¤U¤@¶: Extension Field
¤W¤@¶: ½u©Ê¥N¼ÆªºÀ³¥Î
«e¤@¶: ½u©Ê¥N¼Æ°ò¥»©Ê½è.
§Ú̺¥ý¨Ó¬Ý¤@¨Ç¨Ò¤l,
¥Bpºâ¨ä dimension.
°²³] F ¬O¤@Ó field, §Ú̦Ҽ F[x] ³o¤@Ó polynomial ring.
«Ü®e©ö¬Ý¥X¨Ó F[x] ©M F º¡¨¬ Definition 9.3.1 ¤¤ (VS1) ¨ì
(VS4) ©Ê½è, ¬Gª¾ F[x] ¬O¤@Ó vector space over F. ¦Ü©ó F[x]
·|¤£·|¬O finite dimensional vector space over F ©O?
Proposition 9.3.5
°²³]
F ¬O¤@Ó field, Y±N
F[
x] ¬Ý¦¨¬O¤@Ó vector space over
F,
«h
F[
x] ¤£¬O finite dimensional vector space over
F.
µý ©ú.
§Ú̧Q¥Î¤ÏÃÒªk. °²³]
F[
x] ¬O finite dimensional over
F ¥B
dim
F(
F[
x]) =
n, «h¦Ò¼
1,
x,
x2,...,
xn F[
x], §ÚÌnÅçÃÒ
1,
x,
x2,...,
xn ¬O linearly independent over
F.
³o¬O¦]¬°¹ï¥ô·N¤£¥þ¬° 0 ªº
c0,
c1,...,
cn §Ú̪¾
c0 . 1 +
c1 . x +
... +
cn . xn0.
ª`·N
1,
x,
x2,...,
xn ¦@¦³
n + 1 Ó¤¸¯À, ¬G§Q¥Î Lemma
9.3.4 (2) ª¾
n + 1
dim
F(
F[
x]) =
n,
¦]¦Ó±o¨ì¥Ù¬Þ. ©Ò¥H
F[
x]
¤£¥i¯à¬O finite dimensional over
F.
±µµÛ§Ú̦Ҽ¥t¤@Ó ring. °²³]
f (x) F[x] ¥B
deg(f (x))1,
§Ú̦Ҽ
R = F[x]/f (x) ³o¤@Ó quotient ring. ¦^ÅU¤@¤U R
¤¤ªº¤¸¯À³£¬O
ªº§Î¦¡, ¨ä¤¤
g(x) F[x]. ¹ï¥ô·Nªº c F,
R, §ÚÌ©w¸q
c . =
.
³oÓ¹Bºâ¬O well-defined. ¦]¬°Y
= ªí¥Ü,
g(x) - h(x) f (x). ¤S¦]¬°
c F F[x] ¥B
f (x) ¬O F[x] ªº¤@Ó ideal, §ÚÌ·íµM¦³
c . (g(x) - h(x)) f (x), ¬Gª¾
c . = c . . §Q¥Î³oÓ F ¹ï R
ªº¹Bºâ§ÚÌ«Ü®e©öÅçÃÒ R ¬O¤@Ó vector space over F. ¨º»ò R
·|¤£·|¬O finite dimensional vector space over F ©O?
Lemma 9.3.6
°²³]
F ¬O¤@Ó field, Y
f (
x)
F[
x] ¥B
deg(
f (
x))
1, «h
R =
F[
x]/
f (
x)
³o¤@Ó quotient ring ¬O¤@Ó finite dimensional
vector space over
F ¦Ó¥B
dim
F(
R) = deg(
f (
x)).
µý ©ú.
°²³]
deg(
f (
x)) =
n, §ÚÌnÃÒ©ú
,
,...,
R ¬O
R over
F ªº¤@²Õ basis.
º¥ýÃÒ©ú
,,..., span R over F. ¥ô¨ú
R, ¨ä¤¤
g(x) F[x], §ÚÌn§ä¨ì
c0, c1,..., cn - 1 F ¨Ï±o
=
c0 . +
c1 . +
... +
cn - 1 . .
¥Ñ Theorem
7.2.4, §Ú̪¾¹D¦s¦b
h(
x),
r(
x)
F[
x] º¡¨¬
g(
x) =
f (
x)
. h(
x) +
r(
x), ¨ä¤¤
r(
x) = 0 ©Î
deg(
r(
x)) < deg(
f (
x)). ¦]¬°
g(
x) -
r(
x) =
f (
x)
. h(
x)
f (
x)
, ¥Ñ quotient ring ªº©w¸qª¾
=
. ²Y
r(
x) = 0, ª¾
=
, ¬G¨ú
c0 =
c1 =
... =
cn - 1 = 0 ®É¥i±o
=
=
c0 . +
c1 . +
... +
cn - 1 . .
