¤U¤@¶: Finite Fields
¤W¤@¶: ¤¤¯Å Field ªº©Ê½è
«e¤@¶: Algebraic Closure
³o¤@¸`¤¤§Ú̱N°Q½×¤@Ó polynomial ¦b¤@Ó field ¤¤¥¦ªº®Úªº©Ê½è.
º¥ý§ÚÌÁÙ¬O¨Ó¬Ý¤j®a³Ì¼ô±xªº¾l¦¡©w²z.
Lemma 10.3.1
°²³]
F ¬O¤@Ó field. Y
f (
x)
F[
x], ¨ä¤¤
deg(
f (
x)) =
n, ¥B
a F º¡¨¬
f (
a) = 0, «h¦s¦b
h(
x)
F[
x], ¨ä¤¤
deg(
h(
x)) =
n - 1, ¨Ï±o
f (
x) = (
x -
a)
. h(
x).
µý ©ú.
¥Ñ©ó
F ¬O¤@Ó field, ¦Ò¼
f (
x)
F[
x] ¥H¤Î
(
x -
a)
F[
x],
§Q¥Î Euclid's Algorithm (Theorem
7.2.4) ª¾¦s¦b
h(
x),
r(
x)
F[
x] º¡¨¬
f (
x) = (
x -
a)
. h(
x) +
r(
x), ¨ä¤¤
r(
x) = 0 ©Î
deg(
r(
x)) < deg(
x -
a) = 1. ¦pªG
r(
x)
0 ¥Ñ
deg(
r(
x)) < 1 ª¾
r(
x) =
c F ¬O¤@Ó±`¼Æ. ¦ý¥Ñ©ó
f (
a) = 0 ¬G±N
a ¥N¤J
f (
x) = (
x -
a)
. h(
x) +
c ±o
c = 0, ¦¹©M
r(
x)
0 ¬Û¥Ù¬Þ¬Gª¾
r(
x) = 0. ¤]´N¬O
f (
x) = (
x -
a)
. h(
x). ¦Ü©ó
deg(
h(
x)) =
n - 1, ¥i¥Ñ
Lemma
7.2.2 ª½±µ±oª¾.
¥Ñ©ó
deg(x - a) = 1, §Ú̪¾¹D x - a ¬O F[x] ¤¤ªº irreducible
element. ¦]¦¹ Lemma 10.3.1 §i¶D§ÚÌY f (a) = 0, «h x - a ·|¬O
f (x) ªº¤@Ó irreducible divisor. §Q¥Î F[x] ¬O unique
factorization domain (Theorem 7.2.14), §Ú̪¾¦s¦b
k
¥H¤Î
q(x) F[x] ¨Ï±o
f (x) = (x - a)k . q(x), ¨ä¤¤ q(a) 0
(§Y x - a ¤£¬O q(x) ªº divisor). §Ų̦́¹¨Ó©w¸q a ¦b f (x)
ªº«®Ú¼Æ.
Definition 10.3.2
°²³]
F ¬O¤@Ó field. Y
f (
x)
F[
x] ¥B
a F º¡¨¬
f (
a) = 0,
«hºÙ
a ¬O¤@Ó
root of
f (
x). ¤S¦pªG
f (
x) = (
x -
a)
k . q(
x), ¨ä¤¤
q(
a)
0, «hºÙ
a ¬O¤@Ó
root of
multiplicity k of
f (
x).
±µ¤U¨Ó¤]¬O¤j®a¼ô±xªº©w²z: ¤@Ó n ¦¸¦h¶µ¦¡¦b¤@Ó field
¤¤pºâ«®Ú¦b¤º¦Ü¦h¦³ n Ó®Ú. ³o¸Ì«üªºpºâ«®Ú¦b¤º¬O»¡¦pªG a ¬O
k «®Ú, «hnºâ¦¨¬O k Ó®Ú.
Theorem 10.3.3
°²³]
F ¬O¤@Ó field. Y
f (
x)
F[
x] ¥B
deg(
f (
x)) =
n1,
«h¦b
F ¤¤±N multiplicity pºâ¦b¤º,
f (
x) ¦Ü¦h¦³
n Ó roots.
