º¥ýª`·N·í a L ¬O algebraic over F ®É, ¨Æ¹ê¤Wº¡¨¬ f (x) F[x] ¥B f (a) = 0 ªº¦h¶µ¦¡¦³µL½a¦hÓ. ¤£¹L³o¨ä¤¤¦³¤@Ó¬Û·í¯S§O. §Ú̺¥ý¥i¥H¦Ò¼º¡¨¬ f (a) = 0 ªº f (x) F[x] ¤¤ degree ³Ì¤pªº polynomials. ³o¼Ëªº polynomials ¦³¥H¤U¨âÓ«nªº©Ê½è.
(2) °²³] f (x) ¦b F[x] ¤¤¤£¬O irreducible, §Y¦s¦b h(x), l (x) 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) F[x] ¥B a L, §Ú̪¾ h(a), l (a) 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.
Y f (x) F[x] ¬O F[x] ¤¤²Å¦X f (a) = 0 degree ³Ì¤pªº polynomial ¥B g(x) F[x] º¡¨¬ g(a) = 0, «h¥Ñ Lemma 10.1.1 (1) ª¾ g(x) f (x). ²¦b¦pªG g(x) ¤]¬O F[x] ¤¤²Å¦X g(a) = 0 degree ³Ì¤pªº polynomial, «h¥i±o f (x) = g(x). ¥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 F ¨Ï±o f (x) = c . g(x). ©Ò¥H¦pªG§Ú̱N³o¨Ç¦¸¼Æ³Ì§C¦Óº¡¨¬ f (a) = 0 ªº polynomial °£¥H¥¦Ìªº³Ì°ª¦¸¶µ«Y¼Æ©Ò±oªº monic polynomial ´N°ß¤@¤F. ¦]¦¹§Ú̦³¥H¤U¤§©w¸q.
§Ú̪¾¹D·í [L : F] ¬O¦³ªº®ÉÔ, L ¤¤ªº¤¸¯À³£¬O algebraic over F. Y [L : F] = n ¥B a L, «h¥Ñ©ó 1, a,..., an ¤@©w linearly independent over F, ¬Gª¾¦s¦b f (x) F[x] ¥B deg(f (x))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))deg(f (x))n. ´«¨¥¤§§Ú̱o a ªº degree ¤p©ó©Îµ¥©ó [L : F]. §Ú̱N³oÓµ²ªG¼g¦¨¥H¤U¤§ Lemma.
·í L ¤£¬O finite extension over F ®É, L ¤¤·íµM¦³¥i¯à¦s¦b¤¸¯À¬O algebraic over F. ¦pªG a 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 K?
¤°»ò¬O ker() ©O? ¥Ñ©ó F[x] ¬O¤@Ó principle ideal domain ¥B ker() ¬O F[x] ªº¤@Ó ideal, §Ú̪¾¦s¦b p(x) f[x] ¨Ï±o ker() = p(x). ¨Æ¹ê¤W §ÚÌ¥i¥H¦³ ker() = p(x) ¨ä¤¤ p(x) ¬O a ªº minimal polynomial. ³o¬O¦]¬°Y f (x) ker(), «hª¾ (f (x)) = f (a) = 0. ¬G¥Ñ Lemma 10.1.1 ª¾ f (x) p(x). ¤Ï¤§, ¹ï¥ô·N f (x) p(x), ¦s¦b h(x) 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() = p(x), ¨ä¤¤ p(x) ¬O a ªº minimal polynomial.
²¥Ñ First Isomorphism Theorem (6.4.2) ª¾
¦Ü©ó¤°»ò¬O im() ©O? ¥Ñ©w¸qª¾
Y¶È¥Ñ©w¸q¨Ó¬Ý Proposition 10.1.5 ¤¤ªº im() = {f (a) | f (x) 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() L ¦ÛµM¬O integral domain, ¦Ó¥Ñ Proposition 10.1.5 ªºÃÒ©ú¤]ª¾ dimF(im()) = n.
§Ṳ́]«Ü®e©öÀˬd {f (a) | f (x) 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) 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) R. ´«¨¥¤§§Ú̱o {f (a) | f (x) F[x]} R, ¦A¥[¤W {f (a) | f (x) 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.
«e±¤wª¾ F[a] ´N¬O im() = {f (a) | f (x) 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
±µ¤U¨Ó§ÚÌ´N¨Ó¬Ý©M a ¬O algebraic over F µ¥»ùªº±ø¥ó¬O¤°»ò?
(2) (3): Y K ¬O L over F ªº subextension (§Y F K L), «h¥Ñ°²³] a K ª¾ F[a] 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) (1): °²³] F[a] = F(a), ¤]´N¬O»¡ F[a] ¬O¤@Ó field. ¦pªG a = 0 F, ¨º·íµM a ¬O algebraic over F (ª`·N F ¤¤ªº¤¸¯À·íµM¬O algebraic over F). ¦pªG a 0, «h¥Ñ a F[a] ¥B F[a] ¬O¤@Ó field ª¾ a-1 F[a]. §O§Ñ¤F F[a] ¸Ìªº¤¸¯À³£¬O f (a), ¨ä¤¤ f (x) F[x] ³oºØ§Î¦¡, ©Ò¥H§Ú̦³ a-1 = f (a), ¨ä¤¤
Theorem 10.1.9 µ¹¤F§Ṳ́@ӫܦnªº¤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) F[x] ¨Ï±o f (a) = 0. ·íµM¤Fn¥Î¤°Ì¤èªk·|¦]°ÝÃD¦Ó¦³©Ò®t§O. ¤ñ¤è»¡Y a2 L ¥B§Ú̪¾ a2 ¬O algebraic over F, ¦pªG f (x) 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 K. µM¦Ó K ¬O¤@Ó field ¥B a K, ©Ò¥H·íµM a2 K, ©Ò¥H¦A¥Î¤@¦¸ Theorem 10.1.9 (©Î¬O§Q¥Î Lemma 10.1.3) §Ú̱oÃÒ a2 ¤]¬O algebraic over F. ¥H«á§Ú̱`·|¥ÎÃþ¦üªº¤èªk¨Ó³B²z¬ÛÃöªº°ÝÃD.