¤U¤@¶: ¤¤¯Å Field ªº©Ê½è
¤W¤@¶: ªì¯Å Field ªº©Ê½è
«e¤@¶: ±N ring ¬Ý¦¨¬O vector
µ¹©w¤@Ó field F, §ÚÌ·íµM¥i¥H°Q½×¨ä subfield, ¤£¹L¦]¤@¯ë field
ªº²z½×Ãö¤ßªº¬Oµ¹©w
f (x) F[x] ¦pªG¦b F ¤¤ f (x) ¨S¦³®Ú,
¨º»ò¦p¦ó¦b¤ñ F ¤jªº field §ä¨ì®Ú. ©Ò¥H§Ṳ́ñ¸ûÃö¤ßªº´N¬O©Ò¿× F
ªº extension field.
Definition 9.4.1
µ¹©w
F ¬O¤@Ó field, Y
L F ¤]¬O¤@Ó field ¦Ó¥B
L
ªº¹Bºâ¨î¦b
F ¤¤´N¬O쥻
F ªº¹Bºâ, «h§Ú̺Ù
L ¬O
F ªº¤@Ó
extension (©ÎºÙ
extension field). ·íµM¤F§Ṳ́]¥i¥HºÙ
F
¬O
L ªº¤@Ó
subfield.
°²³] F ¬O¤@Ó field ¥B L ¬O F ªº¤@Ó extension field, ¥Ñ Lemma
9.1.1 ª¾ L ¬O¤@Ó integral domain, ¬G¥Ñ«e¤@¸`ªº°Q½×§Ú̪¾
L ¬O¤@Ó vector space over F. §ÚÌ·íµM¥i¥H°Q½× L over F ªº
dimension.
Definition 9.4.2
°²³] F ¬O¤@Ó field ¥B L ¬O F ªº¤@Ó extension field. ¦pªG±N
L ¬Ý¦¨¬O over F ªº¤@Ó vector space ¬O¤@Ó finite dimensional
vector space over F, «hºÙ L ¬O F ªº¤@Ó finite
extension. ³q±`§ÚÌ·|±N dimF(L) ¥Î [L : F] ¨Óªí¥Ü, ºÙ¤§¬° the
degree of L over F (¦Ó¤£¬O»¡ the dimension of L over
F).
§ÚÌ¥i¥H§Q¥Î Theorem 9.3.7 ±o¨ì¥H¤U¦³½ìªºµ²ªG:
Proposition 9.4.3
°²³]
F ¬O¤@Ó field ¥B
L ¬O
F ªº¤@Ó finite extension. ¦pªG
R ¬O
L ªº¤@Ó subring ¥B²Å¦X
F R L, «h
R
¬O¤@Ó field.
µý ©ú.
§Ṳ́£¥´ºâ¥Î©w¸qª½±µÃÒ©ú
R ¬O¤@Ó field, ¦Ó¬O·Q®M¥Î Theorem
9.3.7 ¨Ó±o¨ì. n®M¥Î Theorem
9.3.7, §ÚÌ¥²¶·»¡©ú
R
¬O¤@Ó integral domain ¥B dim
F(
R) ¬O¦³ªº.
¦]¬° L ¤w¸g¬O¤@Ó integral domain (Lemma 9.1.1), ¦Ó R ¬O
L ªº subring, ©Ò¥H R ·íµM¬O integral domain. ¥t¤@¤è±,
§ÚÌ¥i¥H§â R ¬Ý¦¨¬O L ªº¤@Ó subspace over F. ¬G§Q¥Î L ¬O
F ªº¤@Ó finite extension ªº°²³]¥H¤Î Lemma 9.3.4 ª¾
dimF(R)dimF(L), ´«¥y¸Ü»¡ R ¬O¤@Ó finite dimensional
vector space over F. ¦]¦¹§Q¥Î Theorem 9.3.7 (2) ±oÃÒ R
¬O¤@Ó field.
Y L ¬O F ªº¤@Ó finite extension, «hª½±µ±N Theorem
9.3.7 (1) ®M¥Î¦b L ¤W, §ÚÌ°¨¤Wª¾¹ï¥ô·Nªº a L
¬Ò¦s¦b¤@Ó F[x] ¤¤ªº polynomial f (x) 0 º¡¨¬ f (a) = 0.
³o¼Ëªº¤¸¯À§Ú̵¹¥¦¤@Ó¯S®íªº¦W¦r.
