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¤U¤@­¶: ¤¤¯Å Field ªº©Ê½è ¤W¤@­¶: ªì¯Å Field ªº©Ê½è «e¤@­¶: ±N ring ¬Ý¦¨¬O vector

Extension Field

µ¹©w¤@­Ó field F, §Ú­Ì·íµM¥i¥H°Q½×¨ä subfield, ¤£¹L¦]¤@¯ë field ªº²z½×Ãö¤ßªº¬Oµ¹©w f (x) $ \in$ F[x] ¦pªG¦b F ¤¤ f (x) ¨S¦³®Ú, ¨º»ò¦p¦ó¦b¤ñ F ¤jªº field §ä¨ì®Ú. ©Ò¥H§Ú­Ì¤ñ¸ûÃö¤ßªº´N¬O©Ò¿× F ªº extension field.

Definition 9.4.1   µ¹©w F ¬O¤@­Ó field, ­Y L $ \supseteq$ F ¤]¬O¤@­Ó field ¦Ó¥B L ªº¹Bºâ­­¨î¦b F ¤¤´N¬O­ì¥» F ªº¹Bºâ, «h§Ú­ÌºÙ L ¬O F ªº¤@­Ó extension (©ÎºÙ extension field). ·íµM¤F§Ú­Ì¤]¥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×§Ú­Ìª¾ 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 $ \subseteq$ R $ \subseteq$ L, «h R ¬O¤@­Ó field.

µý ©ú. §Ú­Ì¤£¥´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 dimF(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)$ \le$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. $ \qedsymbol$

­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 $ \in$ L ¬Ò¦s¦b¤@­Ó F[x] ¤¤ªº polynomial f (x)$ \ne$ 0 º¡¨¬ f (a) = 0. ³o¼Ëªº¤¸¯À§Ú­Ìµ¹¥¦¤@­Ó¯S®íªº¦W¦r.

Definition 9.4.4   °²³] F ¬O¤@­Ó field ¥B L ¬O F ªº¤@­Ó extension field. °²³] a $ \in$ 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, §Ú­ÌºÙ³o­Ó 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§Ú­Ì¦A¬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§Ú­Ì¦³ F $ \subseteq$ K $ \subseteq$ 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, §Ú­Ì¦ÛµM·|°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 $ \subseteq$ K $ \subseteq$ 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 dimF(K) = m, §Y¦s¦b a1,..., am $ \in$ K ¬O K over F ªº¤@²Õ basis. ¦P¼Ëªº¦s¦b b1,..., bn $ \in$ L ¬O L over K ªº¤@²Õ basis. §Ú­Ì·QÃÒ©ú

{ai . bj},    $\displaystyle \mbox{$i=1,\dots,m$ ¥B
$j=1,\dots,n$}$

¬O L over F ªº¤@²Õ basis. ¦p¦¹¦ÛµM±oÃÒ¥»©w²z. ­º¥ý­nª`·Nªº¬O¦]¬° K $ \subseteq$ L ©Ò¥H¥Ñ ai $ \in$ K, bj $ \in$ L ¦ÛµM¥i±o ai . bj $ \in$ 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 $ \alpha$ $ \in$ L, §Ú­Ì­n§ä¨ì ci, j $ \in$ F ¨Ï±o

$\displaystyle \alpha$ = $\displaystyle \sum_{j=1}^{n}$$\displaystyle \sum_{i=1}^{m}$ci, j . (ai . bj).

µM¦Ó¦] b1,..., bn span L over K, §Ú­Ì¥i¥H§ä¨ì d1,..., dn $ \in$ K ¨Ï±o

$\displaystyle \alpha$ = d1 . b1 + ... + dn . bn. (9.4)

¦A§Q¥Î a1,..., am span K over F, ¹ï¥ô¤@ dj $ \in$ K, §Ú­Ì³£¥i¥H§ä¨ì c1, j,..., cm, j $ \in$ 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 $ \in$ F ¨Ï±o $ \sum$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 $ \in$ F, ai $ \in$ K ¥B F $ \subseteq$ K, §Ú­Ì¦³ dj $ \in$ 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, §Ú­Ì±o³o¨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. $ \qedsymbol$

­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. §Ú­Ì¦ÛµM·|°Ý¤Ï¹L¨Ó¹ï¶Ü? ¤]´N¬O»¡¦ý¦pªG¤wª¾ L ¬O F ªº finite extension, §Ú­Ì¬O§_¥i±o K ¬O F ªº finite extension ¥B L ¬O K ªº finite extension ©O? µª®×¬OªÖ©wªº, ¨Æ¹ê¤W§Ú­Ì¦³¥H¤Uªºµ²ªG:

Corollary 9.4.7   °²³] F ¬O¤@­Ó field, L ©M K ³£¬O F ªº extensions ¥B²Å¦X F $ \subseteq$ K $ \subseteq$ 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 $ \subseteq$ K $ \subseteq$ L ³o­ÓÃö«Y¦¡, §Ú­Ì¥i±N K ¬Ý¦¨¬O L over F ªº subspace, ©Ò¥H¥Ñ Lemma 9.3.4 (3) ª¾ dimF(L)$ \ge$dimF(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] = dimF(L) = n, ¤]´N»¡¦s¦b a1,..., an $ \in$ L ¬O¤@²Õ L over F ªº basis, ¥Ñ©ó a1,..., an span L over F ¦A¥[¤W F $ \subseteq$ K, §Ú­Ì·íµMª¾ a1,..., an ¤] span L over K. ©Ò¥H§Q¥Î Lemma 9.3.4 (1) ª¾ dimK(L)$ \le$n = dimF(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].

$ \qedsymbol$


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¤U¤@­¶: ¤¤¯Å Field ªº©Ê½è ¤W¤@­¶: ªì¯Å Field ªº©Ê½è «e¤@­¶: ±N ring ¬Ý¦¨¬O vector
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