
cng c limn,m
|xn(t) xm(t)| = 0Vy vi mi t [a, b] th {xn(t)} l dy Cauchy trong R, do l dy hi t. Lp hm x xc nh bi x(t) = limxn(t), t [a, b].Ta cn chng minh x C[a,b] v lim d(xn, x) = 0.Cho > 0 ty . Do {xn} l dy Cauchy, ta tm c n0 tha
n,m n0 d(xn, xm) < Nh vy ta c
|xn(t) xm(t)| < , n n0,m n0, t [a, b]C nh n, t v cho m trong bt ng thc trn ta c
|xn(t) x(t)| , n n0, t [a, b]Nh vy, ta chng minh rng
> 0,n0 : n n0 supatb
|xn(t) x(t)|
T y suy ra:
Dy hm lin tc {xn(t)} hi t u trn [a, b] v hm x(t), do hm x(t) lin tc trn[a, b].
limn
d(xn, x) = 0.
y l iu ta cn chng minh.
B. Bi tp
Bi 1. Cho khng gian metric (X, d). Ta nh ngha
d1(x, y) =d(x, y)
1 + d(x, y), x, y X
1. Chng minh d1 l metric trn X.
2. Chng minh xnd1 x xn d x
3. Gi s (X, d) y , chng minh (X, d1) y .
Gii
1. Hin nhin d1 l mt nh x t X X vo R. Ta kim tra d1 tha mn cc iukin ca metric
4
(i) Ta c: d1(x, y) 0 do d(x, y) 0d1(x, y) = 0 d(x, y) = 0 x = y
(ii) d1(y, x) =d(y, x)
1 + d(y, x)=
d(x, y)
1 + d(x, y)= d(x, y)
(iii) Ta cn chng minh
d(x, y)
1 + d(x, y) d(x, z)
1 + d(x, z)+
d(z, y)
1 + d(z, y)
gn, ta t a = d(x, y), b = d(x, z), c = d(z, y).Ta c a b+ c; a, b, c 0 (do tnh cht ca d)
a1 + a
b+ c1 + b+ c
(do hm
t
1 + ttng trn [0,))
a1 + a
b1 + b+ c
+c
1 + b+ c
b1 + b
+c
1 + c(pcm)
2. Gi s xn d x. Ta clim d(xn, x) = 0
d1(xn, x) =d(xn, x)
1 + d(xn, x)
Do , lim d1(xn, x) = 0 hay xnd1 x
Gi s xn d1 x. Tlim d1(xn, x) = 0
d(xn, x) =d1(xn, x)
1 d1(xn, x)ta suy ra lim d(xn, x) = 0 hay xn
d x.3. Xt ty dy Cauchy {xn} trong (X, d1), ta cn chng minh {xn} hi t trong
(X, d1).
Ta clim
n,md1(xn, xm) = 0
d(xn, xm) =d1(xn, xm)
1 d1(xn, xm) lim
n,md(xn, xm) = 0 hay {xn} l dy Cauchy trong (X, d)
{xn} l hi t trong (X, d) (v (X, d) y ) t x = lim
nxn (trong (X, d)), ta c x = lim
nxn trong (X, d1) (do cu 2).
5
Bi 2. Cho cc khng gian metric (X1, d1), (X2, d2). Trn tp X = X1 X2 ta nh ngha
d((x1, x2), (y1, y2)) = d1(x1, y1) + d2(x2, y2)
1. Chng minh d l metric trn X
2. Gi s xn = (xn1 , xn2 ) (n N), a = (a1, a2). Chng minh
xnd a
{xn1
d1 a1xn2
d2 a2
3. Gi s (X1, d1), (X2, d2) y . Chng minh (X, d) y .
Bi 3. K hiu S l tp hp cc dy s thc x = {ak}k. Ta nh ngha
d(x, y) =k=1
1
2k.|ak bk|
1 + |ak bk| , x = {ak}, y = {bk}
1. Chng minh d l metric trn X
2. Gi s xn = {ank}k, n N, x = {ak}k. Chng minh
xnd x lim
nank = ak , k N
3. Chng minh (S, d) y .
Bi 4. Trn X = C[0,1] xt cc metric
d(x, y) = sup0x1
|x(t) y(t)|
d1(x, y) =
10
|x(t) y(t)| dt
1. Chng minh: (xnd x) (xn d1 x)
2. Bng v d dy xn(t) = n(tn tn+1), chng minh chiu "" trong cu 1) c th
khng ng.
3. Chng minh (X, d1) khng y .
6
GII TCH (C S)
Phn 1. Khng gian metricPhin bn chnh sa - c phn b sung ca bi trc
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
Ni dung chnh ca mn C sChuyn ngnh: Ton Gii tch
Phng php Ging dy Ton
Phn 1: Khng gian metric
1. Metric trn mt tp hp. S hi t. Khng gian y .
2. Tp m. Tp ng. Phn trong, bao ng ca tp hp.
3. nh x lin tc gia cc khng gian metric. Cc tnh cht:
Lin h vi s hi t Lin h vi nh ngc ca tp m, tp ng. nh x m, nh x ng, nh x ng phi.
4. Tp compc. Cc tnh cht cn bn:
H c tm cc tp ng. Tnh cht compc v s hi t. nh ca tp compc qua nh x lin tc.
Phn 2: o v tch phn.
1. i s trn tp hp.
o v cc tnh cht cn bn.
2. Cc tnh cht ca o Lebesgue trn R (khng xt cch xy dng).
3. Hm s o c. Cc tnh cht cn bn.
1
Cc php ton s hc, ly max,min trn 2 hm o c. Ly gii hn hm o c (khng xt: hi t theo o, nh l Egoroff, Lusin).
4. Tch phn theo mt o. Cc tnh cht cn bn (khng xt tnh lin tc tuyt i).
5. Cc nh l Levi, Lebesgue v qua gii hn di du tch phn.
Phn 3: Gii tch hm.
1. Chun trn mt khng gian vect. Chun tng ng. Khng gian Banach.
2. nh x tuyn tnh lin tc. Khng gian cc nh x tuyn tnh lin tc (khng xt nhx lin hp, nh x compc, cc nguyn l c bn).
3. Khng gian Hilbert. Phn tch trc giao. Chui Fourier theo mt h trc chun. H trcchun y .
1 Metric trn mt tp hp. S hi t.Khng gian y
Phn ny c thm phn b sung ca bi trc
1. Tm tt l thuyt
1.1 Khng gian metric
nh ngha 1 Cho tp X 6= . Mt nh x d t X X vo R c gi l mt metric trnX nu cc iu kin sau c tha mn x, y, z X:
i. d(x, y) > 0d(x, y) = 0 x = y
ii. d(x, y) = d(y, x)
iii. d(x, y) 6 d(x, z) + d(z, y) (bt ng thc tam gic)
Nu d l metric trn X th cp (X, d) gi l mt khng gian metric.
Nu d l metric trn X th n cng tha mn tnh cht sau
|d(x, y) d(u, v)| 6 d(x, u) + d(y, v) (bt ng thc t gic)
V d. nh x d : Rm Rm R, nh bi
d(x, y) =
[mi=1
(xi yi)2]1/2
, x = (x1, x2, . . . , xm), y = (y1, y2, . . . , ym)
2
l mt metric trn Rm, gi l metric thng thng ca Rm.Khi m = 1, ta c d(x, y) = |x y|. Trn Rm ta cng c cc metric khc nh
d1(x, y) =mi=1
|xi yi|
d2(x, y) = max16i6m
|xi yi|
V d. K hiu C[a,b] l tp hp cc hm thc x = x(t) lin tc trn [a, b]. nh x
d(x, y) = supa6t6b
|x(t) y(t)|, x, y C[a,b]
l metric trn C[a,b], gi l metric hi t u.
1.2 S hi t
nh ngha 2 Cho khng gian metric (X, d). Ta ni dy phn t {xn} X hi t (hi ttheo metric d, nu cn lm r) v phn t x X nu lim
nd(xn, x) = 0.
Khi ta vit
limn
xn = x trong (X, d)
xnd x
xn xlimxn = x
Nh vy, limn
limxn = x trong (X, d) c ngha
> 0,n0 : n N, n > n0 d(xn, x) < Ta ch rng, cc metric khc nhau trn cng tp X s sinh ra cc s hi t khc nhau.
Tnh cht.
1. Gii hn ca mt dy hi t l duy nht.
2. Nu dy {xn} hi t v x th mi dy con ca n cng hi t v x.3. Nu lim
nxn = x, lim
nyn = y th lim
nd(xn, yn) = d(x, y)
V d. Trong Rm ta xt metric thng thng. Xt phn t a = (a1, . . . , am) v dy {xn} vixn = (xn1 , x
n2 , . . . , x
nm). Ta c
d(xn, a) =
mi=1
(xni ai)2 > |xni ai|, i = 1, 2, . . . ,m
T y suy ra:
limn
xn = a trong (Rm, d) limn
xni = ai trong R, i = 1, 2, . . . , n
3
V d. Trong C[a,b] ta xt metric hi t u. Ta c
xnd x ( > 0,n0 : n > n0 sup
a6t6b|xn(t) x(t)| < )
dy hm {xn(t)} hi t u trn [a, b] v hm x(t)= lim
nxn(t) = x(t), t [a, b]
Nh vy, limn
xn(t) = x(t), t [a, b] l iu kin cn limxn = x trong C[a,b] vi metric hit u. Ch ny gip ta d on phn t gii hn.
1.3 Khng gian metric y
nh ngha 3 Cho khng gian metric (X, d). Dy {xn} X c gi l dy Cauchy (dy cbn) nu
limn,m
d(xn, xm) = 0
hay > 0,n0 : n,m > n0 d(xn, xm) <
Tnh cht.
1. Nu {xn} hi t th n l dy Cauchy.2. Nu dy {xn} l dy Cauchy v c dy con hi t v x th {xn} cng hi t v x.
nh ngha 4 Khng gian metric (X, d) gi l y nu mi dy Cauchy trong n u ldy hi t.
V d. Khng gian Rm vi metric d thng thng l y .Tht vy, xt ty dy Cauchy {xn}, xn = (xn1 , . . . , xnm). V
{d(xn, xk) > |xni xki | (i = 1, . . . ,m)lim
n,kd(xn, xk) = 0 limn,k |x
ni xki | = 0,
nn ta suy ra cc dy {xni }n (i = 1, . . . ,m) l dy Cauchy trong R, do chng hi t v Ry .
t ai = limn
xni (i = 1, 2, . . . ,m) v xt phn t a = (a1, . . . , am), ta c limn
xn = a
trong (Rm, d).
V d. Khng gian C[a,b] vi metric hi t u d l y .Gi s {xn} l dy Cauchy trong (C[a,b], d).Vi mi t [a, b], ta c |xn(t) xm(t)| 6 d(xn, xm). T gi thit lim
n,md(xn, xm) = 0 ta
cng c limn,m
|xn(t) xm(t)| = 0.Vy vi mi t [a, b] th {xn(t)} l dy Cauchy trong R, do l dy hi t.
4
Lp hm x xc nh bi x(t) = limxn(t), t [a, b]. Ta cn chng minh x C[a,b] vlim d(xn, x) = 0.
Cho > 0 ty . Do {xn} l dy Cauchy, ta tm c n0 tha
n,m > n0 d(xn, xm) <
Nh vy ta c|xn(t) xm(t)| < , n > n0,m > n0, t [a, b]
C nh n, t v cho m trong bt ng thc trn ta c
|xn(t) x(t)| , n > n0, t [a, b]
Nh vy, ta chng minh rng
> 0,n0 : n > n0 supa6t6b
|xn(t) x(t)| 6
T y suy ra:
Dy hm lin tc {xn(t)} hi t u trn [a, b] v hm x(t), do hm x(t) lin tc trn[a, b].
limn
d(xn, x) = 0.
y l iu ta cn chng minh.
2. Bi tp
Bi 1 Cho khng gian metric (X, d). Ta nh ngha
d1(x, y) =d(x, y)
1 + d(x, y), x, y X
1. Chng minh d1 l metric trn X.
2. Chng minh xnd1 x xn d x
3. Gi s (X, d) y , chng minh (X, d1) y .
Gii.
1. Hin nhin d1 l mt nh x t X X vo R. Ta kim tra d1 tha mn cc iu kin cametric
(i) Ta c: d1(x, y) > 0 do d(x, y) > 0d1(x, y) = 0 d(x, y) = 0 x = y
5
(ii) d1(y, x) =d(y, x)
1 + d(y, x)=
d(x, y)
1 + d(x, y)= d(x, y)
(iii) Ta cn chng minh
d(x, y)
1 + d(x, y)6 d(x, z)
1 + d(x, z)+
d(z, y)
1 + d(z, y)
gn, ta t a = d(x, y), b = d(x, z), c = d(z, y).
Ta c a 6 b+ c; a, b, c > 0 (do tnh cht ca metric d)
a1 + a
6 b+ c1 + b+ c
(do hm
t
1 + ttng trn [0,))
a1 + a
6 b1 + b+ c
+c
1 + b+ c6 b
1 + b+
c
1 + c(pcm)
2. Gi s xn d x. Ta clim d(xn, x) = 0
d1(xn, x) =d(xn, x)
1 + d(xn, x)
Do , lim d1(xn, x) = 0 hay xnd1 x
Gi s xn d1 x. Tlim d1(xn, x) = 0
d(xn, x) =d1(xn, x)
1 d1(xn, x)ta suy ra lim d(xn, x) = 0 hay xn
d x.3. Xt ty dy Cauchy {xn} trong (X, d1), ta cn chng minh {xn} hi t trong (X, d1). Ta c
limn,m
d1(xn, xm) = 0
d(xn, xm) =d1(xn, xm)
1 d1(xn, xm) lim
n,md(xn, xm) = 0 hay {xn} l dy Cauchy trong (X, d)
{xn} l hi t trong (X, d) (v (X, d) y ) t x = lim
nxn (trong (X, d)), ta c x = lim
nxn trong (X, d1) (do cu 2).
