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7/25/2019 Chng 5 - Logarit ri rc.doc
1/9
Chng 5
NHNG THUT TON LOGARITH RI RC
5.1M u. Phng h! n "nh
Cho G l nhm nhn Abel, Gba , . Bi ton tm kim nghim ca phng tnh
bax =
g!i l bi ton logaith "i #c tong nhm G. $ghim % ca phng tnh g!i l logaith
"i #c c &' a ca b, k( hi) l balog , n) nh c &' a c' *+nh n) nh nghim ca
phng tnh t-n t#i Ga Zb log , n) nh mo4 pbax >.H=
tong nhm mo4 pa bKng pLH.
Mhi * phng tnh giIi *;c, nghim % l mt phNn tJ ca HpZ . Oong phNn n?
chFng ta mi@) tI phng php *n *+nh * %c *+nh nghim ca >.H=.
$) 2i &P giFp *Q ca phng php ch!n th c th giIi phng tnh >.H= cNn
=>pO lnh &' h!c.
$ghim balog ca phng tnh >.H= c th tm theo cRng th3c &a)
=H>mo4=H>log H pbab jja ,
th nhng * ph3c t#p n) tSnh theo cRng th3c n? thi &T t-i hn cch lPa ch!n.
Oh)6t ton tip theo giIi phng tnh >.H= c * ph3c t#p l =log> UVH ppO lnh &'
h!c.
Thu#$ $%!n $ng h&.
'() 1. Gn HW UVH += pH .
'() *. Om =>mo4pac H .
'() +. X6p bIng gi t+ Hupcu H=,>mo4 , &Yp %p n.
'() ,.X6p bIng gi t+ Hvpab v Z=,>mo4. , &Yp %p n.
7/25/2019 Chng 5 - Logarit ri rc.doc
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'() 5.Om &P t[ng nha) phNn tJ t\ bIng th3 nh1t a bIng th3 hai. 8 lm *i]) n?
=>mo4. pabc vu ,
t\ *? =>mo4pba vHu .
'() -.8a a gi t+ =.H>mo4 pvHux
/$ $h0) $hu#$ $%!n.
ChFng ta ch3ng minh &P *Fng *Yn ca th)6t ton. B1t k^ &' ng)?@n %, UZ px ,
c th bi) 4i_n 42i 4#ng =H>mo4 pvHux , ` *? HvHu Z,H , ng Kng
t6p &' ,LH,LU,,L, U, ULH,, UH , HU H ,, HH U ch3a tong mnh t6p &'
Z,H,,pLU, b`i pH >U . O\ *? 4dn *n &P *Fng *Yn ca th)6t ton. 8nh gi * ph3c
t#p ca th)6t ton cng ng *Fng, b`i t6p t\ $ phNn tJ c th &Yp %p cNn
=log> NNO
lnh &' h!c.5.* Thu#$ $%!n P%h2g3467n
B? gi" giI &J chFng ta bit *;c &P phn tSch thnh nhn tJ ca pLH a th\a &'
=
=s
i
iiqp
H
H
XFc n? phng tnh >.H= c th giIi cNn
+
=
=>logH
i
s
i
i qpO lnh &' h!c 2i &P giFp
*Q ca th)6t ton &a).Thu#$ $%!n P%h2g3467n
BIn ch1t ca th)6t ton nKm ` ch, tm &' l;ng * l2n phng tnh % theo mo4)lo
i
iq 2i t1t cI i, &a) * tm nghim ca phng tnh ban *N) bKng *+nh l( phNn 4 t)ng
hoa. 8 tm % theo mt tong cc mo4)lo nh th, chFng ta phIi giIi *-ng 4 th3c
==>mo4>=>
HH
paa i
ii
i q
p
xq
p
fhng tnh n? giIi *;c 2i * ph3c t#p th"i gian l *a th3c tong t"ng h;p n)
nh iq khRng :) l2n >c ngha l khRng ;t :Fa cp=>log , c l mt hKng &' no *=.
'() 1.8'i 2i t\ng &' ng)?@n t' H/, pqq , ta l6p bIng gi t+
=>mo4V=H>,
par qpjjq
, H,...,Z = qj .
