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¤U¤@­¶: Algebraic Closure ¤W¤@­¶: ¤¤¯Å Field ªº©Ê½è «e¤@­¶: ¤¤¯Å Field ªº©Ê½è

Algebraic Elements

°²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension. ­nª¾¹D F ¤¤ªº¤@­Ó¤¸¯À a ¬O§_ algebraic over F, ¨Ì©w¸q´N¥²¶·ÅçÃÒ¬O§_¦s¦b¤@­Ó¤£¬° 0 ªº f (x) $ \in$ F[x] ¨Ï±o f (a) = 0. ¤@¯ë¨Ó»¡¥Î³oºØ¤èªk¨ÓÅçÃÒ¤@­Ó¤¸¯À¬O§_¬O algebraic over F, §Þ³N¤W¬O¬Û·í§xÃøªº. ³o¤@¸`¤¤§Ú­Ì±N°Q½×´XºØ©M­ì¥ý algebraic element ªº©w¸qµ¥»ùªº©Ê½è, ³o¼Ë¥H«á§Ú­Ì­nÅçÃÒ¤@­Ó¤¸¯À¬O§_¬O algebraic over F ´N¦³¦h¤@ÂIªº¤èªk¨Ó³B²z.

­º¥ýª`·N·í a $ \in$ L ¬O algebraic over F ®É, ¨Æ¹ê¤Wº¡¨¬ f (x) $ \in$ F[x] ¥B f (a) = 0 ªº¦h¶µ¦¡¦³µL½a¦h­Ó. ¤£¹L³o¨ä¤¤¦³¤@­Ó¬Û·í¯S§O. §Ú­Ì­º¥ý¥i¥H¦Ò¼º¡¨¬ f (a) = 0 ªº f (x) $ \in$ F[x] ¤¤ degree ³Ì¤pªº polynomials. ³o¼Ëªº polynomials ¦³¥H¤U¨â­Ó­«­nªº©Ê½è.

Lemma 10.1.1   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension. ­Y a $ \in$ L ¬O algebraic over F ¥B f (x) $ \in$ F[x] ¬O F[x] ¤¤º¡¨¬ f (a) = 0 ªº«D 0 ¦h¶µ¦¡¤¤ degree ³Ì¤pªº¤@­Ó polynomial, «h f (x) ¦³¥H¤U¨â­Ó©Ê½è:
  1. ­Y g(x) $ \in$ F[x] ¥B g(a) = 0, «h¦s¦b h(x) $ \in$ F[x] º¡¨¬ g(x) = f (x) . h(x).
  2. f (x) ¬O F[x] ¤¤ªº irreducible element.

µý ©ú. (1) ¥Ñ©ó F ¬O¤@­Ó field, §Q¥Î Euclid's Algorithm (Theorem 7.2.4) ª¾¦s¦b h(x), r(x) $ \in$ F[x] ¨Ï±o

g(x) = f (x) . h(x) + r(x) (10.1)

¨ä¤¤ r(x) = 0 ©Î deg(r(x)) < deg(f (x)). ±N a ¥N¤J¦¡¤l (10.1) ±o

g(a) = f (a) . h(a) + r(a).

¥Ñ©ó f (a) = g(a) = 0, §Ú­Ì±o r(a) = 0. ¦pªG r(x)$ \ne$ 0, «h±o¨ì r(x) $ \in$ F[x] º¡¨¬ deg(r(x)) < deg(f (x)) ¥B r(a) = 0. ³o©M f (x) ·íªìªº¿ï¨ú¬Û¥Ù¬Þ, ¬Gª¾ r(x) = 0. ¤]´N¬O»¡ g(x) = f (x) . h(x).