Bi 2 Cho cc khng gian metric (X1, d1), (X2, d2). Trn tp X = X1 X2 ta nh ngha
d ((x1, x2), (y1, y2)) = d1(x1, y1) + d2(x2, y2)
6
1. Chng minh d l metric trn X.
2. Gi s xn = (xn1 , xn2 ), (n N), a = (a1, a2). Chng minh xn d a
{xn1
d1 a1xn2
d2 a23. Gi s (X1, d1), (X2, d2) y . Chng minh (X, d) y .
Gii.
1. Ta kim tra tnh cht i), iii) ca metric. Gi s x = (x1, x2), y = (y1, y2), z = (z1, z2), tac:
i) d(x, y) = d1(x1, y1) + d2(x2, y2) > 0
d(x, y) = 0{
d1(x1, y1) = 0d2(x2, y2) = 0
{
x1 = y1x2 = y2
x = y
iii) Cng tng v cc bt ng thc:
d1(x1, y1) 6 d1(x1, z1) + d1(z1, y1)d2(x2, y2) 6 d2(x2, z2) + d2(z2, y2)
ta cd(x, y) 6 d(x, z) + d(z, y)
2. Ta cd1(x
n1 , a1), d2(x
n2 , a2) 6 d(xn, a) = d1(xn1 , a1) + d2(xn2 , a2)
Do :
lim d(xn, a) = 0{
lim d1(xn1 , a1) = 0
lim d2(xn2 , a2) = 0
3. Gi s {xn} l dy Cauchy trong (X, d), xn = (xn1 , xn2 ). Ta c {xni } l dy Cauchy trong(Xi, di) (v di(x
ni , x
mi ) 6 d(xn, xm)). Suy ra
ai Xi : xni di ai (do (Xi, di) y ) xn d a := (a1, a2) (theo cu 2))
Bi 3 K hiu S l tp hp cc dy s thc x = {ak}k. Ta nh ngha
d(x, y) =k=1
1
2k.|ak bk|
1 + |ak bk| , x = {ak}, y = {bk}
1. Chng minh d l metric trn X.
2. Gi s xn = {ank}k, n N, x = {ak}k. Chng minh
xnd x lim
nank = ak , k N
7
3. Chng minh (S, d) y .
Gii.
1. u tin ta nhn xt rng chui s nh ngha s d(x, y) l hi t v s hng th k nhhn 1/2k.
Vi x = {ak}, y = {bk}, z = {ck}, cc tnh cht i), iii) kim tra nh sau:i) Hin nhin d(x, y) > 0,
d(x, y) = 0 ak = bk k N x = y
iii) T l lun bi 1 ta c
|ak bk|1 + |ak bk| 6
|ak ck|1 + |ak ck| +
|ck bk|1 + |ck bk| k N
Nhn cc bt ng thc trn vi 1/2k ri ly tng, ta c
d(x, y) 6 d(x, z) + d(z, y)
2. Ta c
d (xn, x) =k=1
1
2k.|ank ak|
1 + |ank ak|n N
Gi s xn x. Ta c: k N
1
2k.|ank ak|
1 + |ank ak|6 d(xn, x) ()
|ank ak| 62kd (xn, x)
1 2kd (xn, x)(
khi n ln d (xn, x) 0 ty . Ta chn s k0 sao cho
k=k0+1
12k
< 2. Xt dy s:
sn =
k0k=1
1
2k.|ank ak|
1 + |ank ak|, n N
Do lim sn = 0 nn c n0 sao cho sn n0.
Vi n > n0, ta c
d(xn, x) = sn +
k=k0+1
(. . . ) 6 sn +
k=k0+1
1
2k<
8
Nh vy ta chng minh
> 0 n0 : n > n0 d(xn, x) <
hay lim d(xn, x) = 0.
3. Xt ty dy Cauchy {xn} trong (S, d), xn = {ank}k. L lun tng t () ta c
|ank amk | 62kd(xn, xm)
1 2kd(xn, xm) 0 khi m,n
Suy ra {ank}n l dy Cauchy trong R, do hi t.t ak = lim
nank v lp phn t a := {ak}. p dng cu 2) ta c xn a trong (S, d).
Bi 4 Trn X = C[0,1] xt cc metric
d(x, y) = sup06x61
|x(t) y(t)|
d1(x, y) =
10
|x(t) y(t)| dt
1. Chng minh: (xnd x) (xn d1 x)
2. Bng v d dy xn(t) = n(tn tn+1), chng minh chiu trong cu 1) c th khng
ng.
3. Chng minh (X, d1) khng y .
Gii.
1. Ta c
|x(t) y(t)| 6 d(x, y) t [0, 1]
10
|x(t) y(t)| dt 6 d(x, y) 10
dt = d(x, y)
d1(x, y) 6 d(x, y) x, y C[0,1]Do , nu lim d(xn, x) = 0 th cng c lim d1(xn, x) = 0.
2. K hiu x0 l hm hng bng 0 trn [0, 1]. Ta c:
d1(xn, x0) = 10|xn(t) x0(t)| dt =
10n (tn tn+1) dt = n
(n+1)(n+2) 0 khi n.
9
d(xn, x0) = sup06t61
n(tn tn+1) = n ( nn+1
)n. 1n+1
(hy lp bng kho st hm n(tn tn+1)trn [0, 1]). Do
limn
d(xn, x0) = limn
(n
n+ 1
)n.
n
n+ 1=
1
e6= 0
Suy ra xnd
/ x0.3. Xt dy {xn} C[0,1] xc nh nh sau:
xn(t) =
0 t [0, 1
2]
n(t 12) t [1
2, 12+ 1
n]
1 t [12+ 1
n, 1]
(n > 2)
Trc tin ta chng minh {xn} l dy Cauchy trong(C[0,1], d1
).
Tht vy, vi m < n, ta c:
d1(xn, xm) = 10|xn(t) xm(t)| dt
= 1/2+1/m1/2
|xn(t) xm(t)| dt6 1/2+1/m1/2
1.dt = 1m
Do limm,n
d1(xn, xm) = 0
Ta chng minh {xn} khng hi t trong(C[0,1], d1
).
Gi s tri li:x C[0,1] : lim d1(xn, x) = 0
Khi
d1(xn, x) > 1/20
|xn(t) x(t)| dt = 1/20
|x(t)| dt , n N
1/20
|x(t)| dt = 0 x(t) 0 trn [0, 12].
Mt khc, vi mi a (12, 1)ta c 1
2+ 1
n< a khi n ln.
Do
d1(xn, x) > 1a
|xn(t) x(t)| dt = 1a
|1 x(t)| dt
x(t) = 1 t [a, 1] (l lun nh trn)Do a > 1
2ty , ta suy ra x(t) = 1 t (1
2, 1].
Ta gp mu thun vi tnh lin tc ca hm x.
10
2 Tp m, tp ng. Phn trong, baong ca mt tp hp
1. Tp m. Phn trong
Cho khng gian metric (X, d).Vi x0 X, r > 0, ta k hiu B(x0, r) = {x X : d(x, x0) < r}gi l qu cu m tm x0, bn knh r.
nh ngha 1 Cho tp hp A X.1. im x c gi l im trong ca tp hp A nu r > 0 : B(x, r) A
2. Tp hp tt c cc im trong ca A gi l phn trong ca A, k hiu IntA hayA. Hin
nhin ta c IntA A.3. Tp A gi l tp m nu mi im ca n l im trong. Ta qui c l m. Nh vy,
A m A = IntA (x A r > 0 : B(x, r) A)Tnh cht.
1. H cc tp m c ba tnh cht c trng sau:
i) , X l cc tp m.ii) Hp ca mt s ty cc tp m l tp m.
iii) Giao ca hu hn cc tp m l tp m.
2. Phn trong ca A l tp m v l tp m ln nht cha trong A.
Nh vy:(B A,B m) B IntA
V d. Qu cu m B(x0, r0) l tp m.Tht vy, x B(x0, r0) ta c r = r0 d(x, x0) > 0. Ta s ch ra B(x, r) B(x0, r0). Vi
y B(x, r), ta c d(y, x0) 6 d(y, x) + d(x, x0) < r + d(x, x0) = r0 nn y B(x0, r0).V d. Trong R vi metric thng thng, cc khong m l tp m.
Tht vy, trong R ta c B(x, r) = (x r, x+ r). Mi khong hu hn (a, b) l qu cu tm a+b
2, bn knh ba
2nn l tp m.
(a,+), (a R) l tp m v x (a,+) ta t r = xa th (x r, x+ r) (a,+).V d. Trong R2 vi metric thng thng mi hnh ch nht m A = (a, b) (c, d) l tp m.
Tht vy, xt ty x = (x1, x2) A. Ta t r = min{x1 a, b x1, x2 c, d x2} th cB(x, r) A.nh l 1
1. Mi tp m trong R l hp ca khng qu m c cc khong m i mt khng giaonhau.
2. Mi tp m trong R2 l hp ca khng qu m c cc hnh ch nht m.
11
2. Tp ng. Bao ng ca mt tp hp
nh ngha 2
1. Tp hp A X gi l tp ng nu X \ A l tp m.2. im x c gi l mt im dnh ca tp A nu A B(x, r) 6= ,r > 0.3. Tp tt c cc im dnh ca A gi l bao ng ca A, k hiu l A hay ClA.
Hin nhin ta lun c A A.
Tnh cht.
1. , X l cc tp ng.Giao ca mt s ty cc tp ng l tp ng.
Hp ca hu hn tp ng l tp ng.
2. A l tp ng v l tp ng nh nht cha A.
Nh vy (B A,B ng) B A3. A ng A = A.
nh l 2
1. x A ({xn} A : limxn = x)2. Cc tnh cht sau l tng ng:
a) A l tp ng;
b) {xn} A (limxn = x x A).
V d. Qu cu ng B(x0, r) := {x X : d(x, x0) 6 r} l tp ng.
Chng minh. Do s tng ng ca tnh cht a), b) nn ta chng minh B(x0, r) c tnhcht b). Xt ty dy {xn} m {xn} B(x0, r), xn x, ta phi chng minh x B(x0, r).Tht vy: {
d(xn, x0) 6 r n = 1, 2, . . .lim d(xn, x0) = d(x, x0) (do tnh cht 3) ca s hi t)
d(x, x0) 6 r (pcm)
12
Bi tp
Bi 1 Chng minh rng trong mt khng gian metric ta c
1. A B A B;2. A B = A B;
3. A = A
Gii.
1. Ta c: (B l tp ng, B A) B A.2. Ta c: A A B,B A B (do cu 1)) nn A B A B
Mt khc: {A B l tp ng (do A,B ng)
A B A B A B A B (do tnh cht nh nht ca bao ng)
3. Ta c A l tp ng nn n bng bao ng ca n.
Bi 2 Trong C[a,b] ta xt metric hi t u. Gi s x0 C[a,b]. Ta xt cc tp sau:M1 ={x C[a,b] : x(t) > x0(t)t [a, b]}M2 ={x C[a,b] : x(t) > x0(t)t [a, b]}M3 ={x C[a,b] : t [a, b] : x(t) > x0(t)}
Chng minh M1 m, M2 v M3 ng.
Gii.
Chng minh M1 m. Xt ty x M1, ta cx(t) x0(t) > 0 t [a, b]
r := infa6t6b
[x(t) x0(t)] > 0 (v t0 [a, b] : r = x(t0) x0(t0) > 0)
Ta s chng minh B(x, r) M1. Tht vy, vi y B(x, r) ta c:supa6t6b
|y(t) x(t)| < r
|y(t) x(t)| < r t [a, b]y(t) > x(t) r t [a, b]y(t) x0(t) > x(t) x0(t) r > r r = 0 t [a, b]y M1
13
Chng minh M2 ng.Gi s {xn} M2, xn d x, ta cn chng minh x M2. Ta c limnxn(t) = x(t) t [a, b]
(do xn
d x)
xn(t) > x0(t) t [a, b], n N (do xn M2)
Suy ra x(t) > x0(t)t [a, b] , do x M2. Chng minh M3 ng.
Cch 1. t M4 ={x C[a,b] : x(t) < x0(t) t [a, b]
}. Ta c M3 = C[a,b] \M4 v M4 l
tp m (chng minh tng t M1 m) nn M3 ng.
Cch 2. Gi s {xn} M3, xn d x ta cn chng minh x M3.Do xn M3 nn tn ti tn [a, b] tha xn(tn) > x0(tn). Dy {tn} b chn nn c dy con{tnk}k hi t v mt t0 [a, b]. Ta s chng minh x(t0) > x0(t0). u tin ta chng minh
limk
xnk(tnk) = x(t0) (1)
Tht vy:
|xnk(tnk)x(t0)| 6 |xnk(tnk)x(tnk)|+ |x(tnk)x(t0)| 6 d(xnk , x)+ |x(tnk)x(t0)| (2)
v v v phi ca (2) hi t v 0 khi k nn (1) ng.T xnk(tnk) > x0(tnk) v (1) ta c x(t0) > x0(t0). Ta chng minh t0 [a, b] : x(t0) >x0(t0) hay x M3.
Bi 3 Trong C[a,b] vi metric hi t u ta xt cc tp hp sau:
M1 ={x C[a,b] : x l n nh, 0 6 x(t) 6 1 t [a, b]
}M2 =
{x C[a,b] : x l ton nh, 0 6 x(t) 6 1 t [a, b]
}Chng minh M1 khng l tp ng, M2 l tp ng.
14
GII TCH (C S)
Chuyn ngnh: Gii Tch, PPDH Ton
Phn 1. Khng gian metric
3. nh x lin tc(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 20 thng 12 nm 2004
Tm tt l thuyt
1 nh ngha
Cho cc khng gian metric (X, d), (Y, ) v nh x f : X Y
Ta ni nh x f lin tc ti im x0 X nu > 0, > 0 : x X, d(x, x0) < = (f(x), f(x0)) <
Ta ni f lin tc trn X nu f lin tc ti mi x X
2 Cc tnh cht
Cho cc khng gian metric (X, d), (Y, ) v nh x f : X Y .
nh l 1. Cc mnh sau tng ng
1. f lin tc ti x0 X
2. {xn} X (limxn = x0) = lim f(xn) = f(x0)
1
H qu. Nu nh x f : X Y lin tc ti x0 v nh x g : Y Z lin tc ti y0 = f(x0)th nh x hp g f : X Z lin tc ti x0.
nh l 2. Cc mnh sau tng ng
1. f lin tc trn X
2. Vi mi tp m G Y th tp nghch nh f1(G) l tp m trong X.
3. Vi mi tp ng F Y th tp f1(F ) l tp m trong X.
3 nh x m, nh x ng, nh x ng phi
Cho cc khng gian metric X, Y v nh x f : X Y .
nh x f gi l nh x m (ng) nu vi mi tp m (ng) A X th nh f(A) ltp m (ng).
nh x f gi l nh x ng phi nu f l song nh lin tc v nh x ngc f1 : Y Xlin tc.