'() *.8'i 2i t\ng &' ng)?@n t' :, H// pq , chFng ta tm =>mo4log qba .
89t
=>mo4...=>mo4log HHHZ qqxqxxqbx a +++ , 2i HZ qxi . XFc n? t\ >.H=
4dn *n Kng
7/25/2019 Chng 5 - Logarit ri rc.doc
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=>mo4V=H>V=H> Z pab
qpxqp .
2i &P giFp *Q ca bIng tong b2c H chFng ta tm a Zx .XFc n? ng ta c
=>mo4=> V=H>V=H> H
UZ paba
qpxqpx .
Oheo bIng tong b2c H ta tm a gi t+ ca Hx tip t5c nh th. Gi t+ ca ix
*;c tm th1? t\ phng tnh
=>mo4=> V=H>V=H>... H
HHHZ paba
qpxqpqxqxx iii
i +
.
'() +.Mhi tm siqb iia ,...,H=,>mo4log = , chFng ta tm =H>mo4log pba theo *+nh
l( phNn 4 t)ng hoa.
/$ $h0) $hu#$ $%!n
ChFng ta ch3ng minh *nh gi * ph3c t#p ca th)6t ton. O6p phNn tJ
=>mo4V=H>
pa iqp cNn
=
s
i
pOH
=>log lnh &' h!c. a) * t6p jqir , *'i 2i t1t cI jqi, *;c
tSnh ton cNn =
s
i
iqOH
=> lnh &' h!c. 8 tm gi t+ ix tong b2c j cNn nng b6c>c ngha
tmH
H
i
i qxa =, tm phNn tJ ngh+ch *Io,nhn, nng b6c tin hnh theo bIng. O1t cI kt
h;p l#i l * ph3c t#p ca th)6t ton *;c n@) ` [email protected] . Oh)6t ton fol)gLellman c * ph3c t#p l *a th3c ==>>log HcpO tong t"ng
h;p khi t1t cI cc 2c ng)?@n t' iq ca p khRng ;t :) U=>log cp , ` *? UH,cc hKng &'
4ng.
5.+ Phng h! 3 P%728 92 :(2 %g7;2$h6 ;mo4HH
++ puu ii , n) nh jVZ pzi mo4UH
+ puu ii , n) nh pzp i jVUjV mo4H + puu ii , n) nh pzp i
7/25/2019 Chng 5 - Logarit ri rc.doc
4/9
=H>mo4UH + pvv ii , n) nh pzp i jVUjV mo4HH ++ pvv ii , n) nh pzp i UUU iiiiii vuzvuz , ,...,j,U,H=i chFng ta tm + tS
i, &ao cho ii zz U= . O\ *ng th3c c)'i c[ng ta Ft a
=>mo4UU pab iiii vvuu
.
$) nh H=H,gc4> U = puu ii , th khi =H>mo4H=>, U puulZl ii chFng ta th)
*;c
=>mo4=> U pab ii
vvl ,
t\ *? gi t+ % cNn tm bKng =H=>mo4>log U pvvlb iia
5., L%g7;2$h ;mo4 pbax , >.U=
` *? p l &' ng)?@n t'. Oh)6t ton n? c * ph3c t#p l
cLp
U
H2i mt &' gi t+ ca
hKng &' c. ChFng ta cho Kng =>mo4 pa c b6c l pLH.
Thu#$ $%!n A867n
Tng 1. nh thnh c &` nhn tJ, bao g-m t1t cI cc &' ng)?@n t' :,ppcns!
"#q logloglog
=
Tng *. BKng cch ch!n lPa chFng ta tm &' tP nhi@n ir &ao cho
#q
rpqa iqi =>mo4
, : l &' ng)?@n t'
O\ *? 4dn *n
#q
aiqi pqr =H>mo4log
. >.j=, : l &' ng)?@n t'Tng +. Ch!n &' l;ng * l2n bi) th3c >.j=, giIi h phng tnh t)?n tSnh th)
*;c 3ng 2i cc n qalog Llogaith "i #c ca phNn tJ ca c &` nhn tJ.