(2) °²³] f (x) ¦b F[x] ¤¤¤£¬O irreducible, §Y¦s¦b h(x), l (x) $ \in$ F[x] º¡¨¬ deg(h(x)) < deg(f (x)), deg(l (x)) < deg(f (x)) ¥B f (x) = h(x) . l (x). ±N a ¥N¤J¤W¦¡, ¥Ñ f (a) = 0 ª¾ h(a) . l (a) = 0. ¥Ñ©ó h(x), l (x) $ \in$ F[x] ¥B a $ \in$ L, §Ú­Ìª¾ h(a), l (a) $ \in$ L. ¬G¥Ñ L ¬O integral domain (Lemma 9.1.1) ±o h(a) = 0 ©Î l (a) = 0. ³o¦A¦¸©M f (x) ªº¿ï¨ú¬Û¥Ù¬Þ, ¬Gª¾ f (x) ¬O F[x] ¤¤ªº irreducible element. $ \qedsymbol$

­Y f (x) $ \in$ F[x] ¬O F[x] ¤¤²Å¦X f (a) = 0 degree ³Ì¤pªº polynomial ¥B g(x) $ \in$ F[x] º¡¨¬ g(a) = 0, «h¥Ñ Lemma 10.1.1 (1) ª¾ g(x) $ \in$ $ \bigl($f (x)$ \bigr)$. ²¦b¦pªG g(x) ¤]¬O F[x] ¤¤²Å¦X g(a) = 0 degree ³Ì¤pªº polynomial, «h¥i±o $ \bigl($f (x)$ \bigr)$ = $ \bigl($g(x)$ \bigr)$. ¥Ñ©ó F[x] ¤¤ªº unit ³£¬O F ¤¤ªº«D 0 ¤¸¯À (Proposition 7.2.3) §Q¥Î Lemma 8.1.3 ª¾¦s¦b c $ \in$ F ¨Ï±o f (x) = c . g(x). ©Ò¥H¦pªG§Ú­Ì±N³o¨Ç¦¸¼Æ³Ì§C¦Óº¡¨¬ f (a) = 0 ªº polynomial °£¥H¥¦­Ìªº³Ì°ª¦¸¶µ«Y¼Æ©Ò±oªº monic polynomial ´N°ß¤@¤F. ¦]¦¹§Ú­Ì¦³¥H¤U¤§©w¸q.

Definition 10.1.2   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field ¥B a $ \in$ L ¬O algebraic over F. ­Y p(x) $ \in$ F[x] ¬O F[x] ªº«D 0 polynomial ¤¤º¡¨¬ p(a) = 0 degree ³Ì¤pªº monic polynomial, «hºÙ p(x) ¬O a over F ªº minimal polynomial. ¤S¦pªG deg(p(x)) = n, «hºÙ a ¬O algebraic over F of degree n.

§Ú­Ìª¾¹D·í [L : F] ¬O¦³­­ªº®É­Ô, L ¤¤ªº¤¸¯À³£¬O algebraic over F. ­Y [L : F] = n ¥B a $ \in$ L, «h¥Ñ©ó 1, a,..., an ¤@©w linearly independent over F, ¬Gª¾¦s¦b f (x) $ \in$ F[x] ¥B deg(f (x))$ \le$n ¨Ï±o f (a) = 0 (¸Ô¨£ Theorem 9.3.7 ªºµý©ú) ¬G¥Ñ minimal polynomial ªº©w¸qª¾: ­Y p(x) ¬O a ªº minimal polynomial, «h deg(p(x))$ \le$deg(f (x))$ \le$n. ´«¨¥¤§§Ú­Ì±o a ªº degree ¤p©ó©Îµ¥©ó [L : F]. §Ú­Ì±N³o­Óµ²ªG¼g¦¨¥H¤U¤§ Lemma.

Lemma 10.1.3   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó finite extension, «h L ¤¤¥ô·Nªº¤¸¯À³£¬O algebraic over F ¥B¨ä degree ¤p©ó©Îµ¥©ó [L : F].

·í L ¤£¬O finite extension over F ®É, L ¤¤·íµM¦³¥i¯à¦s¦b¤¸¯À¬O algebraic over F. ¦pªG a $ \in$ L ¬O algebraic over F, §Ú­Ì·Qª¾¹D F ©M L ¤§¶¡¬O§_¥i¥H§ä¨ì¤@­Ó field K ¬O F ªº¤@­Ó finite extension º¡¨¬ a $ \in$ K?