4 Mt s cc h thc v nh v nh ngc
Cho cc tp X, Y khc trng v nh x f : X Y . Vi cc tp A,Ai X v B,Bi Y , tac
1. f(iI
Ai) =iI
f(Ai), f(iI
Ai) iI
f(Ai)
2. f1(iI
Bi) =iI
f1(Bi), f1(iI
Bi) =iI
f1(Bi)
f1(B1 \B2) = f1(B1) \ f1(B2)
3. f(f1(B)) B ("=" nu f l ton nh)f1(f(A)) A ("=" nu f l n nh)
Bi tp
Bi 1. Trong khng gian C[a,b], ta xt metric d(x, y) = supatb
|x(t) y(t)| v trong R ta xtmetric thng thng. Chng minh cc nh x sau y lin tc t C[a,b] vo R.
2
1. f1(x) = infatb
x(t)
2. f2(x) =ba
x2(t)dt
Gii. 1. Ta s chng minh |f1(x) f1(y)| d(x, y) (*)Tht vy
f1(x) x(t) = y(t) + (x(t) y(t)) y(t) + d(x, y) t [a, b]= f1(x) d(x, y) y(t), t [a, b]= f1(x) d(x, y) f1(y) hay f1(x) f1(y) d(x, y)Tng t, ta c f1(y) f1(x) d(x, y) nn (*) ng. T y, ta thy{xn}, lim
nxn = x = lim
nf1(xn) = f1(x)
2. Xt ty x C[a,b], {xn} C[a,b] m limxn = x, ta cn chng minh lim f2(xn) = f2(x)Ta c
|x2n(t) x2(t)| = |xn(t) x(t)|.|xn(t) x(t) + 2x(t)| d(xn, x).[d(xn, x) +M ] (M = sup
atb2|x(t)|)
= |f2(xn) f2(x)| b
a
|x2n(t) x2(t)|dt
d(xn, x)[d(xn, x) +M ](b a)
Do lim d(xn, x) = 0 nn t y ta c lim f2(xn) = f2(x) (pcm)
Ghi ch. Ta c th dng cc kt qu v nh x lin tc gii bi tp 3 (2). V d, chngminh tp
M = {x C[a,b] : x(t) > x0(t), t [a, b]} (x0 C[a,b] cho trc )
l tp m, ta c th lm nh sau. Xt nh x
f : C[a,b] R, f(x) = infatb
(x(t) x0(t))
Ta c:
f lin tc (l lun nh khi chng minh f1 lin tc)
3
M = {x C[a,b] : f(x) > 0} = f1((0,+)), (0,) l tp m trong R
Bi 2. Cho cc khng gian metric X, Y v nh x f : X Y . Cc mnh sau l tngng
1. f lin tc trn X
2. f1(B) f1(B) B Y
3. f(A) f(A) A X
Gii. 1) 2) Ta c{f1(B) l tp ng (do f lin tc v B Y l tp ng)f1(B) f1(B)
= f1(B) f1(B) (do tnh cht "nh nht" ca bao ng)
2) 3) t B = f(A) trong 2), ta c f1(f(A) ) f1(f(A)) ADo f(f1(f(A) )) f(A) = f(A) f(A)
3) 1) Xt ty tp ng F Y , ta cn chng minh f1(F ) l tp ng.t A = f1(F ), ta c
f(A) f(A) = f(f1(F )) F = F (do F ng)= f1(f(A)) f1(F )= A AVy A = A nn A l tp ng.
Bi 3. Trong C[a,b] ta xt metric d(x, y) = sup{|x(t) y(t)|, a t b}. Cho : [a, b]R Rl hm lin tc. Chng minh nh x sau y lin tc
F : C[a,b] C[a,b], F (x)(t) = (t, x(t))
Gii. C nh x0 C[a,b], ta s chng minh F lin tc ti x0.t M = 1 + sup
atb|x0(t)|. Cho > 0 ty .
Hm lin tc trn tp compact D := [a, b] [M,M ] nn lin tc u trn D. Do ,tn ti s 1 > 0 sao cho
(t, s), (t, s) D, |t t| < 1, |s s| < 1 = |(t, s) (t, s)| <
4
t = min(1, 1). Vi mi x C[a,b], d(x, x0) < , ta c|x(t) x0(t)| < t [a, b]x(t) [M,M ] (do |x(t) x0(t)| < 1, t [a, b])
Do , |(t, x(t)) (t, x0(t))| < , t [a, b]= |F (x)(t) F (x0)(t)| < , t [a, b]= d(F (x), F (x0)) <
Nh vy, ta chng minh
> 0, > 0 : x C[a,b], d(x, x0) < d(F (x), F (x0)) < hay F lin tc ti x0.
Bi 4. Cho cc khng gian metric X, Y v song nh f : X Y . Chng minh cc mnh sau tng ng
1. f1 : Y X lin tc
2. f l nh x ng
Gii. Ta c (f1 : Y X lin tc) (A X,A ng (f1)1(A) ng trong Y ) (A X,A ng f(A) ng) (f : X Y l nh x ng)
Bi 5. Cho khng gian metric (X, d). Vi x X, 6= A X, ta nh ngha
d(x,A) = infyA
d(x, y)
Chng minh cc khng nh sau y
1. nh x f : X R, f(x) = d(x,A) lin tc
2. x A d(x,A) = 0
3. Nu F1, F2 l cc tp ng, khc v F1F2 = th tn ti cc tp m G1, G2 sao cho
F1 G1, F2 G2, G1 G2 =
Gii. 1. Ta s chng minh |f(x) f(x)| d(x, x) (*)Tht vy, ta c d(x, y) d(x, x) + d(x, y) y A
= infyA
d(x, y) d(x, x) + infyA
d(x, y)
= d(x,A) d(x, A) d(x, x)
5
2. Ta c
d(x,A) = 0 ({xn} A : limn
d(x, xn) = 0) (do tnh cht ca inf v d(x,A) 0) ({xn} A : limxn = x) x A
3. Ta xt nh x g : X R, g(x) = d(x, F1) d(x, F2)Ta c g lin tc theo cu 1)
t G1 = {x X : g(x) < 0}, G2 = {x X : g(x) > 0}, ta c
G1 G2 = G1, G2 l cc tp m (do G1 = g1((, 0)), G2 = g1((0,+)), (0,+),(, 0)l cc tp m v g lin tc).
F1 G1 v x F1 {
d(x, F1) = 0
d(x, F2) > 0 (do x / F2 v kt qu cu 2)) g(x) < 0
Tng t, F2 G2
Bi tp t gii c hng dn
Bi 6. Cho cc khng gian metric X, (Y1, d1), (Y2, d2). Trn Y1 Y2, ta xt metricd((y1, y2), (y
1, y
2)) = d1(y1, y
1) + d2(y2, y
2)
Gi s rng f1 : X Y1, f2 : X Y2 l cc nh x lin tc. Chng minh rng nh xf : X Y1 Y2, f(x) = (f1(x), f2(x)) lin tc.
Hng dn
S dng nh l 1 v iu kin hi t trong khng gian metric tch trong bi tp 1.
Bi 7. Cho cc khng gian metric X, Y v nh x f : X Y . Chng minh cc mnh sautng ng:
1. f lin tc trn X
2. f1(IntB) Int f1(B) B Y
6
Hng dn
1) 2) p dng nh l 2 v tnh cht "ln nht" ca phn trong.
2) 1) p dng nh l 2 v tnh cht G = IntG nu G m.
Bi 8. Cho cc khng gian metric (X, d), (Y, ) v cc nh x lin tc f, g : X Y . Ta nhngha nh x
h : X R, h(x) = (f(x), g(x)), x X
1. Chng minh h lin tc
2. Suy ra rng tp A := {x X : f(x) = g(x)} l tp ng.
Hng dn
1. Chng minh rng nu dnd x th h(xn) h(x) trong R, s dng tnh cht yn y,
zn z th (yn, zn) (y, z)
2. A = h1({0}), {0} l tp ng trong R
Bi 9. Cho khng gian metric (X, d) v A, B l cc tp ng khc , khng giao nhau. Chngminh rng tn ti nh x lin tc f : X R sao cho
0 f(x) 1, x X,f(x) = 0, x A,f(x) = 1, x B
Hng dn
Chng minh hm f(x) =d(x,A)
d(x,A) + d(x,B)cn tm.
7
GII TCH (C S)
Chuyn ngnh: Gii Tch, PPDH Ton
Phn 1. Khng gian metric
4. Tp compact, khng gian compact(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 20 thng 12 nm 2004
Tm tt l thuyt
1 nh ngha
Cho cc khng gian metric (X, d)
1. Mt h {Gi : i I} cc tp con ca X c gi l mt ph ca tp A X nu A iI
Gi
Nu I l tp hu hn th ta ni ph l hu hn.
Nu mi Gi l tp m th ta ni ph l ph m.
2. Tp A X c gi l tp compact nu t mi ph m ca A ta lun c th ly ra cmt ph hu hn.
3. Tp A c gi l compact tng i nu A l tp compact.
1
2 Cc tnh cht
2.1 Lin h vi tp ng
Nu A l tp compact trong khng gian metric th A l tp ng.
Nu A l tp compact, B A v B ng th B l tp compact.
2.2 H c tm cc tp ng
H {Fi : i I} cc tp con ca X c gi l h c tm nu vi mi tp con hu hn J IthiJ
Fi 6= .
nh l 1. Cc mnh sau l tng ng:
1. X l khng gian compact.
2. Mi h c tm cc tp con ng ca X u c giao khc .
nh l 2. Gi s f : X Y l nh x lin tc v A X l tp compact. Khi , f(A) ltp compact.
H qu. Nu f : X R l mt hm lin tc v A X l tp compact th f b chn trn Av t gi tr ln nht, nh nht trn A, ngha l:
x1, x2 A : f(x1) = inf f(A), f(x2) = sup f(A)
nh l 3 (Weierstrass). Trong khng gian metric X, cc mnh sau l tng ng:
1. Tp A X l compact.
2. T mi dy {xn} A c th ly ra mt dy con hi t v phn t thuc A.
2.3 Tiu chun compact trong Rn
Trong khng gian Rn (vi metric thng thng), mt tp A l compact khi v ch khi n ngv b chn.
2.4 Tiu chun compact trong C[a,b]
nh ngha. Cho tp A C[a,b].
2
1. Tp A c gi l b chn tng im trn [a, b] nu vi mi t [a, b] tn ti s Mt > 0sao cho |x(t)| Mt, x A.Tp A c gi l b chn u trn [a, b] nu tn ti s M > 0 sao cho
|x(t)| M , t [a, b], x A.
2. Tp A gi l ng lin tc tc trn [a, b] nu vi mi > 0, tn ti s > 0 sao cho vi
mi t, s [a, b] m |t s| < v vi mi x A th ta c |x(t) x(s)| < .
V d. Gi s A C[a,b] l tp cc hm x = x(t) c o hm trn (a, b) v |x(t)| 2,t (a, b).
Tp A l lin tc ng bc. Tht vy, do nh l Lagrange ta c|x(t) x(s)| = |x(c)(t s)| 2.|t s|
Do , cho trc > 0, ta chn =
2th c:
x A, t, s [a, b], |t s| < |x(t) x(s)| <
Nu thm gi thit A b chn ti im t0 [a, b] th A b chn u trn [a, b]. Tht vy|x(t)| |x(t) x(t0)|+ |x(t0)| = |x(c).(t t0)|+ |x(t0)|
2(b a) +Mt0 t [a, b],x A
nh l 4 (Ascoli - Arzela). Tp A C[a,b] (vi metric hi t u) l compact tng i khiv ch khi A b chn tng im v ng lin tc trn [a, b].
Bi tp
Bi 1. 1. Cho X l khng gian metric compact, {Fn} l h cc tp ng, khc rng, thamn Fn Fn+1 (n = 1, 2, . . . ). Chng minh
n=1
Fn 6=
2. Gi s {Fn} l h c tm cc tp ng, b chn trn R. Chng minhn=1
Fn 6=
Gii. 1. Ta chng minh {Fn} l h c tm. Nu J N l tp hu hn, ta t n0 = max Jth s c
nJ
Fn = Fn0 6=
Ghi ch. Dng khc ca cu 1) l: Cho F1 l tp compact, Fn (n 2) l cc tp ngkhc v F1 F2 . Khi
n=1
Fn 6=
3
2. Ta xy dng dy tp hp {Kn} nh sau:
K1 = F1, Kn =n
k=1
Fk (n 2)
Th th ta c
Kn compact, Kn 6= (do h {Fn} c tm)
F1 F2 ,n=1
Kn =n=1
Fn
Do , theo ghi ch trn ta cn=1
Kn 6=
Bi 2. Cho X l khng gian compact v f : X R lin tc. Chng minh f b chn trn Xv t gi tr nh nht.
Gii. t a = inf f(x), ta c a (ta hiu cn di ng ca tp khng b chn di l). Ta lun c th tm c dy s {an} sao cho an > an+1, lim an = a. Ta t Fn = {x X : f(x) an} (n 1), ta c
Fn l tp ng (do Fn = f1((, an]))
Fn 6= (do an > a = inf f(X)
Fn Fn+1 (do an > an+1)
Do , theo bi 1) th tn ti x0 n=1
Fn. Ta c
f(x0) an n = 1, 2, . . . f(x0) a
Vy f(x0) = a, ni ring a 6= . Ta c pcm.
