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¤U¤@­¶: Extension Field ¤W¤@­¶: ½u©Ê¥N¼ÆªºÀ³¥Î «e¤@­¶: ½u©Ê¥N¼Æ°ò¥»©Ê½è.

±N ring ¬Ý¦¨¬O vector space

§Ú­Ì­º¥ý¨Ó¬Ý¤@¨Ç¨Ò¤l, ¥B­pºâ¨ä dimension.

°²³] F ¬O¤@­Ó field, §Ú­Ì¦Ò¼ 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. §Ú­Ì§Q¥Î¤ÏÃÒªk. °²³] F[x] ¬O finite dimensional over F ¥B dimF(F[x]) = n, «h¦Ò¼ 1, x, x2,..., xn $ \in$ F[x], §Ú­Ì­nÅçÃÒ 1, x, x2,..., xn ¬O linearly independent over F. ³o¬O¦]¬°¹ï¥ô·N¤£¥þ¬° 0 ªº c0, c1,..., cn §Ú­Ìª¾

c0 . 1 + c1 . x + ... + cn . xn$\displaystyle \ne$0.

ª`·N 1, x, x2,..., xn ¦@¦³ n + 1 ­Ó¤¸¯À, ¬G§Q¥Î Lemma 9.3.4 (2) ª¾

n + 1$\displaystyle \le$dimF(F[x]) = n,

¦]¦Ó±o¨ì¥Ù¬Þ. ©Ò¥H F[x] ¤£¥i¯à¬O finite dimensional over F. $ \qedsymbol$

±µµÛ§Ú­Ì¦Ò¼¥t¤@­Ó ring. °²³] f (x) $ \in$ F[x] ¥B deg(f (x))$ \ge$1, §Ú­Ì¦Ò¼ R = F[x]/$ \bigl($f (x)$ \bigr)$ ³o¤@­Ó quotient ring. ¦^ÅU¤@¤U R ¤¤ªº¤¸¯À³£¬O $ \overline{g(x)}$ ªº§Î¦¡, ¨ä¤¤ g(x) $ \in$ F[x]. ¹ï¥ô·Nªº c $ \in$ F, $ \overline{g(x)}$ $ \in$ R, §Ú­Ì©w¸q

c . $\displaystyle \overline{g(x)}$ = $\displaystyle \overline{c\cdot g(x)}$.

³o­Ó¹Bºâ¬O well-defined. ¦]¬°­Y $ \overline{g(x)}$ = $ \overline{h(x)}$ ªí¥Ü, g(x) - h(x) $ \in$ $ \bigl($f (x)$ \bigr)$. ¤S¦]¬° c $ \in$ F $ \subseteq$ F[x] ¥B $ \bigl($f (x)$ \bigr)$ ¬O F[x] ªº¤@­Ó ideal, §Ú­Ì·íµM¦³ c . (g(x) - h(x)) $ \in$ $ \bigl($f (x)$ \bigr)$, ¬Gª¾ c . $ \overline{g(x)}$ = c . $ \overline{h(x)}$. §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) $ \in$ F[x] ¥B deg(f (x))$ \ge$1, «h R = F[x]/$ \bigl($f (x)$ \bigr)$ ³o¤@­Ó quotient ring ¬O¤@­Ó finite dimensional vector space over F ¦Ó¥B dimF(R) = deg(f (x)).

µý ©ú. °²³] deg(f (x)) = n, §Ú­Ì­nÃÒ©ú $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ $ \in$ R ¬O R over F ªº¤@²Õ basis.

­º¥ýÃÒ©ú $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ span R over F. ¥ô¨ú $ \overline{g(x)}$ $ \in$ R, ¨ä¤¤ g(x) $ \in$ F[x], §Ú­Ì­n§ä¨ì c0, c1,..., cn - 1 $ \in$ F ¨Ï±o

$\displaystyle \overline{g(x)}$ = c0 . $\displaystyle \overline{1}$ + c1 . $\displaystyle \overline{x}$ + ... + cn - 1 . $\displaystyle \overline{x}^{n-1}_{}$.

¥Ñ Theorem 7.2.4, §Ú­Ìª¾¹D¦s¦b h(x), r(x) $ \in$ 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) $ \in$ $ \bigl($f (x)$ \bigr)$, ¥Ñ quotient ring ªº©w¸qª¾ $ \overline{g(x)}$ = $ \overline{r(x)}$. ²­Y r(x) = 0, ª¾ $ \overline{g(x)}$ = $ \overline{0}$, ¬G¨ú c0 = c1 = ... = cn - 1 = 0 ®É¥i±o

$\displaystyle \overline{g(x)}$ = $\displaystyle \overline{0}$ = c0 . $\displaystyle \overline{1}$ + c1 . $\displaystyle \overline{x}$ + ... + cn - 1 . $\displaystyle \overline{x}^{n-1}_{}$.

¥t¤@¤è­±­Y r(x)$ \ne$ 0, «h¥Ñ deg(r(x))$ \le$n - 1 ª¾¦s¦b a0, a1,..., an - 1 $ \in$ F ¨Ï±o r(x) = a0 + a1x + ... + an - 1xn - 1, ¬G¥O c0 = a0,..., cn - 1 = an - 1 ®É§Ú­Ì¦³

$\displaystyle \overline{g(x)}$ = $\displaystyle \overline{r(x)}$ = c0 . $\displaystyle \overline{1}$ + c1 . $\displaystyle \overline{x}$ + ... + cn - 1 . $\displaystyle \overline{x}^{n-1}_{}$.