Definition 10.1.4   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field. ­Y K ¬O L ªº¤@­Ó extension field ¥B F $ \subseteq$ K $ \subseteq$ L, «hºÙ K ¬O L over F ªº¤@­Ó subextension ©Î¬O intermediate field.

Proposition 10.1.5   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field. ­Y a $ \in$ L ¬O algebraic over F ¥B¨ä degree ¬° n, «h¦s¦b L over F ªº¤@­Ó subextension K º¡¨¬ a $ \in$ K ¥B [K : F] = n.

µý ©ú. ¦Ò¼ $ \phi$ : F[x]$ \to$L ¨ä¤¤¹ï¥ô·Nªº f (x) $ \in$ F[x], $ \phi$(f (x)) = f (a). ¥Ñ©ó a $ \in$ L, ©Ò¥H¦ÛµM¦³ $ \phi$(f (x)) = f (a) $ \in$ L, ¦]¦¹ $ \phi$ ½T¹ê¬O¤@­Ó±q F[x] ¬M®g¨ì L ªº¨ç¼Æ. «Ü®e©öÅçÃÒ $ \phi$ ¬O¤@­Ó ring homomorphism.

¤°»ò¬O ker($ \phi$) ©O? ¥Ñ©ó F[x] ¬O¤@­Ó principle ideal domain ¥B ker($ \phi$) ¬O F[x] ªº¤@­Ó ideal, §Ú­Ìª¾¦s¦b p(x) $ \in$ f[x] ¨Ï±o ker($ \phi$) = $ \bigl($p(x)$ \bigr)$. ¨Æ¹ê¤W §Ú­Ì¥i¥H¦³ ker($ \phi$) = $ \bigl($p(x)$ \bigr)$ ¨ä¤¤ p(x) ¬O a ªº minimal polynomial. ³o¬O¦]¬°­Y f (x) $ \in$ ker($ \phi$), «hª¾ $ \phi$(f (x)) = f (a) = 0. ¬G¥Ñ Lemma 10.1.1 ª¾ f (x) $ \in$ $ \bigl($p(x)$ \bigr)$. ¤Ï¤§, ¹ï¥ô·N f (x) $ \in$ $ \bigl($p(x)$ \bigr)$, ¦s¦b h(x) $ \in$ F[x] ¨Ï±o f (x) = p(x) . h(x), ¦]¦¹¥Ñ p(a) = 0 ±o f (a) = p(a) . h(a) = 0. ¬G±oÃÒ ker($ \phi$) = $ \bigl($p(x)$ \bigr)$, ¨ä¤¤ p(x) ¬O a ªº minimal polynomial.

²¥Ñ First Isomorphism Theorem (6.4.2) ª¾

F[x]/$\displaystyle \bigl($p(x)$\displaystyle \bigr)$ $\displaystyle \simeq$ im($\displaystyle \phi$).

µM¦Ó p(x) ¬O F[x] ªº¤@­Ó irreducible element (Lemma 10.1.1), ¬G¥Ñ $ \bigl($p(x)$ \bigr)$ ¬O F[x] ªº¤@­Ó maximal ideal (Lemma 8.3.2), ±oª¾ F[x]/$ \bigl($p(x)$ \bigr)$ ¬O¤@­Ó field (Theorem 6.5.11). ´«¨¥¤§ im($ \phi$) ¬O¤@­Ó field.

¦Ü©ó¤°»ò¬O im($ \phi$) ©O? ¥Ñ©w¸qª¾

im($\displaystyle \phi$) = {f (a) | f (x) $\displaystyle \in$ F[x]}.