Tng ,. BKng cch lPa ch!n chFng ta tm a mt gi t+ ca , &ao cho
=>mo4...H pppqba#q
$
r q
,
` *? $pp ,...,H L l cc &' ng)?@n t' 2i * l2n t)ng bnh, c ngha H#p# i
7/25/2019 Chng 5 - Logarit ri rc.doc
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Tng 5. BKng cch tSnh ton tng tP nh tNng U j ca th)6t ton, tm a logaithm
"i #c ia plog *'i 2i cc &' ng)?@n t' $pp ,...,H ` tNng .
Tng -. c *+nh gi t+ cNn tm balog W
=++
$
iia
#qaqa
ppqrbH
=H>mo4logloglog
.
/$ $h0) $hu#$ $%!n.
Thu#$ $%!n CO@
Tng 1. 89t
[ ] HZ,,ZW,HW logloglogUUVH UVHpO% = n@n
=>mo4=>==>> UHUHUH pccHcc%cHcH +++++
Xogaith theo c &' a chFng ta th) *;c bi) th3c &a)
+++UVH
=H>mo4log=,>=>log=>log UHUHLq
aqaa pqcccHcH
a c th tSnh theo cRng th3c
UVH
=>mo4Lq
pqa q
O\ *?
UVH
=H>mo4logH
Lq
aq pq
Tng +. O@n tNng U chFng ta tm *;c &' l;ng * l2n phng tnh, chFng ta giIi h
phng tnh t)?n tSnh th) *;c tm a qcH aa log=,>log + .
Tng ,. 8 tm %, chFng ta *a a gi2i h#n m2i UL . BKng cch ch!n ngd)
nhi@n,chFng ta tm mt gi t+ r, thsa m7n bi) th3c
< mo4Lq LuL
hg'puqba uq
,:,) l &' ng)?@n t'Oong bi) th3c n? 2i &P c m9t ca &' ng)?@n t' m2i l ) c * l2n t)ng binh.
7/25/2019 Chng 5 - Logarit ri rc.doc
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Tng 5. BKng cch tng tP nh tNng U j chFng ta tm logaithm ca mt &' &'
ng)?@n t' ), ) %)1t hin tong tNng .
Tng -. ChFng ta tm *p &'
< mo4logloglog
Lq LuL
auaqa puhqg'bx
Oh)6t ton n? c * ph3c t#p lO>e%p>>logploglogp=H V U== lnh &' h!c.
Thu#$ $%!n LOG6%%$h
GiI &J : l &' ng)?@n t', l 2c ca pLH. Mhi * t6p nghim ca phng tnh Ht =qx
tong t"ng pZ g-m cc phNn tJ HU,...,,,H qccc , 2i =>mo4H
pac q
p
. $) nh cho &' 4
bit *;c Kng n thsa m7n *i]) kin phng tnh Ht =qx , th c th lPa ch!n &' t &ao cho
HZ, = q!c( !
.GiI &J lqp $=H , 2i : l ng)?@n t' c[ng nha). ChFng ta &T tm &' $iui ,...,H,Z, = ,
m chFng thsa m7n
=>mo4H=> pbai$
i lqu >.=
Mhi iqk th chFng ta c *-ng 4
=>mo4H=> pba lu$
O\ >.U= &T tng *ng
=>mo4H=> > $lux
qa $
B`i o4>a=qpLH, n@n *ng th3c c)'i c[ng cho ta lux $=> chia ht cho pLH, c ngha
=>mo4 $
$ qux
ChFng ta tm cc *-ng 4 th3c nh 6? *'i 2i cc 2c : ca pLH, c th tm *;c % >mo4 pL
H= bKng *+nh l( phNn 4 t)ng hoa.
1n *] c0n l#i l tm iu th no * thsa m7n phng tnh >.=. ChFng ta c th *9t HZ =u .
$) nh mt &' iu tm *;c, th t\ >.= 4dn *nH
=> i$i lquba thsa m7n phng tnh
=>mo4Ht px q = . XFc n? c th tm t &ao cho
=>mo4=>H
pcba !lqu
i$i
.
ChFng ta *9t iii !quu +=+H . XFc n?
=>mo4H=>HHH
pacba$i$
i !lq!lqu +
$h 6? *i]) n? c ngha thsa m7n >.=
$h" 6? m chFng ta tm $u bKng cch thPc hin theo & *-W =>mo4=>,H
HZ pbarui$
i lqui
=
, iiiiici q!uur! +== +H,log .