µý ©ú.
§Ú̧Q¥Î induction. ¦pªG
deg(
f (
x)) = 1, «h
f (
x) ·íµM¶È¦³ 1
Ó®Ú. °²³] degree ¤p©ó
n ªº polynomial ©w²z¬Ò¦¨¥ß. ²¦Ò¼
f (
x)
F[
x] ¥B
deg(
f (
x)) =
n ªº±¡§Î. ¦pªG
f (
x) ¦b
F ¤¤¨S¦³ root,
«h©w²z·íµM¦¨¥ß. ¦pªG
a F ¬O
f (
x) ªº¤@Ó root of multiplicity
k, §Yªí¥Ü¦s¦b
q(
x)
F[
x] ¨Ï±o
f (
x) = (
x -
a)
k . q(
x), ¨ä¤¤
q(
a)
0. §Q¥Î degree ªº©Ê½è (Lemma
7.2.2) §Ú̦³
deg(
q(
x)) =
n -
k <
n, ¬G§Q¥Î induction ªº°²³]ª¾¦b
F ¤¤±N
multiplicity pºâ¦b¤º,
q(
x) ¦Ü¦h¦³
n -
k Ó roots. µM¦ÓY
b F
¬O
f (
x) ªº¤@Ó root, §Ú̦³
0 = f (b) = (b - a)k . q(b).
§Q¥Î
F
¬O integral domain, §Ú̪¾
f (
x) ªº roots n¤£¬O
a ´N¬O
q(
x) ªº
roots. ¦]¦¹¦b
F ¤¤
f (
x) roots ªºÓ¼Æ´N¬O
k ¥[¤W
q(
x) ªº
roots ªºÓ¼Æ, ©Ò¥H¦Ü¦h¦³
k + (
n -
k) =
n Ó.
§ÚÌn¬Ý¤@Ó¤¸¯À a ¬O§_¬O f (x) ªº¤@Ó®Ú, ¤j®aª½Ä±ªº·Qªk´N¬O±N
a ¥N¤J f (x) ¬Ý¬O§_¬° 0. ¨Æ¹ê¤W³o¬O¤£¹ïªº, n±N a ¥N¤J f (x)
²o§è¤W a ©M f (x) ªº«Y¼Æ¶¡ªº¥[ªk©M¼ªk. ´«¨¥¤§¦pªG a
®y¸¨¦b¤@Ó¥]§t F ©M a ªº field L (¦Ü¤Ön¬O ring) ¤¤,
³o¼Ë§Ṳ́ ¥i¥H±N a ©M f (x) ªº«Y¼Æ¦Ò¼¦¨¬O L ªº¤¸¯À¦Ó¥[¥H¹Bºâ.
³o¼Ë f (a) (¬Ý¦¨¬O L ªº¤¸¯À) ¤ ¦³·N¸q.
³o´N¬O¬°¬Æ»ò§ÚÌ«e±ªº°Q½×³£·|¥ýµ¹ F ªº¤@Ó extension L,
µM«á¦A½Í½× a L »P F[x] ¤¤ªº polynomials ªºÃö«Y.
©Ò¥H§Ú̦۵M·|°Ý: µ¹©w¥ô¤@«D±`¼Æªº
f (x) F[x] ¬O§_¥i¥H§ä¨ì F
ªº¤@Ó extension L ¨Ï±o f (x) ¦b L ¤¤¦³®Ú? µª®×¬OªÖ©wªº.
¥H¤Uªº©w²z´N¬O¦^µª³oÓ°ÝÃD. §Ú̱N·|«Øºc¤@Ó F ªº extension field
µM«á»¡©ú¦b¨ä¤¤¥i§ä¨ì¤@Ó®Ú. ³oÓ©w²zªºÃÒ©ú¦P¾Ç©Î³|ı±o``µêµê''ªº,
¦]¬°¦n¹³¨S¦³¯uªº¦b§ä®Úªº·Pı.