´«¨¥¤§, im($ \phi$) ¸Ìªº¤¸¯À³£¬O¥Ñ¬Y­Ó F[x] ¸Ìªº polynomial ¥N¤J a ©Ò±o. ©Ò¥H­Y c $ \in$ F, §Ú­Ì¦ÛµM¦³ $ \phi$(c) = c $ \in$ im($ \phi$), ¬G±o F $ \subseteq$ im($ \phi$) $ \subseteq$ L. ¥t¤@¤è­±±N a ¥N¤J x ³o¤@­Ó polynomial ±o¨ì a: ¤]´N¬O»¡ $ \phi$ ±N x °e¨ì a (§Y $ \phi$(x) = a), ¬Gª¾ a $ \in$ im($ \phi$). ©Ò¥H­Y¥O K = im($ \phi$), «hª¾ K ¬O L over F ªº¤@­Ó subextension ¥B a $ \in$ K. ³Ì«á¥Ñ°²³] a over F ªº degree ¬O n, ¤]´N¬O»¡ a ªº minimal polynomial p(x) ªº degree ¬O n, ¦]¦¹¥Ñ Lemma 9.3.6 ª¾ dimF(F[x]/$ \bigl($p(x)$ \bigr)$) = deg(p(x)) = n. ¬G¥Ñ K $ \simeq$ F[x]/$ \bigl($p(x)$ \bigr)$ ª¾ [K : F] = n. $ \qedsymbol$

­Y¶È¥Ñ©w¸q¨Ó¬Ý Proposition 10.1.5 ¤¤ªº im($ \phi$) = {f (a) | f (x) $ \in$ F[x]} ¥u¬O¤@­Ó ring, ¨º¬°¦ó¥¦·|¬O field ©O? ­Y§A°O±o Theorem 9.3.7 ³o´N¤@ÂI³£¤£©_©Ç¤F. ¦]¬° im($ \phi$) $ \subseteq$ L ¦ÛµM¬O integral domain, ¦Ó¥Ñ Proposition 10.1.5 ªºÃÒ©ú¤]ª¾ dimF(im($ \phi$)) = n.

§Ú­Ì¤]«Ü®e©öÀˬd {f (a) | f (x) $ \in$ F[x]} ·|¬O L ¤¤¥]§t F ¥H¤Î a ³Ì¤pªº ring, ³o¬O¦]¬°­Y R ¬O¤@­Ó ring ¥B¥]§t F ¥H¤Î a, «h¹ï¥ô·Nªº f (x) $ \in$ F[x], ¥Ñ©ó f (a) ¶È²o¯A¨ì a ©M F ¤¤ªº¤¸¯À¶¡ªº¥[ªk¥H¤Î­¼ªk, §O§Ñ¤F³o¨Ç³£¬O R ¤¤¤¸¯Àªº¹Bºâ©Ò¥H·íµM±o f (a) $ \in$ R. ´«¨¥¤§§Ú­Ì±o {f (a) | f (x) $ \in$ F[x]} $ \subseteq$ R, ¦A¥[¤W {f (a) | f (x) $ \in$ F[x]} ¥»¨­¬O¤@­Ó ring ©Ò¥H¥¦¦ÛµM¬O¥]§t F ¥H¤Î a ³Ì¤pªº ring ¤F!

¬°¤F¤è«K§Ú­Ì©w¥H¤U¤§²Å¸¹, ¦b¤@¯ëªº¥N¼Æ®Ñ¤W³o­Ó©w¸q¬O¼Ð·Çªº¥B±`³Q¨Ï¥Îªº©w¸q.

Definition 10.1.6   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field ¥B a $ \in$ L. §Ú­Ì¥O F[a] ªí¥Ü L ¤¤¥]§t F ¥H¤Î a ³Ì¤pªº ring; §Ú­Ì¤]¥O F(a) ªí¥Ü L ¤¤¥]§t F ¥H¤Î a ³Ì¤pªº field.

«e­±¤wª¾ F[a] ´N¬O im($ \phi$) = {f (a) | f (x) $ \in$ F[x]}. ¨º»ò F(a) ¤¤ªº¤¸¯À¤S¬O«ç¼Ë©O? §Q¥Î quotient field ªº©Ê½è (Proposition 7.4.2) «Ü®e©öÅçÃÒ

F(a) = {f (a)/g(a) | f (x), g(x) $\displaystyle \in$ F[x] ¥B g(a)$\displaystyle \ne$0}.