Definition 9.4.4
°²³]
F ¬O¤@Ó field ¥B
L ¬O
F ªº¤@Ó extension field. °²³]
a L, ¦pªG¦s¦b
F[
x] ¤¤ªº¤@Ó«D 0 ªº polynomial
f (
x) º¡¨¬
f (
a) = 0, «hºÙ
a ¬O
algebraic over
F.
©Ò¥H Theorem 9.3.7 §i¶D§ÚÌ¥H¤Uµ²ªG:
Lemma 9.4.5
°²³]
F ¬O¤@Ó field ¥B
L ¬O
F ªº¤@Ó finite extension, «h
L
¤¤ªº¤¸¯À³£¬O algebraic over
F.
·í¤@Ó extensional field of F ¤¤ªº¤¸¯À³£¬O algebraic over F ®É,
§Ú̺ٳoÓ extension ¬O¤@Ó algebraic extension. Lemma
9.4.5 §i¶D§ÚÌ¥ô¦óªº finite extension of F ¤]³£¬O algebraic
extension of F. ¤£¹Lnª`·Nªº¬O¤@Ó algebraic extension of F
¤£¤@©w¬O finite extension of F.
³Ì«á§Ú̦A¬Ý¤@Ó¦³Ãö finite extension «nªº©Ê½è. ¦pªG F ¬O¤@Ó
field, K ¬O F ªº ¤@Ó extension field, ¦Ó¤S L ¬O K ªº¤@Ó
extension field. ¤]´N¬O§Ú̦³
F K L ³o¤@ÓÃö«Y.
·íµM¤F L ¤]¥i¬Ý¦¨¬O F ªº¤@Ó extension. ²Y°²³] K over F ©M
L over K ³£¬O finite extension, §Ú̦۵M·|°Ý¨º»ò L ¬Ý¦¨¬O F
ªº extension ®É¬O§_¤]¬O finite extension?
Theorem 9.4.6
°²³]
F ¬O¤@Ó field,
L ©M
K ³£¬O
F ªº extensions ¥B²Å¦X
F K L. Y¤wª¾
K ¬O
F ªº¤@Ó finite extension
¥B
L ¬O
K ªº¤@Ó finite extension, «h
L ¤]¬O
F ªº¤@Ó finite
extension, ¦Ó¥B
[L : F] = [L : K][K : F].
µý ©ú.
°²³] [
K :
F] =
m ¥H¤Î [
L :
K] =
n, §ÚÌ·QÃÒ©ú
L ¬O¤@Ó finite
extension of
F ¥B¨ä degree ¬°
m . n. ¥Ñ [
K :
F] =
m ªº°²³]ª¾
dim
F(
K) =
m, §Y¦s¦b
a1,...,
am K ¬O
K over
F ªº¤@²Õ
basis. ¦P¼Ëªº¦s¦b
b1,...,
bn L ¬O
L over
K ªº¤@²Õ basis.
§ÚÌ·QÃÒ©ú
{
ai . bj},
¬O
L over
F ªº¤@²Õ basis. ¦p¦¹¦ÛµM±oÃÒ¥»©w²z.
º¥ýnª`·Nªº¬O¦]¬°
K L ©Ò¥H¥Ñ
ai K,
bj L
¦ÛµM¥i±o
ai . bj L. §ÚÌnÃÒ©ú³o¨Ç
ai . bj span
L
over
F ¥B¬O linearly independent over
F.
º¥ýÃÒ©ú
{ai . bj} span L over F: ¥ô¨ú
L,
§ÚÌn§ä¨ì
ci, j F ¨Ï±o
µM¦Ó¦]
b1,...,
bn span
L over
K, §ÚÌ¥i¥H§ä¨ì
d1,...,
dn K
¨Ï±o
= d1 . b1 + ... + dn . bn. |
(9.4) |
¦A§Q¥Î
a1,...,
am span
K over
F, ¹ï¥ô¤@
dj K, §Ú̳£¥i¥H§ä¨ì
c1, j,...,
cm, j F ¨Ï±o
dj = c1, j . a1 + c2, j . a2 + ... + cm, j . am.
±N³o¨Ç
dj ±a¤J¦¡¤l (
9.4), ±oÃÒ
{
ai . bj} span
L
over
F.
±µµÛÃÒ©ú
{ai . bj} ¬O linearly independent over F.