Bi 3. Cho khng gian metric (X, d) v A, B l cc tp con khc ca X. Ta nh nghad(A,B) = inf
xA,yBd(x, y)
1. Gi s A, B l cc tp compact, chng minh tn ti x0 A, y0 B sao chod(A,B) = d(x0, y0)
2. Gi s A ng, B compact v A B = , chng minh d(A,B) > 0.Nu v d chng t kt lun khng ng nu thay gi thit B compact bng B ng.
4
Gii. 1. Tn ti cc dy {xn} A, {yn} B sao cho lim d(xn, yn) = d(A,B). Do Acompact nn {xn} c dy con {xnk}k hi t v mt phn t x0 A. Xt dy con tngng {ynk}k ca {yn}. Do B compact nn {ynk}k c dy con {ynki}i hi t v mt phnt y0 B.Ta c:
limi
xnki = x0 (v l dy con ca {xnk}) lim
id(xnki , ynki ) = d(A,B) (v l dy con ca {d(xn, yn)})
limi
d(xnki , ynki ) = d(x0, y0) (h qu ca bt t gic)
Do , d(x0, y0) = d(A,B)
2. Gi s tri li, d(A,B) = 0. Khi , ta tm c cc dy {xn} A, {yn} B saocho lim d(xn, yn) = 0.
Do B compact nn {yn} c dy con {ynk}k hi t v y0 B. Td(xnk , y0) d(xnk , ynk) + d(ynk , y0)
ta suy ra limk
xnk = y0
Do A l tp ng, {xnk} A nn ta suy ra y0 A, mu thun vi gi thitA B = .
Trong R2 ta xt metric thng thng v tA = {(t, 0) : t R},B =
{(t,1
t
): t > 0
}Ta c A, B l cc tp ng, A B = t x = (t, 0), y =
(t,1
t
)(t > 0)
Ta c d(x, y) =1
t 0 (t +)
Do , d(A,B) = 0
Bi 4. Cho khng gian metric (X, d) v A X, l tp compact, V l tp m cha A. Ta khiu B(A, ) := {x X : d(x,A) < }
Chng minh tn ti s > 0 sao cho B(A, ) V .
Gii. Cch 1Do A V v V l tp m nn x A,rx > 0 : B(x, 2rx) V
5
H {B(x, rx) : x A} l mt ph m ca tp compact A nn tn ti x1, . . . , xn sao cho
A n
k=1
B(xk, rxk)
t = min{rx1 , . . . , rx2}, ta s chng minh B(A, ) V .Xt ty y B(A, ), ta c
d(y, A) <
x A : d(y, x) < k = 1, n : x B(xk, rxk)
Khi , d(y, xk) d(y, x) + d(x, xk) < + rxk 2rxkDo , y B(xk, 2rxk) V
Cch 2t B = X \ V , ta c B ng v A B = nn theo bi 3 ta c d(A,B) > 0. Chn = d(A,B). Ta s chng minh B(A, ) V hay ch cn chng t B(A, ) B = Tht vy, nu c y B(A, ) B, th ta c
d(y, A) < x A : d(y, x) < Mt khc x A, y B nn d(x, y) d(A,B) = . V l.
Bi 5. Cho X, Y l cc khng gian metric, vi X l khng gian compact v f : X Y lsong nh lin tc. Chng minh f l nh x ng phi.
Gii. Ta cn chng minh nh x ngc f1 lin tc. Do mt bi tp 3, ch cn chng t fl nh x ng.
Vi A X l tp ng, ta cA compact f(A) compact
f(A) ngVy f l nh x ng.
Cc bi tp t gii
Bi 6. Cho cc khng gian metric compact X, Y v nh x f : X Y . Chng minh cc mnh sau tng ng:
1. f lin tc
2. f1(K) l tp compact vi mi tp compact K Y
6
Hng dn
S dng lin h gia tnh compact v tnh ng.
Bi 7. Cho khng gian metric (X, d) v cc tp A, B khc , trong A compact. Chngminh tn ti im x0 A sao cho d(x0, B) = d(A,B).
Hng dn
S dng d(A,B) = infxA
d(x,B)
Bi 8. Cho khng gian metric (X, d) v f : X X l nh x lin tc. im x gi l im btng ca f nu f(x) = x.
1. Chng minh tp im bt ng ca f l tp ng.
2. Gi s X l compact v f khng c im bt ng no. Chng minh tn ti s c > 0 sao
cho d(f(x), x) c x X
Hng dn
t h(x) = d(f(x), x), x X th h : X R lin tc.
1. Ch rng: x bt ng h(x) = 0
2. Cn chng minh infxX
h(x) > 0
Ngoi ra, cu 1) c th chng minh trc tip da vo lin h gia tnh cht ng v s hi
t, cu 2) c th dng phn chng gii.
7
GII TCH (C S)Ti liu n thi cao hc nm 2005
Phin bn chnh sa
PGS TS Nguyn Bch Huy
Ngy 26 thng 1 nm 2005
5. Bi n tpBi 1:
Trn X = C[0,1] ta xt metric hi t u. Cho tp hp A = {x X : x(1) = 1, 0 x(t) 1 t [0, 1]} v nh x f : X R, f(x) =
10
x2(t) dt.
1. Chng minh inf f(A) = 0 nhng khng tn ti x A f(x) = 0.2. Chng minh A khng l tp compact.
Gii1. t = inf f(A). Ta c f(x) 0 x A nn 0.Vi xn(t) = t
n, ta c xn A
f(xn) = 10
t2n dt =1
2n+ 1 0 (n)
Do = 0.
Nu f(x) = 0, ta c:( 10
x2(t) dt = 0, x2(t) 0, x2(t) lin tc trn [0, 1])
= x(t) = 0 t [0, 1]= x / A.
2. Ta c: {f lin tc trn X, nhn gi tr trong R (xem bi tp 3)f(x) 6= inf f(A) x A
= A khng compact (xem l thuyt 4).
1
Bi 2:Cho (X, d) l khng gian metric compact v nh x X X tha mn
d(f(x), f(y)) < d(x, y) x, y X, x 6= y. (1)Chng minh tn ti duy nht im x0 X tha mn x0 = f(x0) (ta ni x0 l im bt ng canh x f).
GiiTa xt hm g : X R, g(x) = d(f(x), x), x X. Ta ch cn chng minh tn ti duy nht
x0 X sao cho g(x0) = 0.p dng bt ng thc t gic v iu kin (1), ta c
|g(x) g(y)| = |d(f(x), x) d(f(y), y)| 2d(x, y)nn g lin tc. T y v tnh compact ca X ta c:
x0 X : g(x0) = inf g(X) (2)Ta s chng minh g(x0) = 0. Gi s g(x0) 6= 0; ta t x1 = f(x0) th x1 6= x0, do :
d(f(x1), f(x0)) < d(x1, x0) d(f(x1), x1) < d(f(x0), x0) g(x1) < g(x0), mu thun vi (2).
Vy g(x0) = 0 hay f(x0) = x0. chng minh s duy nht ta gi s tri li, c x 6= x0 v x = f(x). Khi :
d(x, x0) = d(f(x), f(x0)) < d(x, x0)
Ta gp mu thun.
Bi 3:Cho cc khng gian metric (X, d), (Y, ) v nh x f : X Y . Trn X Y ta xt metric
d1((x, y), (x, y)) = d(x, x) + (y, y), (x, y), (x, y) X Y.
v xt tp hp G = {(x, f(x)) : x X}.
1. Gi s f lin tc, chng minh G l tp ng.
2. Gi s G l tp ng v (Y, ) l khng gian compact, chng minh f lin tc.
Gii1. Xt ty dy {(xn, f(xn))} G m lim(xn, f(xn)) = (a, b) (1)Ta cn chng minh (a, b) G hay b = f(a).
T (1), ta c
limxn = a (2), lim f(xn) = b (3).
2
T (2) v s lin tc ca f ta c lim f(xn) = f(a); kt hp vi (3) ta c b = f(a) (pcm).
2. Xt ty tp ng F Y , ta cn chng minh f1(F ) l tp ng trong X: chng minh f1(F ) ng, ta xt ty dy {xn} f1(F ) m limxn = a v cn chng t
a f1(F ).Ta c: {
f(xn) F, n NF l tp compact (do F ng, Y compact)
= {xnk} : limk
f(xnk) = b F .Khi :
limk
(xnk , f(xnk)) = (a, b), (xnk , f(xnk)) G,G ng= (a, b) G hay b = f(a).
Vy f(a) F hay a f1(F ) (pcm).
Bi 4:Cho khng giam metric compact (X,d) v cc nh x lin tc fn : X R (n N) tha mn
cc iu kin sau:
f1(x) f2(x) . . . , limn
fn(x) = 0 x X ()
Chng minh dy {fn} hi t u trn X v khng, ngha l:
> 0 n0 : n n0 = supxX
|fn(x)| < ()
p dng phng php sau: vi > 0 cho, t
Gn = {x X : fn(x) < }, n NCh cn chng minh tn ti n0 sao cho Gn0 = X.
GiiTrc tin t gi thit (*) ta suy ra rng fn(x) 0 x X, n N. Ta c:Gn l tp m (do fn lin tc v Gn = f
1n (, ))
Gn Gn+1, (do fn(x) fn+1(x))X =
n=1
Gn (do x X nx : n nx fn(x) < )
Do X l khng gian compact ta tm c n1, n2, . . . , nk sao cho
X =k
i=1
Gni
3
t n0 = max{n1, . . . , nk} ta c X = Gn0 . Khi n n0 ta c Gn Gn0 nn Gn = X. T yta thy (**) ng.
Bi 5:Cho khng gian metric compact (X, d) v nh x lin tc f : X X. Ta nh ngha
A1 = f(X), An+1 = f(An), n = 1, 2, . . . , A =n=1
An.
Chng minh A 6= v f(A) = A.
GiiTa c
6= A1 X,A1 compact (do X compact v f lin tc).Dng quy np, ta chng minh c rng
6= An An+1, An compact n = 1, 2, . . .T y ta c {An} l h c tm cc tp ng trong khng gian compact. Do A 6= 0.
Bao hm thc f(A) A c suy t
f(A) f(An1) = An n = 1, 2, . . . ( do A An1, vi quy c A0 = X).
chng minh A f(A), ta xt ty x A. V x An+1 = f(An) nn
n = 1, 2, . . . xn An : x = f(xn).
Do X compact nn c dy con {xnk}, limk
xnk = a. Khi
x = limk
f(xnk) (do cch xy dng {xn})= f(a) (do f lin tc)
Ta cn phi chng minh a A. C nh n, ta c
xnk An khi nk n (do xnk Ank An)= a = lim
kxnk An (do An ng).
Vy a An n = 1, 2, . . . ; do a A v x = f(a) f(A). (pcm).
4
GII TCH (C S)
Chuyn ngnh: Gii Tch, PPDH Ton
Phn 2. Khng gian nh chun
nh x tuyn tnh lin tc
1. Khng gian nh chun(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
L thuyt
1 Chun
Gi s X l mt khng gian vect (k.g.v.t) trn trng s K (K = R hoc K = C). Mt nhx p : X R c gi l mt chun trn X nu tha mn cc iu kin sau cho mi x, y X,mi K:
i) p(x) 0p(x) = 0 x = ( ch phn t khng trong X)
ii) p(x) = ||p(x)
iii) p(x+ y) p(x) + p(y)
S p(x) gi l chun ca phn t x.
Thng thng, ta dng k hiu ||x|| thay cho p(x).
Mnh 1. Nu p l mt chun trn k.g.v.t X th ta c:
1
1. |p(x) p(y)| p(x y) (hay |||x|| ||y||| ||x y||) x, y X.
2. d(x, y) := p(x y) l mt mtric trn X, gi l mtric sinh bi chun p (hay d(x, y) =||x y||)
V d 1. Trn Rn nh x
x = (x1, . . . , xn) 7 ||x|| =(
nk=1
x2k
)1/2l chun, gi chun Euclide. Mtric sinh bi chun ny chnh l mtric thng thng ca Rn.
V d 2. Trn C[a, b], nh x x 7 ||x|| := supatb |x(t)| l mt chun mtric sinh bi chunny l mtric hi t u trn C[a, b]
2 Khng gian nh chun
nh ngha 1.
Khng gian vect X cng vi chun || || trong n, c gi l mt khng gian nh chun(kgc), k hiu (X, || ||).
Cc khi nim hi t, tp m, ng, compact, dy Cauchy, trong (X, || ||) c hiul cc khi nim tng ng i vi mtric sinh bi chun.
Ni ring, trong (X, || ||) ta c
B(x0, r) = {x X : ||x x0|| < r}
( limn
xn = x(cng vit xn|||| x)) lim
n||xn x|| = 0
({xn} l dy Cauchy) limn,m
||xn xm|| = 0.
nh ngha 2. Kgc (X, || ||) c gi l khng gian Banach nu X vi mtric sinh bi || ||l khng gian y .
V kgc l trng hp c bit ca khng gian mtric nn tt c cc kt qu v khng gian
mtric cng ng cho kgc. Ngoi ra, ta c cc kt qu sau v kgc.
Mnh 2. Cho Kgc (X, .) trn trng s K v cc dy {xn}, {yn} X, {n} K,limxn = x, lim yn = y, limn = . Khi :
1. lim xn = x
2. lim(xn + yn) = x+ y, limnxn = x.
H qu. Cc nh x f, g : X X, f(x) = x0 + x, g(x) = 0x (0 K\{0}) l ng phi.
2
3 Chun tng ng
nh ngha 3. Hai chun .1, .2 trn kgvt X gi l tng ng (vit .1 .2) nu tnti cc hng s dng a, b sao cho
x1 ax2 , x2 bx1 x X
Mnh 3. Gi s .1, .2 l hai chun tng ng trn kgvt X. Khi :
1. (limxn = x theo .1) (limxn = x theo .2)
2. (X, .1) y (X, .2) y .
4 Mt s khng gian nh chun
4.1 Khng gian nh chun con
Cho kgc (X, .) v X0 l mt kgvt con ca X. K hiu .0 l thu hp ca . trn X0 th.0 l mt chun trn X0. Cp (X0, .0) gi l kgc con ca (X, .).