©Ò¥H R ¤¤ªº¤¸¯À³£¥i¥Ñ $ \overline{1}$,...,$ \overline{x}^{n-1}_{}$ span over F ±o¨ì.

±µµÛÃÒ©ú $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ ¬O linearly independent over F. §Ú­Ì§Q¥Î¤ÏÃÒªk. °²³]¦s¦b¤£¥þ¬° 0 ªº c0, c1, ... , cn - 1 $ \in$ F ¨Ï±o

c0 . $\displaystyle \overline{1}$ + c1 . $\displaystyle \overline{x}$ + ... + cn - 1 . $\displaystyle \overline{x}^{n-1}_{}$ = $\displaystyle \overline{0}$,

ªí¥Ü g(x) = c0 + ... + cn - 1xn - 1 ³o­Ó«D 0 ªº¦h¶µ¦¡²Å¦X $ \overline{g(x)}$ = $ \overline{0}$. ´«¥y¸Ü»¡ g(x) $ \in$ $ \bigl($f (x)$ \bigr)$. ¦] g(x)$ \ne$ 0, ¬Gª¾¦s¦b h(x) $ \in$ F[x] ¥B h(x)$ \ne$ 0 ¨Ï±o g(x) = f (x) . h(x). Æ[¹î degree ª¾

deg(g(x)) = deg(f (x)) + deg(h(x))$\displaystyle \ge$deg(f (x)) = n,

¤£¹L¥Ñ·íªì g(x) ªº¿ï¨ú, §Ú­Ìª¾¹D deg(g(x))$ \le$n - 1, ¦]¦¹±o¨ì¥Ù¬Þ. ¬Gª¾ $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ ¬O linearly independent over F.

§Ú­Ì¤wÃÒ±o $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ $ \in$ R ¬O R over F ªº¤@²Õ basis. ¤S¦] $ \overline{1}$,$ \overline{x}$,...,$ \overline{x}^{n-1}_{}$ ¤¤¦@¦³ n ­Ó¤¸¯À, ¬Gª¾ dimF(R) = n = deg(f (x)). $ \qedsymbol$

·í R ¬O¤@­Ó integral domain ¥B F ¬O¤@­Ó¥]§t©ó R ªº field ®É, §Ú­Ì¤]¥i¥H±N R ¬Ý¦¨¬O¤@­Ó vector space over F. ¨Æ¹ê¤W¥Ñ ring ªº©Ê½è¥[¤W F $ \subseteq$ R, Definition 9.3.1 ¤¤ªº (VS1), (VS2) ¥H¤Î (VS3) ¦ÛµM³£²Å¦X, §Ú­Ì°ß¤@­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 $ \in$ 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, §Ú­Ì¦³ 1F . c = c; ¥t¤@¤è­±±N¥¦¬Ý¦¨¬O R ªº¤¸¯À, §Ú­Ì¦³ 1R . c = c. µ²¦X¤W­±¨â­Óµ¥¦¡±o: (1F - 1R) . c = 0. ¥Ñ©ó R ¬O integral domain ¥B c$ \ne$ 0, ©Ò¥H§Ú­Ì¦³ 1F = 1R.

¬JµM R ¬O¤@­Ó over F ªº vector space, §Ú­Ì¨Ó¬Ý·í 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 $ \subseteq$ R. ¤S°²³] R ¬Ý¦¨¬O¤@­Ó vector space over F ®É¬O finite dimensional over F, «h
  1. ¹ï¥ô·N a $ \in$ R, ¬Ò¦s¦b¤@­Ó«D 0 ªº f (x) $ \in$ F[x] ¨Ï±o f (a) = 0.
  2. R ¬O¤@­Ó field.

µý ©ú. §Ú­Ì°²³] dimF(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 = dimF(R)$\displaystyle \ge$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 $ \in$ F, º¡¨¬

c0 . 1 + c1 . a + ... + cn . an = 0.

¬G¥O f (x) = c0 + c1x + ... + cnxn, §Ú­Ì±o f (x)$ \ne$ 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 $ \in$ R ¥B a$ \ne$ 0, ¬Ò¦s¦b b $ \in$ 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 $\displaystyle \in$ 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$ \ne$ 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 $ \in$ 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$ \ne$ 0. ²±N f (a) = 0 ªº c0 ²¾¦Üµ¥¦¡ªº¥t¤@Ãä, §Ú­Ì±o

(c1 + c2 . a + ... + cm . am - 1) . a = - c0.

¦]¦¹­Y¥O

b = (- c0)-1 . (c1 + c2 . a + ... + cm . am - 1),

«h§Ú­Ì¦³ a . b = 1. ª`·N¥Ñ©ó - c0 $ \in$ F ¥B - c0$ \ne$ 0 ¥H¤Î F ¬O¤@­Ó field, §Ú­Ì¦³ (- c0)-1 $ \in$ F $ \subseteq$ R, ¦A¥[¤W c1 + c2 . a + ... + cm . am - 1 $ \in$ R §Ú­Ì±o b $ \in$ R, ¬Gª¾ a ¬O R ªº¤@­Ó unit. $ \qedsymbol$

§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 §Ú­Ì±o F[x] ·|¬O¤@­Ó field. ¦ý³o¬O¤£¥i¯àªº, ¦]¬° F[x] ¤¤¥u¦³ degree ¬° 0 ªº¤¸¯À¤ ¬O unit.


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¤U¤@­¶: Extension Field ¤W¤@­¶: ½u©Ê¥N¼ÆªºÀ³¥Î «e¤@­¶: ½u©Ê¥N¼Æ°ò¥»©Ê½è.
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