§Q¥Î¤ÏÃÒªk, °²³]¦s¦b¤@²Õ¤£¥þ¬° 0 ªº
ci, j F ¨Ï±o
ci, j . (ai . bj) = 0. ³oªí¥Ü
0 |
= |
(c1, 1 . a1 + c2, 1 . a2 + ... + cm, 1 . am) . b1 |
|
|
|
+ ... + (c1, n . a1 + c2, n . a2 + ... + cm, n . am) . bn |
|
ª`·N¹ï¥ô·Nªº
j = 1,...,
n, Y¥O
dj = c1, j . a1 + c2, j . a2 + ... + cm, j . am,
¦]¬°
ci, j F,
ai K ¥B
F K, §Ú̦³
dj K ¥B
0 = d1 . b1 + d2 . b2 + ... + dn . bn.
¦]¬°
b1,...,
bn ¬O
linearly independent over
K, ¬G±o
d1 =
d2 =
... =
dn = 0.
´«¥y¸Ü»¡¹ï¥ô·Nªº
j = 1,...,
n, ¬Ò¦³
0 = dj = c1, j . a1 + c2, j . a2 + ... + cm, j . am.
¦A§Q¥Î
a1,...,
am ¬O linearly independent over
F ¥H¤Î³o¨Ç
ci, j
¬ÒÄÝ©ó
F, §Ú̱o³o¨Ç
ci, j ¬Òµ¥©ó 0. ¦¹©M·íªì°²³]
ci, j
¤£¥þ¬° 0 ¬Û¥Ù¬Þ, ¬G±oÃÒ
{
ai . bj} ¬O linearly independent
over
F.
nª`·N Theorem 9.4.6 ¤¤ªº±ø¥ó¬On¨D K ¬O F ªº finite
extension ¥B L ¬O K ªº finite extension ¤ ¯à±À±o L ¬O F ªº
finite extension. §Ú̦۵M·|°Ý¤Ï¹L¨Ó¹ï¶Ü? ¤]´N¬O»¡¦ý¦pªG¤wª¾
L ¬O F ªº finite extension, §Ú̬O§_¥i±o K ¬O F ªº finite
extension ¥B L ¬O K ªº finite extension ©O? µª®×¬OªÖ©wªº,
¨Æ¹ê¤W§Ú̦³¥H¤Uªºµ²ªG:
Corollary 9.4.7
°²³]
F ¬O¤@Ó field,
L ©M
K ³£¬O
F ªº extensions ¥B²Å¦X
F K L. Y¤wª¾
L ¬O
F ªº¤@Ó finite
extension, «h
K ¬O
F ªº¤@Ó finite extension ¥B
L ¬O
K
ªº¤@Ó finite extension, ¦Ó¥B
[L : F] = [L : K][K : F].
µý ©ú.
¥Ñ
F K L ³oÓÃö«Y¦¡, §ÚÌ¥i±N
K ¬Ý¦¨¬O
L
over
F ªº subspace, ©Ò¥H¥Ñ Lemma
9.3.4 (3) ª¾
dim
F(
L)
dim
F(
K), ´«¥y¸Ü»¡Y
L over
F ¬O¤@Ó finite
extension ¨º»ò
K over
F ·íµM¤]¬O finite extension.
¥t¤@¤è±Y°²³]
[
L :
F] = dim
F(
L) =
n, ¤]´N»¡¦s¦b
a1,...,
an L
¬O¤@²Õ
L over
F ªº basis, ¥Ñ©ó
a1,...,
an span
L over
F
¦A¥[¤W
F K, §ÚÌ·íµMª¾
a1,...,
an ¤] span
L over
K. ©Ò¥H§Q¥Î Lemma
9.3.4 (1) ª¾
dim
K(
L)
n = dim
F(
L).
¦]¦¹±o
L ¬O
K ªº¤@Ó finite extension.
¤W±¤wÃÒY L ¬O F ªº¤@Ó finite extension, «h K ¬O F ªº¤@Ó
finite extension ¥B L ¬O K ªº¤@Ó finite extension. ¦]¦¹¥i®M¥Î
Theorem 9.4.6 ±oÃÒ
[L : F] = [L : K][K : F].
¤U¤@¶: ¤¤¯Å Field ªº©Ê½è
¤W¤@¶: ªì¯Å Field ªº©Ê½è
«e¤@¶: ±N ring ¬Ý¦¨¬O vector
Administrator
2005-06-18