¤£¹L³o´N¬O¼Æ¾Ç¦b½Í¦s¦b©Ê©ÒÃö¤ßªº«ÂI,
§ÚÌ¥unª¾¹DªF¦è¦s¦b¦Ó¤£¥²¯u¥¿§i¶D§AªF¦è¬O¤°»ò.
Theorem 10.3.4
°²³]
F ¬O¤@Ó field ¥B
p(
x)
F[
x] ¬O
F[
x] ¤¤ªº irreducible
element, «h¦s¦b¤@Ó field
L ¬O
F ªº finite extension, ¨ä¤¤
[
L :
F] = deg(
p(
x)) ¥B
L ¤¤¦s¦b
a L º¡¨¬
p(
a) = 0.
µý ©ú.
¥O
L =
F[
x]/
p(
x)
. ¥Ñ©ó
p(
x) ¬O irreducible, §Ú̪¾
p(
x)
¬O
F[
x] ¤¤ªº maximal ideal, ¬Gª¾
L ¬O¤@Ó field.
º¥ý§ÚÌnÅçÃÒ L ¤¤¦s¦b¤@Ó subfield ©M F ¬O isomorphic ªº,
¦]¦¹§ÚÌ¥i¥H±N L ¬Ý¦¨¬O F ªº¤@Ó extension. ¨Æ¹ê¤W¦Ò¼
: FF[x]/p(x), ©w¸q¦¨
(c) = , «Ü®e©öÅçÃÒ ¬O¤@Ó
ring homomorphism. ¤]«Ü®e©öÅçÃÒ ¬O¤@¹ï¤@ªº: ³o¬O¦]¬°¦pªG
c ker(), ªí¥Ü
= , §Y
c p(x). ¦ý¬O
p(x) ¤¤°£¤F 0 ¥H¥ ¨S¦³¨ä¥Lªº±`¼Æ, ¬G±o c = 0 (¤]¥i®M¥Î
Proposition 9.1.5 (2) ±o¨ì ¬O¤@¹ï¤@). ¦]¦¹±oÃÒ
im() ¬O L ªº subfield ¥B©M F ¬O isomorphic ªº.
²¦bnÃÒ©ú L ¤¤¦s¦b¤@¤¸¯À¬O p(x) ªº®Ú. ¦Ò¼
a = L,
§ÚÌn»¡©ú
p() = (ª`·N
¬O
L = F[x]/p(x)
ªº 0). °²³]
p(x) = anxn + ... + a1x + a0, ¨ä¤¤ ai F. ¨º»ò
p(a) ·|¬O¤°»ò©O? §O§Ñ¤F§ÚÌ´£¹L³o¸Ì¥N¤J a ¥²¶·¥Î¨ìªº¬O L
¤¤ªº¹Bºâ, ¦Ó¦b L ¤¤ c F ¬O»Ý¸g¹L °e¨ì L ªº,
´«¥y¸Ü»¡§ÚÌ¥²¶·¦Ò¼ªº¬O
. ¦]¦¹¦³
p(a) |
= |
p() |
|
|
= |
an . + ... + a1 . + a0 |
|
|
= |
. + ... + . + () |
|
|
= |
|
|
|
= |
= |
|
©Ò¥H
L ¤¤¯uªº¦s¦b¤@Ó¤¸¯À¥N¤J
p(
x) µ¥©ó
L ¤¤ªº 0.
³Ì«á¥Ñ Lemma 9.3.6 ª¾
[L : F] = dimF(L) = dimF(F[x]/p(x)) = deg(p(x)).
¥Ñ Theorem 10.3.4 §ÚÌ«Ü®e©ö±o¨ì¥H¤U¤@¯ëªºª¬ªp.
Corollary 10.3.5
°²³]
F ¬O¤@Ó field ¥B
f (
x)
F[
x], ¨ä¤¤
deg(
f (
x)) =
n1,
«h¦s¦b¤@Ó field
L ¬O
F ªº finite extension, ¨ä¤¤
[
L :
F]
n
¥B
L ¤¤¦s¦b
a L º¡¨¬
f (
a) = 0.
µý ©ú.