4.2 Tch ca hai kgc
Cho cc kgc (X1, .1), (X2, .2). Tch cc X1 X2 s tr thnh kgvt nu ta nh nghacc php ton
(x1, x2) + (y1, y2) = (x1 + y1, x2 + y2) (x1, x2) = (x1, x2)
Kgvt X1 X2 vi chun(x1, x2) := x11 + x22 ()
hoc vi chun tng ng vi (*), gi l kgc tch ca cc kgc (X1, .1), (X2, .2).Ta d dng kim tra c cc tnh cht sau:
Dy (xn1 , xn2 ) hi t v phn t (x1, x2) trong kgc tch khi v ch khi cc dy {xni } hi tv xi trong kgc (Xi, .i), i = 1, 2.
Nu (Xi, .i)(i = 1, 2) l cc khng gian Banach th kgc tch cng l khng gian Banach.
4.3 Kgc hu hn chiu
Gi s X l kgvt m chiu v e = {e1, . . . , em} l mt c s ca X. Khi nh x
x =mk=1
kek 7 xe :=(
mk=1
|k|2)1/2
l mt chun, gi l chun Euclide sinh bi c s e.
3
Mnh 4.
1. Trn mt khng gian hu hn chiu, hai chun bt k lun tng ng vi nhau.
2. Trn kgc hu hn chiu, mt tp l compact khi v ch khi n ng v b chn.
3. Mt khng gian nh chun hu hn chiu lun l khng gian y . Do , mt kgvt
con hu hn chiu ca mt kgc l tp ng trong khng gian .
nh l 1 (Riesz). Nu qu cu B(, 1) := {x X : x 1} ca cc kgc X l tp compactth X l khng gian hu hn chiu.
5 Chui trong kgc
Nh c php ton cng v ly gii hn, trong kgc ta c th a ra khi nim chui phn t
tng t khi nim chui s.
nh ngha 4. Cho kgc (X, .) v dy {xn} cc phn t ca X. Ta ni chui phn tn=1
xn ()
hi t v c tng bng x nu nh x = limn sn, trong : s1 = x1, sn = x1+ +xn (n N) Nu chui s n=1 xn hi t th ta ni chui (**) hi t tuyt i.
Mnh 5. Nu X l khng gian Banach th mi chui hi t tuyt i l chui hi t
4
Bi tpBi 1. K hiu C1[a,b] l khng gian cc hm thc x = x(t) c o hm lin tc trn [a, b]. C
1[a,b]
l kgvt trn R vi cc php ton thng thng v cng hai hm v nhn hm vi s thc. Tanh ngha p1(x) = |x(a)|+ sup
atb|x(t)| , p2(x) = sup
atb|x(t)|, p3(x) = sup
atb{|x(t)|+ |x(t)|}
1. Chng minh p1, p2, p3 l cc chun trn C1[a,b].
2. Chng minh p2 6 p33. Chng minh p1 p3
Gii.
1. lm v d, ta kim tra p1 l chun.
i) Hin nhin ta c p1(x) 0 x C1[a,b]; hn na
p1(x) = 0{
x(a) = 0
x(t) = 0 t [a, b] {
x(a) = 0
x(t) l hm hng s x(t) = 0t [a, b].
ii) p1(x) = |x(a)|+ supatb
|x(t)| = ||(|x(a)|+ sup
atb|x(t)|
)= ||p1(x)
iii) Vi x, y C1[a,b] ta c
|x(a) + y(a)|+ |(x(t) + y(t))| |x(a)|+ |y(a)|+ |x(t)|+ |y(t)| p1(x) + p1(y) t [a, b]
= p1(x+ y) p1(x) + p1(y).
2. D thy p2(x) p3(x) x C1[a,b]. Ta s chng minh khng tn ti s c > 0 sao cho
p3(x) cp2(x) x C1[a,b] ()
Xt dy xn(t) = (t a)n, n N. D dng tnh c:p2(xn) = (b a)np3(xn) = (b a)n + n(b a)n1
Do , nu tn ti c > 0 (*) ng th ta c
(b a)n + n(b a)n1 c(b a)n n = 1, 2, b a+ n c(b a) n = 1, 2,
Ta gp mu thun.
5
3. Ta d dng kim tra p1(x) p3(x) x C1[a,b] Mt khc ta c:|x(t)| |x(a)|+ |x(t) x(a)| = |x(a)|+ |x(c)(t a)|(p dng nh l Lagrange)
|x(a)|+ (b a) supatb
|x(t)|Mp1(x) t [a, b] (M = max{1, b a})
|x(t)| p1(x) t [a, b].Do p3(x) (M + 1)p1(x) x C1[a,b].Vy p1 p3.
Bi 2. K hiu l2 l khng gian cc dy s thc x = {k}k tha mn iu kink=1
2k <
vi cc php ton thng thng v cng hai dy s v nhn dy s vi s thc. Trn l2 ta xt
chun x =(
k=1
2k
)1/2nu x = {k} l2
1. Xt cc dy s en = {n,k}k (n N) trong n,k = 1 nu n = k, n,k = 0 nu n 6= k.Chng minh rng nu x = {k} l2 th x =
n=1
nen
2. Chng minh l2 y .
Gii.
1. t sn = 1e1 + + nen, ta cn chng minh limn
sn = x
Ta c:
sn = (1, , n, 0, 0, )
x sn = (0, , 0, n+1, n+2, ), x sn =( k=n+1
2k
)1/2
V chuik=1
2k hi t nn limn
k=n+1
2k = 0.
Vy limn
x sn = 0 (pcm).
2. Gi s {xn} l dy Cauchy trong l2, xn = {nk}k, n N.
Vi mi k N, ta c:
|nk mk | (
k=1
|nk mk |2)1/2
= xn xm (1)
6
v {xn} l dy Cauchy nn {nk}n l dy Cauchy trong R, do hi t.t ak = lim
nnk (k N) v lp dy s a = {ak}
Tip theo ta s chng minh a l2 v limn
xn a = 0Cho > 0 ty . Do {xn} l dy Cauchy ta c n0 tha mn
n,m N, n,m n0 xn xm < . (1)T (1) ta c
Nk=1
|nk mk |2 < 2 N N,n,m n0
Nk=1
|nk ak|2 2 N N,n n0(ta cho m trong bt trn)
k=1 |nk ak|2 2 n n0 (2)T (2) ta suy ra xn a l2 (n n0) v do a = xn (xn a) cng thuc l2. Hnna, ta chng minh:
> 0n0 : n n0 = xn a hay l lim xn a = 0
Ghi ch
trn ta khng kim tra l2 l kgvt v cc iu kin ca chun. lm v d, ta s chng
minh rng nu x = {k} l2, y = {k} l2 th x + y l2 v x + y x + y. Tht vy,ta c theo bt ng thc Bunhiakowski:N
k=1(k + k)2 =
Nk=1
2k + 2
Nk=1 kk +
Nk=1
2k
x2 + 2x.y+ y2 N N.Cho N ta c pcm.Bi 3. Gi m l khng gian cc dy s thc x = {k}k b chn vi chun x = sup{|k| : k N}.
1. Chng minh m l khng gian Banach.
2. K hiu C l tp hp cc dy s hi t. Chng minh C l khng gian con ng ca m.Gii. 1. Gi s {xn} l dy Cauchy trng m,xn = {nk}k, n N
Vi mi k N, ta c:
|nk mk | sup{|nk mk | : k N} = xn xmv do {xn} l dy Cauchy nn {nk}n l dy Cauchy trong Rv do vy, hi t.t ak = lim
nnk v lp dy s a = {ak}k.
7
Ta chng minh a m v lim xn a = 0Cho > 0, ta tm c n0 sao cho
n,m n0 xn xm <
Ta c:
|nk nk | < k N,n,m n0 |nk ak| k N,n n0(cho m trong bt trn) sup
k|nk ak| n n0.
Nh vy, ta chng minh:
* (xn a) m, do a = xn (xn a) m.* > 0 n0 : n n0 xn a hay lim xn a = 0.
2. Gi s ta c dy {xn} C, xn = {nk}k m xn hi t v a = {ak} m ta cn chng minha C. Mun vy, ta ch cn chng minh a l dy Cauchy.Cho > 0, ta tm c n sao cho
supk|nk ak| = xn a < /3(do a = lim xn trong m)
V xn = {nk }k C nn n l dy Cauchy, do c k0 sao cho:
k, l k0 |nk n
l | < /3.
Vi k0 ny, ta c:
k, l k0 |ak al| |ak nk |+ |nk nl |+ |nl al|< /3 + /3 + /3 =
Vy {ak} l dy Cauchy (pcm).
Bi 4. Cho kgc X v cc tp A,B X khc . Chng minh
1. Nu A m th A+B m
2. Nu A,B compact th A+B compact.
3. Nu A ng, B compact th A+B ng
Gii.
8
1. Trc tin ta chng minh rng b B th A+ b l tp m.Tht vy, nh x f : X X, f(x) = x+ b l ng phi nn
A m f(A) m hay A+ b m
Do A+B =bB
(A+ b) nn A+B m.
2. Xt ty dy {xn} A + B, ta chng minh {xn} c dy con hi t v phn t thucA+B.
Ta c: xn = an + bn vi an A, bn B.Do A compact nn {an} c dy con {ank}k hi t v mt a A. Do B compact nn dy{bnk}k c dy con {bnkl}l hi t v b B. Tng ng vi dy {bnkl}l ta c dy {ankl}lvn hi t v a.
Suy ra dy con xnkl = ankl + bnkl hi t v a+ b (pcm).
Ghi ch: Cu ny c th gii nh sau:
Xt kgc tch X X v nh x f : X X X, f(x, y) = x+ y. Ta c:(f lin tc, AB l tp compact trong X X) = f(AB) l tp compact trong X.Do f(AB) = A+B ta c pcm.
3. Xt dy ty {xn} A+ B, xn = an + bn, an A, bn B m limxn = x, ta cn chngminh x A+BDo B compact nn {bn} c dy con {bnk} hi t v mt b B. Khi ank = xnk bnkhi t v x b v v A ng nn x b A.Ta c x = (x b) + b nn x A+B (pcm).
Bi 5. Cho kgc (X, .) v X0 l khng gian con hu hn chiu ca X. Chng minh tn tix0 X0 sao cho
a x0 = infxX0
a x
Gii. t d = inf{a x : x X0} v chn dy {xn} X0 tha mn lim a xn = d.Ta c: xn a+ a xn nn {xn} b chn
M > 0 : {xn} B(,M)
Tp B(,M)X0 compact (do dimX0
Ghi ch: Bi ny cn c th gii bng cch tm s M > 0 sao cho
infxX0
a x = infxX0B(,M)
a x
Sau s dng tnh compact ca tp X0 B(,M) v tnh lin tc ca hm x 7 a x
Bi 6. Cho kgc X v A X l tp li. Chng minh tc tp A, IntA cng li.
Gii (Hng dn). C nh s t (0, 1)
chng minh tA+ (1 t)A A ta dng lin h gia im dnh v s hi t.
chng minh t IntA + (1 t) IntA IntA ch cn kim tra v tri l tp m, chatrong A.
Bi 7. Gi s trong kgc X, tp S = {x X : x = 1} l compact. Chng minh dimX
GII TCH (C S)
Chuyn ngnh: Gii Tch, PPDH Ton
Phn 2. Khng gian nh chun
nh x tuyn tnh lin tc
2. nh X Tuyn Tnh Lin Tc(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
PHN L THUYT
1. S lin tc ca ca nh x tuyn tnh :
nh x tuyn tnh lin tc gia cc khng gian nh chun c tt c cc tnh cht camt nh x lin tc gia cc khng gian metric. Ngoi ra n cn c cc tnh cht c bitnu trong nh l sau :nh l 1 :Gi s X, Y l cc khng gian nh chun trn cng mt trng s v A : X Y lmt nh x tuyn tnh. Cc mnh sau l tng ng :
(a) A lin tc ti mt im no ca X.
(b) A lin tc trn X.
(c) Tn ti s M > 0 sao cho A(x)Y 6M ||x||X x X2. Chun ca nh x tuyn tnh lin tc. Khng gian L(X,Y )
(a) Nu A : (X, ||.||X) (Y, ||.||Y ) l nh x tuyn tnh lin tc th ta nh nghachun ca A bi :
||A|| = supxXx 6=0
||A(x)||Y||x||X
T nh ngha ny, ta d thy cc tnh cht sau :
i. ||A|| = sup||x||X1
||A(x)||Y = sup||x||X=1
||A(x)||Y
1
ii. Nu A tuyn tnh lin tc th ||A(x)||Y 6 ||A||.||x||X , x Xiii. Nu A tuyn tnh v tn ti s dngM sao cho ||A(x)||Y 6M.||x||X , x X
th A lin tc v ||A|| 6M(b) Ta k hiu L(X, Y ) l tp tt c cc nh x tuyn tnh lin tc t X vo Y .
L(X, Y ) tr thnh khng gian nh chun nu ta nh ngha chun ca mi A L(X, Y ) nh trn v cc php ton nh sau :
(A+B)(x) = A(x) +B(x)
(A)(x) = A(x), x Xnh l 2 :Nu Y l khng gian Banach th L(X, Y ) l khng gian Banach.
3. Phim hm tuyn tnh lin tc
Mt nh x tuyn tnh t khng gian nh chun X vo trng s K cng cn gil mt phim hm tuyn tnh.nh l 3 :Cho f : (X, ||.||) K l phim hm tuyn tnh. Cc mnh sau l tng ng:
(a) f lin tc ti mt im no ca X.
(b) f lin tc trn X.
(c) M > 0 : |f(x)| 6M.||x|| x X(d) Kerf = {x X : f(x) = 0} l khng gian con ng.