¥Ñ³o¸Ì¥i¬Ý¥X: ¤@¯ë¨Ó»¡ F[a] ©M F(a) ¬O¤£¬Û¦Pªº; ¤£¹L«e­±´£¹L­Y a ¬O algebraic over F, «h F[a] ·|¬O¤@­Ó field, ©Ò¥H F[a] ¦ÛµM¬O¥]§t F ¥H¤Î a ³Ì¤pªº field. ´«¥y¸Ü»¡·í a ¬O algebraic over F ®É, §Ú­Ì¦³ F[a] = F(a). ¦]¦¹ F(a) ´N¬O Proposition 10.1.5 ¤¤©Ò­n§äªº K, ©Ò¥H§Ú­Ì±N Proposition 10.1.5 ­«¾ã¥H«á¥i¥H±o:

Corollary 10.1.7   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field. ­Y a $ \in$ L ¬O algebraic over F ¥B p(x) $ \in$ F[x] ¬° a over F ªº minimal polynomial, «h

F(a) $\displaystyle \simeq$ F[x]/$\displaystyle \bigl($p(x)$\displaystyle \bigr)$    and    [F(a) : F] = deg(p(x)).

Remark 10.1.8   ¦P¾Ç©Î³|©_©Ç F[a] ¸Ìªº¤¸¯Àªøªº¬O f (a) ¨ä¤¤ f (x) $ \in$ F[x] ³oºØ¼Ë¤l, ¦Ó F(a) ¸Ìªº¤¸¯Àªøªº¬O f (a)/g(a) ¨ä¤¤ f (x), g(x) $ \in$ F[x] ³oºØ¼Ë¤l: ¨â­Ó¼Ë¤l®t³o»ò¦h, «ç»ò¥i¯à·| F[a] = F(a) ©O? ³o¬O¦]¬°·í a ¬O algebraic over F ®É, F[a] (©Î F(a)) ¸Ìªº¤¸¯À¨äªí¥Üªk¬O¤£°ß¤@ªº. ¨Ò¦p­Y p(x) $ \in$ F[x] ¬O a ªº minimal polynomial, ¦pªG¥O g(x) = f (x) + p(x), «h g(a) = f (a). ©Ò¥H·íµM¦³¥i¯à¥Î¤£¦Pªº§Î¦¡¼g¤U¨Óªº¤¸¯À¥¦­Ìªº­È¬O¬Û¦Pªº.

±µ¤U¨Ó§Ú­Ì´N¨Ó¬Ý©M a ¬O algebraic over F µ¥»ùªº±ø¥ó¬O¤°»ò?

Theorem 10.1.9   °²³] F ¬O¤@­Ó field, L ¬O F ªº¤@­Ó extension field ¥B a $ \in$ L, «h¤U­±¦³Ãö©ó a ªº±Ô­z¬Oµ¥»ùªº.
  1. a ¬O algebraic over F.
  2. ¦s¦b K ¬O L over F ªº subextension º¡¨¬ a $ \in$ K ¥B [K : F] ¬O¦³­­ªº.
  3. F[a] = F(a).

µý ©ú. ¥Ñ«e­± Proposition 10.1.5 ¥iª¾ (1) $ \Rightarrow$ (2), ©Ò¥H§Ú­Ì¶È­nÅçÃÒ (2) $ \Rightarrow$ (3) ¥H¤Î (3) $ \Rightarrow$ (1).

(2) $ \Rightarrow$ (3): ­Y K ¬O L over F ªº subextension (§Y F $ \subseteq$ K $ \subseteq$ L), «h¥Ñ°²³] a $ \in$ K ª¾ F[a] $ \subseteq$ K. ¦A¥Ñ°²³] K ¬O F ªº¤@­Ó finite extension, ®M¥Î Proposition 9.4.3 ¥i±o F[a] ¬O¤@­Ó field. ¬Gª¾ F[a] = F(a).