¥Ñ©ó
f (
x)
F[
x] ¦Ó¥B
deg(
f (
x))
1, ©Ò¥H
f (
x) ¤£¬O
F[
x]
¤¤ªº unit. §Q¥Î
F[
x] ¬O unique factorization domain, §Ú̪¾¦s¦b
p(
x)
F[
x] ¬O
F[
x] ¤¤ªº irreducible element º¡¨¬
p(
x) |
f (
x). ª`·N¦pªG
p(
a) = 0, «h·íµM±o
f (
a) = 0. ¦]¦¹¥Ñ Theorem
10.3.4 ª¾¦s¦b
L, ¨ä¤¤
[
L :
F] = deg(
p(
x))
deg(
f (
x)) ¥B
a L, º¡¨¬
f (
a) =
p(
a) = 0.
§ÚÌ¥i¥H¤@ª½®M¥Î Corollary 10.3.5 §ä¨ì¤@Ó F ªº finite
extension L' ¨Ï±o f (x) ¦b L' ¤¤¥i¥H§¹¥þ¤À¸Ñ. ³o¸Ì©Ò¿×ªº f (x)
¦b L' §¹¥þ¤À¸Ñ´N¬O»¡: ¦pªG
deg(f (x)) = n, «h f (x) ¦b L'[x]
¤¤¥i¥H¼g¦¨
f (x) = c . (x - a1) ... (x - an), ¨ä¤¤ ai L'.
¦¹®É§Ú̳q±`ºÙ f (x) splits into linear factors in L'.
Theorem 10.3.6
°²³]
F ¬O¤@Ó field ¥B
f (
x)
F[
x], ¨ä¤¤
deg(
f (
x)) =
n1,
«h¦s¦b¤@Ó field
L' ¬O
F ªº finite extension, ¨ä¤¤
[
L' :
F]
n!, ¨Ï±o
f (
x) splits into linear factors in
L'.
µý ©ú.
§Q¥Î Corollary
10.3.5 ª¾¦s¦b
L1 ¬O
F ªº¤@Ó extension
º¡¨¬
[
L1 :
F]
n ¥B
a1 L1 ¨Ï±o
f (
a1) = 0. ¬G¥Ñ Lemma
10.3.1 ª¾¦s¦b
f1(
x)
L1[
x] ¥B
deg(
f1(
x)) =
n - 1 ¨Ï±o
f (
x) = (
x -
a1)
. f1(
x). ¹ï
f1(
x) ¦A®M¥Î¤@¦¸ Corollary
10.3.5 ª¾¦s¦b
L2 ¬O
L1 ªº¤@Ó extension º¡¨¬
[
L2 :
L1]
n - 1 ¥B
a2 L2 ¨Ï±o
f1(
a2) = 0. ª`·N¦¹®É
[
L2 :
F] = [
L2 :
L1][
L1 :
F]
n(
n - 1),
¥B¦s¦b
f2(
x)
L2[
x]
¨Ï±o
f (x) = (x - a1) . (x - a2) . f2(x).
©Ò¥H³o¼Ë¤@ª½§@¤U¥h
(©Î¬O¹ï degree §@ induction) §Ú̱oÃÒ¥»©w²z.
³Ì«á§Ú̱j½Õ¤@¤U Theorem 10.3.6 ¸Ìªº L' ·íµM·|¦] f (x)
¤£¦P¦Ó¤£¦P, ¤£¹L¨Æ¹ê¤W§ÚÌ¥i¥H§ä¨ì¤@Ó F ªº extension
¨Ï±o F[x] ¤¤ªº©Ò¦³ polynomial ¦b ¤¤³£¥i¥H splits into
linear factors (·íµM¦¹®É ¦³¥i¯à¤£¬O F ªº finite
extension). ¤£¹L¥Ñ©ó³oÓ©w²zªºÃÒ©ú»Ý¥Î¨ì©Ò¿×ªº Zorn's Lemma,
§ÚÌ´N²¤¥h¤£ÃÒ¤F.
¤U¤@¶: Finite Fields
¤W¤@¶: ¤¤¯Å Field ªº©Ê½è
«e¤@¶: Algebraic Closure
Administrator
2005-06-18