7/25/2019 Chng 5 - Logarit ri rc.doc
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ChFng ta %em S 45 &a)
Om &' n &ao cho =Huj>mo4vpU x
w *? aqU,bqv,pqHuj, pj.UH =p
89t :qj. Mhi * kq lqU. $goi a =Huj>mo4HZpUU GpjH
=
p
c ,chFng ta c th bi)
4i_n th)6t ton :)a bIng &a)
i Z H U j
ir H x H HZ
i! Z U Z H
H+iu H v v j
O\ *? =xH>mo4jpx >.=
B? gi" ch!n :qU. XFc n? kqH,lqxH =Huj>mo4HU UH
p
c . Ong tP ta l6p bIng
i Z
ir LH
i! H
H+iu U
O\ *? chFng ta c =U>mo4Ux >.u=
O\ >.= >.u= &)? a =HuU>mo4jpx
5.5 L%g7;2$h ;
7/25/2019 Chng 5 - Logarit ri rc.doc
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Mhi nh6n *;c &' l;ng * l2n bi) th3c >.v=> *i]) kin l St nh1t l phIi c mt
phNn tJ g, m galog *7 bit=, th chFng ta c th giIi h phng tnh t)?n tSnh 2i n
l ia xlog ja *log tong nh HpZ 2i *i]) kin l &' l;ng n tong h khRng :)
l2n.
fhng php *n giIn * t#o a bi) th3c >.v= | ch!n phNn tJ b1t k^ pZg , tSnh
=>mo4pau g= bKng cch lPa ch!n chFng ta thJ tm &' thsa m7n *i]) kin &a)
= ipu
O\ ( t`ng t@n ta c th)6t ton c5 th &a)W
Thu#$ $%!n 2n8B3)7)uu
Tng 1. >OSnh ton ban *N)=. O"ng z>:= *-ng c1) 2i [ ] =>V=> *+*pF , 2i
[ ]*pF*+ =>=> l *a th3c b1t khI :)? b6c n. Cho n@n b1t k^ thnh phNn ca t"ng z>:=*;c bi) 4i_n 42i 4#ng *a th3c b6c khRng ;t :) nLH. nhn cc *a th3c nh 6?
&T Ft g!n theo mo4)lo }>?=, *i]) n? chFng ta *7 tm hi) ` chng t"ng &'. fhNn tJ
=HV>=H>
H
= pqaa c b6c l pLH t#o thnh pF . 2i &P h t; ca n chFng ta l6p bIng
logaithm hKng &'L c ngha l phNn tJ ca t"ng ng)?@n t' =>=> qFpF . ChFng ta
tSnh UHU
HH
Z
H ,...,,,H = paaaa .
Tng *. >XPa ch!n c &` nhn tJ=. C &` nhn tJ=>qF#
thnh l6p t\ t1t cI cc *ath3c b1t khI :)? g b6t khRng l2n hn t, ` *? t l mt &' tham &', t~n
Tng +. >Om bi) th3c=. XPa ch!n ngd) nhi@n m, UH q) , chFng ta tm cc gi
t+ &ao cho thsa m7n bi) th3c
#g
))*+gca g ==>>mo4
=>
Z
,
2i =>Z pFc , t\ *? chFng ta tm *;c bi) th3c
+
#gaaa qg)c) =H>mo4log=>log Z ,
` *? Zlog ca chFng ta *7 bit, c0n galog chFng ta cha bit * l2n.
Tng ,. >tm th)6t ton cho cc phNn tJ ca c &` nhn tJ=. Mhi tm ` tNng j 2i &'
l;ng * l2n cc bi) th3c >l2n hn /B/=, chFng ta giIi h phng tnh t)?n tSnh tong
nh HqZ tm a galog 2i #g
Tng 5. >Om logaith i@ng=. ChFng ta tm mt gi t+ ca m &ao cho
==>>mo4H
#g
) x+gcab g
,
` *? =>H pFc . O\ *? chFng ta tm a gi t+ cNn tm
7/25/2019 Chng 5 - Logarit ri rc.doc
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++#g
aqaa qgc)b =H>mo4logloglog H
/$ $h0) $hu#$ $%!n.