Khng gian L(X,K) tt c cc phim hm tuyn tnh lin tc trn X thng khiu l X v gi l khng gian lin hp ca X. T nh l 2 ta c X l khnggian Banach vi chun
||f || = supx 6=
|f(x)|||x||
PHN BI TP
Bi 1Cho cc khng gian nh chun (X, ||.||X), (Y, ||.||Y ) vi dimX = n v A : X Y l nhx tuyn tnh. Chng minh :
1. A lin tc.
2. Tn ti dim xo X sao cho :
||xo||X = 1, ||A|| = ||A(xo)||Y
Gii
2
1. Gi s e = {e1, . . . , en} l mt c s ca X v ||.||e l chun Euclide sinh bi c s e(xem1). Vi x =
nk=1
kek, ta c :
||A(x)||Y n
k=1
|k|.||A(ek)|| (
nk=1
|k|2) 1
2
||x||e
.
(n
k=1
||A(ek)||2) 1
2
M
Nh vy tn ti s M 0 tha mn :||A(x)||Y M.||x||e
V X hu hn chiu nn ||.||e ||.||X v c s a > 0 sao cho :||x||e a||x||X , x X.
T y ta c :||A(x)||Y Ma||x||X , x X
Do A lin tc.
2. Ta c :||A|| = sup
||x||X=1||A(x)||Y ,
nh x x 7 ||A(x)||Y lin tc trn (X, ||.||X), tp S = {x X : ||x||X = 1} l tpcompc trong (X, ||.||X) (V X hu hn chiu).Do tn ti xo S sao cho ||A(xo)||Y = sup
xS||A(x)||Y (pcm)
Bi 2Cho cc khng gian nh chun (X1, ||.||1), (X2, ||.||2), (Y, ||.||Y ) v cc nh x tuyn tnhlin tc Ak : Xk Y, k = 1, 2. Trn khng gian nh chun tch X1 X2 ta xt chun||(x1, x2)|| = ||x1||1 + ||x2||2 v xt nh x
A :X1 X2 YA(x1, x2) = A1(x1) + A2(x2), (x1, x2) X1 X2.
Chng minh A tuyn tnh lin tc v ||A|| = max(||A1||, ||A2||)Gii
t M = max(||A1||, ||A2||) Vi x = (x1, x2), x = (x1, x2) X1 X2, , K, ta c :
x+ x = (x1 + x1, x2 + x2)
A(x+ x) = A1(x1 + x1) + A2(x2 + x2)= A1(x1) +
A1(x1) + A2(x2) + A2(x2)
= [A1(x1) + A2(x2)] + [A1(x1) + A2(x
2)]
= A(x) + A(x)
Vy A l nh x tuyn tnh.
3
||A(x1, x2)||Y = ||A1(x1) + A2(x2)||Y
||A1(x1)||Y + ||A2(x2)||Y ||A1||.||x1||1 + ||A2||.||x2||2 (do A1, A2 lin tc )M(||x1||1 + ||x2||2)
||A(x1, x2)||Y M ||(x1, x2)|| (x1, x2) X1 X2 A lin tc v ||A|| M.
Tip theo ta chng minh ||A|| M . Ta c :||A1(x1)||Y = ||A(x1, )||Y ||A||.||(x1, )||
hay ||A1(x1)||Y ||A||.||x1|| x1 X1Do ||A1|| ||A||. Tng t, ||A2|| ||A||. Vy M ||A|| (pcm)
Bi 3
Gi l1 l tp hp cc dy s thc x = {k} sao cho : ||x|| =k=1
|k|
T () ta suy ra |f(x)| M ||x|| x l1Do f lin tc v ||f || M chng minh ||f || M ta xt cc dy en = {kn}k vi kn = 0 nu k 6= n, nn = 1Ta c
||en|| = 1, f(en) = n, ||f(en)|| ||f ||.||en||nn ||f || |n| n N. Suy ra ||f || M = sup
n|n|.
Vy ||f || = M2. Vi en c nh ngha trn ta t n = f(en). Ta c
|n| ||f ||.||en|| = ||f || n = 1, 2, . . .
nn {k}k l dy b chn. Ta s chng minh vi k nh ngha nh trn th () ng.C nh x = {k} l1 ta t xn = 1e1+ . . .+nen, n N Ta d dng thy limxn = xtrong l1 (xem mt bi tp 1), do theo tnh lin tc v tuyn tnh ca f ta c :
f(x) = limn
f(xn)
= limn
nk=1
kf(ek)
= limn
nk=1
kk
T y ta c ()
Bi 4Gi X l khng gian nh chun cc hm thc x = x(t) lin tc trn [0,) vi chun
||x|| = supt[0,)
eat|x(t)| 0 cho trc)
Chng minh phim hm f sau l tuyn tnh lin tc trn X v tnh chun ca n :
f(x) =
+0
tx(t)dt x X.
Gii
Trc tin ta cng kim tra f(x) xc nh. Vi x X ta c
|tx(t)| = eat.|x(t)|.teat ||x||.teat t [0,) (1)
Hm teat kh tch trn [0,) (d tnh+0
teatdt =1
a2) nn hm tx(t) cng kh tch trn
[0,).
5
D dng kim tra c f l tuyn tnh. T bt ng thc () ta c :
|f(x)| +0
teatdt.||x||
=1
a2||x|| x X (2).
Do f lin tc v ||f || 1a2
Ta s chng minh ||f || = 1a2. Trong (1), (2) ta thy du = t c khi x = eat. t
xo = eat, ta c :
xo X, ||xo|| = 1, f(xo) = 1a2
Mt khc |f(xo)| ||f ||.||xo||. Do ta suy ra ||f || 1a2. Vy ||f || = 1
a2
Bi 5Trn C[1, 1] vi chun hi t u ta xt phim hm :
f(x) =
10
x(t)dt0
1
x(t)dt x C[1, 1]
Chng minh f tuyn tnh lin tc v tnh ||f ||.Gii :
D dng kim tra f l tuyn tnh v
|f(x)| 1
0
|x(t)|dt+0
1
|x(t)|dt 2||x|| x C[1, 1]
Do vy f lin tc v ||f || 2. Ta s chng minh ||f || = 2.Ta thy trong bt ng thc trn du = t c khi xo(t) = 1 trn (0, 1], xo(t) = 1 trn[1, 0], nhng hm xo ny khng thuc C[1, 1]. Ta xt dy hm {xn} nh sau :
xn(t) =
1, nu t [1, 1
n]
nt, nu t [ 1n,1
n] (n 2)
1, nu t [ 1n, 1]
(nu v th ca hm xn v hm xo ta s thy ngha ca vic chn xn). Ta c :
xn C[1, 1], ||xn|| = 1, f(xn) = 2 1n.
6
M ta cng c |f(xn)| ||f ||.||xn||.Do ta c ||f || 2 1
nn N
Cho n ta c ||f || 2. Vy ||f || = 2Bi 6Cho cc khng gian nh chun (X, ||.||X), (Y1, ||.||1), (Y2, ||.||2) v cc nh x tuyn tnh lintc Ak : X Yk, k = 1, 2.Ta xt nh x
A : X Y1 Y2A(x) = (A1(x), A2(x)), x X.
Chng minh A tuyn tnh, lin tc v :
1. max(||A1||, ||A2||) ||A|| ||A1||+ ||A2||nu trong Y1 Y2 ta xt chun :
||(y1, y2)|| = ||y1||1 + ||y2||2, (y1, y2) Y1 Y2.2. ||A|| = max(||A1||, ||A2||) nu trong Y1 Y2 ta xt chun :
||(y1, y2)|| = max(||y1||1, ||y2||2)Hng dn
1. Ta c :
||A(x)|| = ||A1(x)||1 + ||A2(x)||2
{ ||A(x)|| (||A1||+ ||A2||)||x||||Ak(x)|| ||A(x)|| ||A||.||x||
{ ||A|| ||A1||+ ||A2||||Ak|| ||A||
2. ||A(x)|| = max(||A1(x)||, ||A2(x)||), s dng cc nh gi tng t.Bi 7Trn C[1, 1] ta xt chun hi t u v xt phim hm :
f(x) =
11
x(t)dt x(0), x C[1, 1].
Tnh ||f ||.Hng dn
|f(x)| 1
1
|x(t|dt+ |x(0)| 3||x||, x C[1, 1].
Do ||f || 3. chng minh ||f || 3 ta ch rng trong bt ng thc trn, du = t c ti hm xo(t) = 1 nu t 6= 0, xo(0) = 1, nhng xo / C[1, 1].
7
GII TCH (C S)
Chuyn ngnh: Gii Tch, PPDH Ton
Phn 2. Khng gian nh chun
nh x tuyn tnh lin tc
3. Khng gian Hilbert(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
I. Phn l thuyt
1 Tch v hng, khng gian Hilbert
1.1 nh ngha
nh ngha 1 1. Cho khng gian vect X trn trng s K(K =
R hoc K = C).Mt nh x t X X vo K, (x, y) x, yc gi l mt tch v hng trn X nu n tha mn cc iu
kin sau:
(a) x, x 0 x Xx, x = 0 x =
(b) y, x = x, y (y, x = x, y nu K = R), x, y X(c) x + x, y = x, y + x, y x, x, y X(d) x, y = x, y x, y X, K
1
T cc tnh cht i) - iv) ta cng c:
x, y + y = x, y + x, y, x, y = x, y2. Nu ., . l mt tch v hng trn X th nh x xx, x
l mt chun trn X, gi l chun sinh bi tch v hng.
3. Nu ., . l tch v hng trn X th cp(X, ., .) gi l mtkhng gian tin Hilbert (hay khng gian Unita, khng gian vi
tch v hng). S hi t, khi nim tp m,...,trong (X, ., .)lun c gn vi chun sinh bi ., .. Nu khng gian nhchun tng ng y th ta ni (X, ., .) l khng gianHilbert.
1.2 Cc tnh cht
1. Bt ng thc Cauchy - Schwartz: |x, y| x.y2. x + y2+x y2 = 2(x2+y2) (ng thc bnh hnh).3. Nu lim xn = a, lim yn = b th limxn, yn = a, b
V d 1 1. Trong C[a, b] cc hm thc lin tc trn [a, b] th nh
x
(x, y) 7 x, y = ba
x(t)y(t)dt
l mt tch v hng. Khng gian (C[a, b], ., .) khng l khnggian Hilbert.(xy dng v d tng t phn khng gian met-
ric)
2. Trong l2, vi x = {k}, y = {k}, ta nh ngha
x, y =k=1
kk
th ., . l tch v hng, (l2, ., .) l khng gian Hilbert.2
2 S trc giao
2.1 nh ngha
nh ngha 2 Cho khng gian vi tch v hng (X, ., .) vx, y X, 6= M X.1. Ta ni x trc giao vi y (vit xy) nu x, y2. Nu xy y M th ta vit xM . Ta k hiu
M = {x X : x M}.
2.2 Cc tnh cht
1. Nu x M th x M(M ch khng gian con sinh bi M)2. Nu x yn n N v lim yn = y th x y. Suy ra nux M th cng c x M .
3. M l mt khng gian con ng.
4. Nu x1, . . . , xn i nt trc giao th
x1 + . . . + xn2 = x12 + . . . + xn2(ng thc Pythagore)
nh l 1 (v phn tch trc giao) Nu M l mt khng gian
con ng ca khng gian Hilbert (X, ., .) th mi x X c duynht phn tch dng
x = y + z, y M, z M (1)Phn t y trong (1) gi l hnh chiu trc giao ca x ln M v
c tnh cht
x y = infyM
x y.
3
3 H trc chun. Chui Fourier
3.1 nh ngha
Cho khng gian Hilbert (X, ., .)1. H {e1, e2, . . .} X gi l mt h trc chun nu
ei, ej ={
0 nu i 6= j1 nu i = j
Nh vy, {en} l h trc chun nu en = 1 n N vei ej(i 6= j).
2. H trc chun {en} gi l y , nu n c tnh cht sau:(x en n = 1, 2, . . .) x = .
3. Nu {en} l h trc chun th chui
n=1x, en en gi l chuiFourier ca phn t x theo h chun {en}.
nh l 2 Cho {en} l h trc chun trong khng gian Hilbert(X, ., .) v {n} l mt dy s. Ta xt chui
n=1
nen (2)
Ta c:
1. Chui (2) hi t khi v ch khi
n=1 |n|2
nh l 3 Chui Fourier ca mi phn t x X theo h trcchun {en} l hi t v ta c
n=1
|x, en|2 x2 (bt dng thc Bessel).
ngha ca h trc chun y c lm r trong nh l sau.
nh l 4 Cho {en} l h trc chun. Cc mnh sau l tngng:
1. H {en} y 2.
x =
n=1
x, enen, x X.
3.
x2 =n=1
|x, en|2 x X (ng thc Parseval)
II. Phn Bi tp
Bi tp 1 Trong khng gian l1 cc dy s thc x = {k},
k=1 |k| < ta nh ngha
x, y =k=1
k k, x = {k} l1, y = {k} l1
1. Chng minh ., . l mt tch v hng trn l1.2. (l1, ., .) khng l khng gian Hilbert.
Gii
5
1. Trc tin ta cn kim tra x, y xc nh x, y l1. Tht vy,v limk = 0 nn {k} b chn: M R, |k| Mk N.Do
k=1
|kk| Mk=1
|k| m:
xn xm = {0, . . . , 0, 1m + 1
, . . . ,1
n, 0, 0, . . .}
xn xm = (n
k=m+1
1
k2)12 0 (khi n,m)
Ta chng minh {xn} khng hi t.Gi s tri li tn ti a = {k} l1 sao cho lim xna = 0.C nh k N, khi n k, ta c
|1k k| xn a
T y ta c k =1k k N, v l v dy {1k}k / l1.
Vy l1 vi tch v hng trn khng l khng gian Hilbert.
Bi tp 2 Cho khng gian Hilbert X v X0 l khng gian con
ng ca X, A : X0 Y l nh x tuyn tnh lin tc(Y l mtkhng gian nh chun). Chng minh tn ti nh x tuyn tnh
lin tc B : X Y sao cho B(x) = A(x) x X0, B = A6
Gii
Ta nh ngha nh x A nh sau. Theo nh l v phn tch trcgiao, mi x X c duy nht phn tch
x = y + z, y X0, z X0 (3)v ta t B(x) := A(y).