(3) $ \Rightarrow$ (1): °²³] F[a] = F(a), ¤]´N¬O»¡ F[a] ¬O¤@­Ó field. ¦pªG a = 0 $ \in$ F, ¨º·íµM a ¬O algebraic over F (ª`·N F ¤¤ªº¤¸¯À·íµM¬O algebraic over F). ¦pªG a$ \ne$ 0, «h¥Ñ a $ \in$ F[a] ¥B F[a] ¬O¤@­Ó field ª¾ a-1 $ \in$ F[a]. §O§Ñ¤F F[a] ¸Ìªº¤¸¯À³£¬O f (a), ¨ä¤¤ f (x) $ \in$ F[x] ³oºØ§Î¦¡, ©Ò¥H§Ú­Ì¦³ a-1 = f (a), ¨ä¤¤

f (x) = cnxn + ... + c1x + c0,    ci $\displaystyle \in$ F.

¬G¥Ñ

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

±o

1 = cn . an + 1 + ... + c1 . a2 + c0 . a.

¦]¦¹­Y¥O

g(x) = cnxn + 1 + ... + c1x2 + c0x - 1,

«h g(a) = 0. ¥Ñ©ó g(x) $ \in$ F[x] ¥B g(x)$ \ne$ 0, ¬Gª¾ a ¬O algebraic over F. $ \qedsymbol$

Theorem 10.1.9 µ¹¤F§Ú­Ì¤@­Ó«Ü¦nªº¤èªk¨ÓÅçÃÒ a ¬O§_¬O algebraic over F. ¤]´N¬O»¡¤µ«á­nÀˬd a ¬O algebraic over F §Ú­Ì¥i¥H¤£¥²¯uªº¥h§ä¤@­Ó f (x) $ \in$ F[x] ¨Ï±o f (a) = 0. ·íµM¤F­n¥Î¤°­Ì¤èªk·|¦]°ÝÃD¦Ó¦³©Ò®t§O. ¤ñ¤è»¡­Y a2 $ \in$ L ¥B§Ú­Ìª¾ a2 ¬O algebraic over F, ¦pªG f (x) $ \in$ F[x] º¡¨¬ f (a2) = 0, «h¥O g(x) = f (x2), §Ú­Ì¥i±o g(a) = f (a2) = 0. ¦]¦¹ª¾ a ¤]¬O algebraic over F. ¤]´N¬O·í a2 ¬O algebraic over F ®É, a ¤]·|¬O algebraic over F. ¦ý¬O¤Ï¹L¨Ó, ¦pªG¤wª¾ a ¬O algebraic over F, §Ú­Ì´NµLªk§Q¥Îº¡¨¬ a ªº polynomial ¨Ó»s³y¤@­Óº¡¨¬ a2 ªº polynomial ¤F. ¦P¾Ç©Î³|·Q­Y f (a) = 0, §Ú­Ì¥i¥H¥O g(x) = f (x1/2), «h g(a2) = f (a) = 0 §r! ³o¬O¤£¹ïªº, ¦]¬° f (x) ­Y¦³©_¼Æ¦¸¶µ, «h g(x) = f (x1/2) ´N¤£¦A¬O¤@­Ó polynomial ¤F. ©Ò¥H¦b³oºØª¬ªp¤U´N¤£¥i¯à§Q¥Î§ä polynomial ªº¤èªk¨ÓÃÒ©ú a2 ¬O algebraic over F. ¨ä¹ê·í a ¬O algebraic over F ®É§Q¥Î Theorem 10.1.9 ª¾¦s¦b¤@­Ó field K ¬O F ªº finite extension ¥B a $ \in$ K. µM¦Ó K ¬O¤@­Ó field ¥B a $ \in$ K, ©Ò¥H·íµM a2 $ \in$ K, ©Ò¥H¦A¥Î¤@¦¸ Theorem 10.1.9 (©Î¬O§Q¥Î Lemma 10.1.3) §Ú­Ì±oÃÒ a2 ¤]¬O algebraic over F. ¥H«á§Ú­Ì±`·|¥ÎÃþ¦üªº¤èªk¨Ó³B²z¬ÛÃöªº°ÝÃD.


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¤U¤@­¶: Algebraic Closure ¤W¤@­¶: ¤¤¯Å Field ªº©Ê½è «e¤@­¶: ¤¤¯Å Field ªº©Ê½è
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