¥t¤@¤è±Y
r(
x)
0, «h¥Ñ
deg(
r(
x))
n - 1 ª¾¦s¦b
a0,
a1,...,
an - 1 F ¨Ï±o
r(
x) =
a0 +
a1x +
... +
an - 1xn - 1, ¬G¥O
c0 =
a0,...,
cn - 1 =
an - 1 ®É§Ú̦³
=
=
c0 . +
c1 . +
... +
cn - 1 . .
©Ò¥H
R
¤¤ªº¤¸¯À³£¥i¥Ñ
,...,
span over
F ±o¨ì.
±µµÛÃÒ©ú
,,..., ¬O linearly independent
over F. §Ú̧Q¥Î¤ÏÃÒªk. °²³]¦s¦b¤£¥þ¬° 0 ªº
c0, c1, ... , cn - 1 F ¨Ï±o
c0 . +
c1 . +
... +
cn - 1 . =
,
ªí¥Ü
g(
x) =
c0 +
... +
cn - 1xn - 1 ³oÓ«D 0 ªº¦h¶µ¦¡²Å¦X
=
. ´«¥y¸Ü»¡
g(
x)
f (
x)
. ¦]
g(
x)
0,
¬Gª¾¦s¦b
h(
x)
F[
x] ¥B
h(
x)
0 ¨Ï±o
g(
x) =
f (
x)
. h(
x).
Æ[¹î degree ª¾
deg(
g(
x)) = deg(
f (
x)) + deg(
h(
x))
deg(
f (
x)) =
n,
¤£¹L¥Ñ·íªì
g(
x) ªº¿ï¨ú, §Ú̪¾¹D
deg(
g(
x))
n - 1, ¦]¦¹±o¨ì¥Ù¬Þ. ¬Gª¾
,
,...,
¬O linearly independent over
F.
§Ṳ́wÃÒ±o
,,..., R ¬O R over F
ªº¤@²Õ basis. ¤S¦]
,,..., ¤¤¦@¦³ n
Ó¤¸¯À, ¬Gª¾
dimF(R) = n = deg(f (x)).
·í R ¬O¤@Ó integral domain ¥B F ¬O¤@Ó¥]§t©ó R ªº field ®É,
§Ṳ́]¥i¥H±N R ¬Ý¦¨¬O¤@Ó vector space over F. ¨Æ¹ê¤W¥Ñ ring
ªº©Ê½è¥[¤W
F R, Definition 9.3.1 ¤¤ªº (VS1), (VS2)
¥H¤Î (VS3) ¦ÛµM³£²Å¦X, §Ṵ́ߤ@nÀˬdªº¬O (VS4). °²³] 1F, 1R
¤À§O¬O F ©M R ¼ªkªº identity, §ÚÌ¥unÀ˹î 1F = 1R §Y¥i.
³o¬O¦]¬° (VS4) ¬O»¡¹ï¥ô·Nªº a R n²Å¦X
1F . a = a.
¦]¦¹Y¯àÃÒ±o 1F = 1R, ¨º»ò¤W¦¡¦ÛµM¦¨¥ß. nª`·N§ÚÌ´¿¸g¬Ý¹L¨Ò¤l¤@Ó
subring ªº identity ¤£¤@©w·|¬Oì¨Óªº ring ªº identity. ¤£¹L¥Ñ©ó²¦b
R ¬O integral domain, ¨Æ±¡´N¨S¦³¨º»ò½ÆÂø¤F. §ÚÌ¥un¥ô¨ú F
¤¤ªº¤@Ó«D 0 ¤¸¯À c, ±N¥¦¦Ò¼¦¨¬O F ªº¤¸¯À, §Ú̦³
1F . c = c; ¥t¤@¤è±±N¥¦¬Ý¦¨¬O R ªº¤¸¯À, §Ú̦³
1R . c = c.
µ²¦X¤W±¨âÓµ¥¦¡±o:
(1F - 1R) . c = 0. ¥Ñ©ó R ¬O integral
domain ¥B c 0, ©Ò¥H§Ú̦³ 1F = 1R.
¬JµM R ¬O¤@Ó over F ªº vector space, §Ų́Ӭݷí R ¬O finite
dimensional over F ®É¥¦¦³¤°»ò«n¯S©Ê.