V phn tch dng (3) ca x X0 l x = x + nn ta c ngayB(x) = A(x) x X0
Ta kim tra B l tuyn tnh: vi x, x X,, K ta vitphn tch (3) v
x = y + z, y X0, z X0Khi :
x + x = (y + y) X0
+ (z + z) X0
B(x + x) = A(y + y)= A(y) + A(y)= B(x) + B(x).
Tip theo ta chng minh B lin tc v B = A.T (3) v nh l Pythagore ta c x2 = y2 + z2, do :
B(x) = A(y) A y (Do A lin tc) B(x) A x, x X
Vy B lin tc v B A. Mt khc ta c:B = supxX,x=1 B(x) supxX0,x=1 B(x)= supxX0,x=1 A(x) = AVy B = A.
7
Bi tp 3 Cho h trc chun {en} trong khng gian HilbertX. Xt dy nh x
Pn :X X
Pn(x) =n
k=1
x, ekek, x X,n N.
1. Chng minh Pn(x) l hnh chiu trc giao ca x ln Xn :=
e1, . . . , en.2. Gi s h {en} y . Chng minh limn ||Pn(x)I(x)|| =
0 x X nhng ||PnI||9 0(I : X X l nh x ngnht)
Gii
1. Ta c: x = Pn(x) + (x Pn(x)), Pn(x) Xn.Do ch cn phi chng minh x Pn(x) Xn hay x Pn(x) Xn. V Xn sinh bi {e1, . . . , en} nn ch cn chngminh x Pn(x) ei i = 1, . . . , n.Tht vy:
xPn(x), ei = x, ein
k=1
x, ekek, ei = x, eix, ei = 0
2. Do ng thc Parseval, ta c x X :
x =k=1
x, ek ek = limn
nk=1
x, ek ek = limnPn(x)
t
Qn(x) = I(x)Pn(x) =
k=n+1
x, ekek, x X,n = 1, 2 . . .
8
Ta c Qn(x) tuyn tnh v
||Qn(x)||2 =
k=n+1
|x, ek|2 ||x||2 (bt Bessel)
Qn lin tc, ||Qn|| 1Mt khc, Qn(en+1) = en+1 v ||Qn|| ||Qn(en+1||||en+1|| = 1 nnta c ||Qn|| = 1 hay ||I Pn|| = 1
Bi tp 4 Cho {en} l h trc chun trong khng gian Hilbert Xv {n} l dy s.
1. Gi s {n} l dy b chn. Chng minh rng
A(x) =
n=1
nx, en en x X (4)
l nh x tuyn tnh lin tc t X vo X v ||A|| = supnN |n|2. Gi s chui trong (4) hi t x X. Chng minh {n} l
dy b chn.
Gii
1. t M = supnN |n|u tin ta kim tra A xc nh hay chng minh chui trong (4)
hi t. Ta cn=1
|nx, en|2 M 2n=1
|x, en|2 M 2 ||x||2 (5)
nn theo nh l 2, chui trong (4) hi t.
D kim tra A l nh x tuyn tnh. T nh l 2 v (5) ta c
9
||A(x)||2 =n=1
|x, en|2 M 2 ||x||2 x X
A lin tc, ||A|| MMc khc ta c:
A(ek) = kek v ||A(ek)|| ||A|| k Nnn ||A|| |k| k N. Do ||A|| M . Vy ||A|| = Mpcm.
2. T gi thit v nh l 2, ta c
n=1
|n|2 |x, en|2 k(k N). Ta c
k=1
1
|nk|2
k=1
1
k2 a :=
k=1
1
nkenk (theo nh l 2)
Ta c
a, en ={
1nk
nu n = nk
0 nu n / {n1, n2, . . .}do :
n=1
|n|2 |a, en|2 =k=1
|nk|2 1
|nk|2=
Ta gp mu thun vi (6).
10
GII TCH (C S)
Phn 3. o V Tch PhnChuyn ngnh: Gii Tch, PPDH Ton
1. o(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
1 PHN L THUYT
1. Khng gian o cnh ngha :
1) Cho tp X 6= ; mt h F cc tp con ca X c gi l mt i s nu n thamn cc iu kin sau :
i. X F v nu A F th Ac F , trong Ac = X \ A.ii. Hp ca m c cc tp thuc F cng l tp thuc F .
2) Nu F l i s cc tp con ca X th cp (X,F ) gi l mt khng gian o c; mi tp A F gi l tp o c (o c i vi F hay F o c)
Tnh chtGi s F l i s trn X. Khi ta c :1) X.
Suy ra hp ca hu hn tp thuc F cng l tp thuc F .2) Giao ca hu hn hoc m c cc tp thuc F cng l tp thuc F .3) Nu A F , B F th A \B F .
2. onh ngha :Cho mt khng gian o c (X,F )
1) Mt nh x : F [0,] c gi l mt o nu :i. () = 0
ii. c tnh cht cng, hiu theo ngha
{An}n F, (An Am = , n 6= m) (n=1
An) =n=1
(An)
2) Nu l mt o xc nh trn i s F th b ba (X,F, ) gi l mt khnggian o
1
Tnh cht :Cho l mt o xc nh trn i s F ; cc tp c xt di y u gi thitl thuc F .
1) Nu A B, th (A) (B), hn na nu (A)
3) Tp A R l (L)o c khi v ch khi vi mi > 0, tn ti tp ng F , tp mG sao cho
F A G, (G \ F ) < 4) Nu A l tp (L)o c th cc tp x+ A, xA cng l (L)o c v :
(x+ A) = (A) (xA) = |x|(A)
5) o Lebesgue l , hu hn
2 PHN BI TP
1. Bi 1 Cho khng gian o (X,F, ), tp Y 6= v nh x : X Y Ta nh ngha :
A = {B Y : 1(B) F}
(B) = (1(B))
Chng minh A l i s trn Y v l o xc nh trn A
Gii
Ta kim tra A tha hai iu kin ca i s :i. Ta c Y A v 1(Y ) = X F
Gi s B A, ta cn chng minh Bc = Y \B A. Tht vy, ta c{1(Y \B) = 1(Y )\1(B) = X\1(B)1(B) F ( do B A) nn X \ 1(B) F
1(Y \B) F hay Y \B Aii. Gi s Bn A(n N) v B =
n=1
Bn. Ta c
1(B) =n=1
1(Bn)
1(Bn) F (n N)
1(B) F hay B A. Tip theo ta kim tra l o.Vi B A ta c 1(B) F nn s [1(B)] xc nh, khng m. Vy s (B) 0,xc nh.
i. Ta c () = [1()] = () = 0
ii. Gi s Bn A (n N), Bn Bm = (n 6= m) v B =n=1
Bn.Ta c
1(B) =n=1
1(Bn),
1(Bn) 1(Bm) = 1(Bn Bm) = (n 6= m). [1(B)] =
n=1
[1(Bn)] (do tnh cng ca )
(B) =n=1
(Bn)
3
2. Bi 2 Cho khng gian o (X,F, ) v cc tp An F (n N). t :B =
k=1
( n=k
An
)(Tp cc im thuc mi An t mt lc no )
C =k=1
( n=k
An
)(Tp cc im thuc v s cc An).
Chng minh
1) (B) limn
(An)
2) (C) limn
(An) Nu c thm iu kin (n=1
An)
t
Ck =n=k
Bn (k = 1, 2 . . .),
ta cCk F,C1 C2 . . .
vB = B1 . . . Bn Cn+1
k=1
Ck = (Xem ngha tp C, bi 2 v gi thit v cc Bn)
(B) =
nk=1
(Bk) + (Cn+1) (2) ( do tnh cht ii.)
limm
(Cn) = 0 ( do tnh cht iii.)
Cho n trong (2) ta c (1).4. Bi 4 : K hiu l o Lebesgue trn R. Cho A [0, 1] l tp (L)o c v
(A) = a > 0. Chng minh rng trong A c t nht mt cp s m hiu ca chng l shu t.
Gii
Ta vit cc s hu t trong [0, 1] thnh dy {rn}n v t An = rn +A (n N). Ta chcn chng minh tn ti n 6= m sao cho An Am 6= . Gi s tri li, iu ny khngng. Khi ta c
(n=1
An) =n=1
(An) (1)
Mt khc, ta c
(An) = (A) = a,n=1
An [0, 2]
Do v phi ca (1) bng + cn v tri 2, v l5. Bi 5 : Cho tp (L) o c A R. Chng minh A c th vit thnh dng A = B \C
vi B l giao ca m c tp m v C l tp (L)o c, c o Lebesgue bng 0.Gii
Do tnh cht 3) ca o Lebesgue, vi mi n Nta tm c tp m Gn A sao cho(Gn \ A) < 1
n
t B =n=1
Gn v C = B \ A.Ta c B l (L) o c v do C cng l (L) o c. V
C Gn \ A n = 1, 2, . . .nn ta c :
(C) 1n
n = 1, 2, . . .Vy (C) = 0.
5
6. Bi 6 : Cho tp L o c A [0, 1] vi (A) = a > 0. Chng minh:1) Hm f(x) = (A [0, x]) lin tc trn [0, 1].2) b (0, a) B A : B (L) o c, (B) = b
Gii
1) Vi 0 x < y 1 ta c
f(y) =(A [0, y])=(A [0, x]) + (A (x, y])
f(y) f(x) = (A (x, y]) 0 f(y) f(x) y x
Do f lin tc trn [0, 1]2) Ta c f(0) = 0, f(1) = a v f lin tc nn tn ti xo (0, 1) tha f(xo) = b hay
(A [0, xo]) = b. Tp B := A [0, xo] cn tm.
6
GII TCH (C S)
Phn 3. o V Tch PhnChuyn ngnh: Gii Tch, PPDH Ton
2. HM O C(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
PHN L THUYT
1. nh ngha:Cho mt khng gian o c (X,F), tp A F v hm f : A R. Vi a R, ta s khiu:
A[f < a] = {x A : f(x) < a}Cc tp hp A[f a], A[f > a], A[f a] c nh ngha tng t.Ta ni hm f o c trn A (o c i vi -i s F hay F -o c) nu:
A[f < a] F , a Rnh l 1:Cc mnh sau tng ng
1) f o c trn A2) A[f a] F , a R3) A[f > a] F , a R4) A[f a] F , a R
2. Mt s lp hm o cCho khng gian o c (X,F). Cc tp hp c xt di y lun gi thit l thucF .1) Hm hng s l o c. Hm c trng 1A ca tp A l o c khi v ch khi A F .2) Nu f o c trn A v B A th f o c trn B
Nu f o c trn mi An (n N) th f o c trnn=1
An
3) Gi s cc hm f, g o c trn A v ch nhn cc gi tr hu hn. Khi cc hmsau cng o c trn A :
|f |, |f | ( > 0), f + g, f.g, fg(nu g(x) 6= 0 x A)
4) Gi s cc hm fn o c trn A (n N). Khi cc hm sau cng o c trn Aa) g(x) = sup{fn(x) : n N}, h(x) = inf{fn(x) : n N}b) f(x) = lim
nfn(x), nu gii hn tn ti ti mi x A.
1
3. Hm o c theo LebesgueHm o c i vi -i s cc tp (L) o c gi l hm o c theo Lebesgue hay(L) o cnh l 2Nu A R l tp (L)-o c v hm f : A R lin tc th f l hm (L)-o c.
4. Hm n ginnh ngha : Cho khng gian o c (X,F) v tp A S.Hm f : A R gi l hm n gin nu n c dng
f(x) =ni=1
ai1Ai(x)
trong : Ai F , (i = 1, n), Ai Aj = (i 6= j),ni=1
An = A v 1Ai l hm c trng
ca tp AiNh vy, hm n gin l hm o c, ch nhn hu hn gi tr.nh l 3Nu f l hm khng m, o c trn A th tn ti dy {sn} cc hm n gin trn Asao cho
i) 0 sn(x) sn+1(x), x Aii) lim
nsn(x) = f(x), x A
2
PHN BI TP
Bi 1 : Cho hm f : X R o c v cc s a, b R, a < b. Chng minh rng hm
g(x) =
f(x) nu a f(x) ba nu f(x) < ab nu f(x) > b
l o c trn X.
GII:Cch 1:t A1 = X[a f b], A2 = X[f < a], A3 = X[f > b], ta c:
Ak F , k = 1, 2, 3, A1 A2 A3 = X
g(x) =
f(x) x A1a x A2b x A3
g o c trn A2 v A3 v l hm hng trn cc tp nyg o c trn A1 v f o c trn A1
Do g o c trn A1 A2 A3 = X.Cch 2:Ta d dng kim tra rng g(x) = min{b,max{a, f(x)}}T cc hm o c qua php ly max, min ta nhn c hm o c. Do go c.
Bi 2 : 1) Cho cc hm f, g : X R o c. Chng minh tpA := {x X : f(x) = g(x)} l o c (ngha l thuc F).
2) Cho dy hm {fn} o c trn X. Chng minh rng tpB := {x X : lim
nfn(x) tn ti} o c.
GII:1) Cch 1:t A1 = {x X : f(x) < g(x)}, A2 = {x X : g(x) < f(x)}Ta chng minh A1, A2 F .. Ta vit tp Q thnh dy {rn}. Ta thy
f(x) < g(x) n : f(x) < rn < g(x)Do :
A1 =n=1
{x X : f(x) < rn < g(x)}
=n=1
(X[f < rn] X[g > rn])nn A1 F . Chng minh A2 F tng t.. Do A = X\(A1 A2) nn A FCch 2:t
A1 = {x X : f(x) = +}, A2 = {x X : f(x) = }A3 = {x X : g(x) = +}, A4 = {x X : g(x) = }Y = X\
4k=1
Ak
Ta c th chng minh Ak, Y F v
3
A = (A1 A3) (A2 A4) Y [f g = 0]Ch rng trn Y th f, g o c, ch nhn gi tr hu hn nn f g o c trnY v do Y [f g = 0] F .
2) t f(x) = limn
fn(x), g(x) = limn
fn(x)
. Theo nh ngha, ta cf(x) = lim
n(infkn
fk(x)), g(x) = limn
(supkn
fk(x))
Cc hm Fn(x) := infkn
fk(x) o c nn f(x) = limn
Fn(x) o c
Tng t, ta c g o c. Ta c B = {x X : f(x) = g(x)} nn p dng cu 1) c B F
Bi 3 : Cho khng gian o (X,F , ), A F v hm f : A R o c.1) t An = {x A : |f(x)| n}, n N. Chng minh
limn
(Bn) = (A).
2) Gi s (A) < . Chng minh rng vi mi > 0, tn ti tp B A, B Fsao cho
(A\B) < , f b chn trn BGII:1) Ta c: An F (v |f | o c), An An+1
A =n=1
An (do f ch nhn gi tr hu hn)
Do limn
(An) = (A)
2) Do (A)
l (L)-o c trn (a, b)
GIIXt cc hm fn : (a, b) R xc nh nh sau
fn(x) =
{n[f(x+ 1
n
) f(x)] , nu x (a, b 1n
)c , nu x [b 1
n, b) , n N
Ta c(1) lim
nfn(x) = f(x) x (a, b)
Tht vy vi x (a, b) ta c x < b 1nkhi n ln, do
limn
fn(x) = limn
n[f(x+ 1
n
) f(x)] = f (x)(2) fn l (L)-o c trn (a, b)Tht vy, trn (a, b 1
n) hm fn lin tc (v f kh vi nn f lin tc) trn
[b 1
n, b)
fn cng l hm lin tc nn fn l (L)- o c)T (1),(2) ta c f l (L)-o c.
5
GII TCH (C S)
Phn 3. o V Tch Phn3. TCH PHN THEO LEBESGUE
Chuyn ngnh: Gii Tch, PPDH Ton(Phin bn chnh sa)
PGS TS Nguyn Bch Huy
Ngy 1 thng 3 nm 2006
1 PHN L THUYT
1. iu kin kh tch theo RiemannNu hm f kh tch trn [a, b] theo ngha tch phn xc nh th ta cng ni f kh tchtheo Riemann hay (R)kh tch.nh l 1Hm f kh tch Riemann trn [a, b] khi v ch khi n tha mn hai iiu kin sau :
i. f b chn.ii. Tp cc im gin on ca f trn [a, b] c o Lebesgue bng 0.
2. nh ngha tch phn theo LebesgueCho khng gian o (X,F, ) v A F, f : A R l hm o c
(a) Nu f l hm n gin, khng m trn A v f =n
i=n
ai.1Ai vi Ai F,Ai Aj =
(i 6= j) vni=1
Ai = A th ta nh ngha tch phn ca f trn A theo o bi :
A
fd :=n
i=n
ai(Ai)
(b) Nu f l hm o c, khng m th tn ti dy cc hm n gin, khng m fnsao cho
fn(x) fn+1(x), limn
fn(x) = f(x) x AKhi ta nh ngha
A
fd = limn
A
fnd
Ch rng, tch phn hm o c khng m lun tn ti, l s khng m v cth bng +
1
(c) Nu f l hm o c th f+(x) = max{f(x), 0}, f(x) = max{f(x), 0} l cchm o c, khng m v ta c f(x) = f+(x) f(x). Nu t nht mt trong cctch phn
A
f+d,
A
fd l s hu hn th ta nh ngha
A
fd =
A
f+dA
fd
Ta ni f kh tch trn A nu
A
fd tn ti v hu hn (hay c hai tch phnA
f+d,
A
fd l s hu hn).
3. Cc tnh chtCho khng gian o (X,F, )
3.1 Mt s cc tnh cht quen thuc :Gi s A F v f, g l cc hm o c, khng m trn A hoc kh tch trn A.Khi ta c
A
(f + g)d =
A
fd+
A
gdA
cfd = c
A
fd c R
Nu f(x) g(x) x A thA
fd A
gd
Nu A = A1 A2 vi A1, A2 F,A1 A2 = thA
fd =
A1
fd+
A2
fd
3.2 S khng ph thuc tp o O. Khi nim "hu khp ni"nh nghaGi s P (x) l mt tnh cht pht biu cho mi x A sao cho x A th hoc P (x)ng hoc P (x) sai. Ta ni tnh cht P (x) ng (hay xy ra) hu khp ni (vit tthkn) trn tp A nu tp
B = {x A : P (x) khng ng}
c cha trong mt tp C F m (C) = 0 (hoc (B) = 0 nu bit B F ).V d1) Gi s f, g o c trn A. Ta c
B := {x A : f(x) 6= g(x)} F
Do vy ta ni "f(x) = g(x) hkn trn A " th c ngha l (B) = 0.2) Nu f o c trn A th tp B = {x A : |f(x)| = +} thuc F . Ta ni "f
hu hn hkn trn A" th c ngha (B) = 0.
2
3) Cho cc hm o c fn, f (n = 1, 2, . . .). Ta ni "Dy {fn} hi t hkn trn Av F th c ngha B = {x A : fn(x) 6 f(x)} c o 0.
S khng ph thuc tp o 0
Nu (A) = 0 v f o c trn A th
A
fd = 0. Do :
Nu f c tch phn trn A B v (B) = 0 thAB
gd =
A
fd
Nu f, g o c trn A, f(x) = g(x) hkn trn A v f c tch phn trn A thA
gd =
A
fd
3.3 Nu f o c, khng m trn A v
A
fd = 0 th f(x) = 0 hkn trn A.
3.4 Nu f kh tch trn A th f(x) hu hn hkn trn A
3.5 Tnh cht cngGi s An F (n N), An Am = (n 6= m) v f l hm o c, khng mhoc kh tch trn A =
n=1
An. Khi
A
fd =n=1
An
fd
3.6 Mt s iu kin kh tch:
Nu f o c trn A th f kh tch trn A khi v ch khi |f | kh tch trn A. Nu f o c, g kh tch trn A v |f(x)| g(x) x A th f cng kh tchtrn A.
Nu f o c, b chn trn A v (A)
ii. limn
fn(x) = f(x) x AKhi
A
fd = limn
A
fd
Ghi chDo s khng ph thuc vo tp o 0 ca tch phn, ta c th gi thit cc diu kini., ii. trong nh l Levi v Lebesgue ch cn ng hkn trn A.
5. Lin h gia tch phn Riemann v tch phn LebesgueNu A R l tp (L)o c th tch phn theo o Lebesgue cng k hiu
(L)
A
f(x)dx hoc (L)
ba
f(x)dx nu A = [a, b].
nh l
1) Nu f kh tch Riemann trn [a, b] th f cng kh tch theo ngha Lebesgue trn [a, b]v ta c
(L)
ba
f(x)dx = (R)
ba
f(x)dx
2) Nu f kh tch Riemann suy rng trn [a, b] (hoc trn [a,]) v l hm khng mth f kh tch theo ngha Lebesgue trn [a, b] (trn [a,]) v ta c :
(R)
ba
f(x)dx = (L)
ba
f(x)dx
(R) a
f(x)dx = (L)
a
f(x)dx
2 PHN BI TP
Trong cc tp di y ta lun gi thit c mt khng gian o (X,F, ). Cc tp c xtlun thuc FBi 1Cho hm f o c trn A, hm g, h kh tch trn A sao cho g(x) f(x) h(x) x A.Chng minh f kh tch trn A.
Gii
Ta c
f+ h+, f g ( v g f h)A
f+d A
h+d,
A
f A
gd
Cc tch phn v phi hu hn nn
A
fd < . Suy ra f kh tch. (Bi ny cng c th
gii da vo bt ng thc |f(x)| |g(x)|+ |h(x)|)Bi 2
4
1. Cho hm s f 0, o c trn A. Xt cc hm
fn(x) =
{f(x), nu f(x) nn, nu f(x) > n
(n N)
Chng minh limn
A
fnd =
A
fd
2. ng dng kt qu trn tnh (L)
10
dxx
Gii
1. Ta d dng kim tra rng fn(x) = min{n, f(x)}. Do : fn(x) o c, khng m. fn(x) = min{n, f(x)} min{n+ 1, f(x)} = fn+1(x) lim
nfn(x) = min{ lim
nn, f(x)} = min{+, f(x)} = f(x)
p dng nh l Levi ta c pcm.
2. t f(x) =1x, x (0, 1], f(0) = +. Ta d dng tm c
fn(x) =
1x, nu x [ 1
n2, 1]
n nu x [0, 1n2]
(L)
10
fn(x)dx = (R)
10
fn(x)dx = 2 1n
Theo cu 1) ta c
(L)
10
f(x)dx = limn
10
fn(x)dx = 2.
Bi 3Cho hm f kh tch trn A. Ta xy dng cc hm fn nh sau :
fn(x) =
f(x), nu |f(x)| nn, nu f(x) > nn, nu f(x) < n
Chng minh limn
A
fnd =
A
fd
Gii
Ta d thy fn(x) = min{n,max{n, f(x)}}. T y ta suy ra :
5
fn o c, |fn| |f | n N
limn
fn(x) = min{+,max{, f(x)}} = f(x) x A
p dng nh l Lebesgue ta c pcm.Bi 4Cho l hm o c, khng m trn X. Ta nh ngha :
(A) =
A
d, A F
1. Chng minh l o.
2. Gi s f l hm o c, khng m trn X. Chng minhX
fd =
X
fd
Gii
1. V l hm o c, khng m nn
A
d tn ti, khng m.
Ch rngA
d = 0 nu (A) = 0, ta c () = 0
S dng tnh cht cng ca tch phn ta suy ra c tnh cng2. u tin ta kim tra rng ng thc ng khi f l hm n gin, khng m :
f =ni=1
ai1Ai , Ai Aj = (i 6= j),ni=1
Ai = X.
Tht vy X
fd =ni=1
ai(Ai)
X
fd =ni=1
ai
X
1Aid =ni=1
ai
Ai
d
T y ta c pcm.
Nu f o c, khng m th tn ti dy cc hm n gin fn0 fn fn+1, lim fn = f.
Ta c : X
fnd =
X
fnf n N (do bc trn)
lim
X
fnd =
X
fd, lim
X
fnd =
X
fd
(Do nh l Levi)T y ta c pcm.
6
Bi 5Cho cc hm f, g kh tch trn A. Vi n N ta t : An = {x A : n |f(x)| < n+ 1}Bn = {x A : |f(x)| n}Chng minh :
1. limn
An
gd = 0
2.n=1
n(An) < +
3. limn
n(Bn) = 0
Gii
Ta d kim tra c An Am = (n 6= m)n=0
An = A
1. Do tnh cht cng, ta c:n=0
An
gd =
A
gd R.
T y ta c pcm (do iu kin cn ca s hi t ca chui)
2. Cng do tnh cht cng, ta c:n=0
An
|f |d =A
|f |d
2. Gi s limn
fn(x) = f(x). Chng minh limn
fn = f trong (M,d).
Gii
1. Trc ht ta kim tra s d(f, g) hu hn vi mi cp f, g M . Tht vy, hm h =|f g|
1 + |f g| o c, b chn trn tp X v (X) < nn l hm kh tch. Kim traiu kin i), iii) ca metric nh sau :
i) Hin nhin d(f, g) 0
d(f, g) = 0 |f(x) g(x)|1 + |f(x) g(x)| = 0 hkn trn X
f(x) = g(x) hkn trn X f = g trong M
iii) Vi f, g, h M ta c :
|f(x) g(x)| |f(x) h(x)|+ |h(x) g(x)|
|f(x) g(x)|1 + |f(x) g(x)|
|f(x) h(x)|1 + |f(x) h(x)| +
|h(x) g(x)|1 + |h(x) g(x)|
(Phng php chng minh bit)Ly tch phn hai v ta c d(f, g) d(f, h) + d(h, g)
2. Ta cn chng minh limn
d(fn, f) = 0
t hn =|fn f |
1 + |fn f | (n N), ta c :
hn o c trn X, |hn| = hn 1, hm g(x) = 1 kh tch trn X (do (X)
V 2k1(Ak) Ak
fd 2k(Ak) ta c
1
2
+k=
2k(Ak) A
fd +
k=2k(Ak)
T y ta c iu phi chng minh.Bi 8Cho dy cc hm {fn} kh tch, hu hn trn A, hi t u trn A v hm f v (A) < .Chng minh f kh tch trn A v
limn
A
fnd =
A
fd
Gii
V cc hm fn o c nn f o c.V dy {fn} hi t u trn A v f nn c s no N tha mn
|fn(x) f(x)| 1 x A,n no (1). T (1) ta c |f(x)| 1 + |fn(x)|. V (A)
|fn(x)| = fn(x) 1 + x2 n Np dng nh l Lebesgue, ta c :
limn
20
fn(x)dx =
20
f(x)dx =10
3
2. t fn(x) l hm trong du tch phn th ta c
limn
fn(x) = f(x) vi f(x) = x, x [1, 0], f(x) = x2, x (0, 1].
|fn(x)| |x|+ x2enx
1 + enx 1 n N,x [1, 1].
3. t
fn(x) =
{ (1 + x
n
)n.e2x , x [0, n]
0 , x (n,+) fn (L)o c trn [0,). Vi mi x [0,) th x [0, n] khi n ln, do :
limn
fn(x) = limn
(1 +
x
n
)n.e2x = ex.e2x = ex.
|fn(x)| (1 +
x
n
)n.e2x ex.e2x = ex. n N,x [0,).
(ta s dng 1 + t et, t 0)Hm g(x) = ex l (L)kh tch trn [0,)p dng nh l Lebesgue ta c
limn
n0
(1 +
x
n
)n.e2x.dx = lim
n
+0
fn(x).dx =
+0
ex = 1
Bi 10
Chng minh limn
n
10
xn
1 + x.dx = 1
2
Gii
y ta khng th p dng nh l Lebesgue cho dy hm fn(x) =nxn
1 + xv khng tm c
hm g kh tch sao cho |fn(x)| g(x) n.Ta tch phn tng phn v c :
n
10
xn
1 + x.dx =
n
n+ 1
xn+11 + x
|10 +1
0
xn+1
(1 + x)2.dx
=
n
n+ 1
(1
2+ In
)p dng nh l Lebesgue ta chng minh c lim
nIn = 0.
10