Theorem 9.3.7
°²³]
R ¬O¤@Ó integral domain,
F ¬O¤@Ó field ¥B
F R.
¤S°²³]
R ¬Ý¦¨¬O¤@Ó vector space over
F ®É¬O finite dimensional
over
F, «h
- ¹ï¥ô·N a R, ¬Ò¦s¦b¤@Ó«D 0 ªº
f (x) F[x] ¨Ï±o f (a) = 0.
- R ¬O¤@Ó field.
µý ©ú.
§ÚÌ°²³]
dim
F(
R) =
n.
(1) ¦Ò¼
1, a, a2,..., an ³o n + 1 Ó R ¤¤ªº¤¸¯À. ¦pªG¥¦Ì¬O
linearly independent over F, «h¥Ñ Lemma 9.3.4 (2) ±o
n = dim
F(
R)
n + 1,
³y¦¨¥Ù¬Þ, ¬Gª¾
1,
a,
a2,...,
an ¤£¬O linearly independent over
F.
´«¥y¸Ü»¡¦s¦b¤£¥þ¬° 0 ªº
c0,
c1,...,
cn F, º¡¨¬
c0 . 1 + c1 . a + ... + cn . an = 0.
¬G¥O
f (
x) =
c0 +
c1x +
... +
cnxn, §Ú̱o
f (
x)
0 ¥B
f (
a) = 0.
(2) ¦] R ¤wª¾¬O integral domain, nÃÒ©ú R ¬O¤@Ó field,
§ÚÌ¥unÃÒ©ú R ¤¤¤£¬° 0 ªº¤¸¯À³£¬O unit. ´«¥y¸Ü»¡nÃÒ©ú¹ï¥ô·N
a R ¥B a 0, ¬Ò¦s¦b b R º¡¨¬
a . b = 1. ¥Ñ (1)
ª¾¦s¦b«D 0 ªº¦h¶µ¦¡ f (x) º¡¨¬ f (a) = 0. §ÚÌ°²³]
f (
x) =
c0 +
c1x +
... +
cmxm F[
x]
¬O
F[
x] ¤¤«D 0 ¥Bº¡¨¬
f (
a) = 0 ªº degree ³Ì¤pªº polynomial. ¥Ñ degree ³Ì¤pªº°²³], §ÚÌ¥i±o
c0 0. ³o¬O¦]¬°Y
c0 = 0, «h¥Ñ
f (a) = c1 . a + ... + cm . am = (c1 + c2 . a + ... + cm . am - 1) . a = 0
¥H¤Î
R ¬O integral domain ±o
g(
a) = 0, ¨ä¤¤
g(
x) =
c1 +
c2x +
... +
cmxm - 1 F[
x] ¤£¬° 0 ¥B
deg(
g(
x)) < deg(
f (
x)). ¦¹©M
f (
x) ¬O degree ³Ì¤pªº§äªk¬Û¥Ù¬Þ,
¬G±o
c0 0. ²±N
f (
a) = 0 ªº
c0 ²¾¦Üµ¥¦¡ªº¥t¤@Ãä, §Ú̱o
(c1 + c2 . a + ... + cm . am - 1) . a = - c0.
¦]¦¹Y¥O
b = (- c0)-1 . (c1 + c2 . a + ... + cm . am - 1),
«h§Ú̦³
a . b = 1. ª`·N¥Ñ©ó
-
c0 F ¥B -
c0 0 ¥H¤Î
F ¬O¤@Ó field, §Ú̦³
(-
c0)
-1 F R, ¦A¥[¤W
c1 +
c2 . a +
... +
cm . am - 1 R §Ú̱o
b R, ¬Gª¾
a ¬O
R
ªº¤@Ó unit.
§Q¥Î Theorem 9.3.7 §ÚÌ¥i¥H«Ü§Öªºµ¹ Proposition 9.3.5
¥t¤@ÓÃÒ©ú: °²¦p F[x] ¬O finite dimensional over F, ¥Ñ©ó F[x]
¬O integral domain §Q¥Î Theorem 9.3.7 §Ú̱o F[x] ·|¬O¤@Ó
field. ¦ý³o¬O¤£¥i¯àªº, ¦]¬° F[x] ¤¤¥u¦³ degree ¬° 0 ªº¤¸¯À¤ ¬O
unit.
¤U¤@¶: Extension Field
¤W¤@¶: ½u©Ê¥N¼ÆªºÀ³¥Î
«e¤@¶: ½u©Ê¥N¼Æ°ò¥»©Ê½è.
Administrator
2005-06-18