¦^ÅU¤@¤U©Ò¿× F ¬O¤@Ó finite field ´N¬O»¡ F ¬O¤@Ó field ¥B F ªº¤¸¯ÀÓ¼Æ (³q±`§Ú̥ΠF ¨Óªí¥Ü) ¬O¦³¦hÓ. ¥Ñ³oÓ©w¸q§ÚÌ°¨¤Wª¾Y F ¬O finite field, «h F ªº characteristic ¤@©w¬O¤@Ó½è¼Æ p (Lemma 9.2.3). ·íªì§ÚÌ©w characteristic ¬O§Q¥Î¤@Ó ring homomorphism : F, ¨ä¤¤¹ï¥ô·N n §ÚÌ©w (n) = n1, ¦Ó (- n) = n(- 1). F ªº characteristic ¬O p ªí¥Ü ker() = p. ¦]¦¹¥Ñ ring ªº 1st Isomorphism Theorem §Ú̪¾ /p im() F. §O§Ñ¤F p ¬O½è¼Æ, ¬Gª¾ p ·|¬O ªº¤@Ó maximal ideal, ¦]¦¹ /p ¬O¤@Ó field. ¤S¦] /p = p, §Ú̱o F ¤¤¦s¦b¤@Ó subfield ©M /p ³oÓ p Ó¤¸¯Àªº finite field ¬O isomorphic ªº. ¬°¤F¤è«K§Ú̱N³oÓ p Ó¤¸¯Àªº finite field °O¬°: p.
¬JµM F ¬O p ªº¤@Ó extension, §ÚÌ·íµM´N¥i¥H§â F ¬Ý¦¨¬O¤@Ó vector space over p. ¨º»ò F ·|¤£·|¬O finite dimensional over p ©O? ¤j®a¥i¯à³£·|²q·Q·|, ¦ý¬O«ç»òÃÒ©O? ¤@¯ë¨Ó»¡§ÚÌnÃÒ©ú¤@Ó vector space V ¬O finite dimensional over ¤@Ó field K, §ÚÌ¥unÃÒ©ú V ¤¤¥i¥H§ä¨ì¦³¦hÓ¤¸¯À span V over K. ²¦b¥Ñ©ó F ¬O finite field, ´N°²³] F = n §a, ¨º»ò F ¤¤©Ò¦³ªº¤¸¯À·íµM span F over p ¤F (¦]¬°¨CÓ a F ³£¥i¥H¬Ý¦¨¬O a = 1 . a). ©Ò¥H¥Ñ Lemma 9.3.4 (1) ª¾ dimp(F)n. ·íµM§Ú̳oÓ¦ôpªº dimension ¬O«D±`²Ê²¤, ¤£¹L§Ú̥ثeªº¥Øªº¥u¬Onª¾¹D F ¬O p ªº¤@Ó finite extension. ºî¦X¥H¤Wªºµ²ªG§ÚÌ¥i¥H±o¨ì¥H¤U finite field ²Ä¤@Ó«nªº©Ê½è.
³o§¹¥þ¬O¤@Ó½u©Ê¥N¼Æªº°ÝÃD. ¥Ñ dimp(F) = [F : p] = k ªº°²³]ª¾¦s¦b a1,..., ak F ¬O¤@²Õ F over p ªº basis. ¥Ñ basis ªº©w¸qª¾¹ï¥ô·Nªº F, ¦s¦b¤@²Õ°ß¤@ªº c1,..., ck p ¨Ï±o = c1 . a1 + ... + ck . ak. (³o¸Ìªº¦s¦b¬O¦]¬° a1,..., ak span F over p, ¦Ó°ß¤@¬O¦]¬° a1,..., ak ¬O linearly independent over p.) ª`·N³o¸Ìªº a1,..., ak ¬O©T©wªº¤@²Õ basis, ¦Ó c1,..., ck p ·|ÀHµÛ F ªº§ïÅܦӧïÅÜ. ´«¨¥¤§ F ¤¤ªº¥ô¤@Ó¤¸¯À³£¥Ñ°ß¤@ªº¤@²Õ c1, ... , ck ©Ò¨M©w. ¦ý¥Ñ©ó³o¨Ç ci ¬Ò¦b p ¤¤¦Ó p = p, ¦]¦¹¹ï¨CÓ i {1,..., k}, ci ³£¦³ p Ó¿ï¾Ü, ¬Gª¾³o¨Ç c1,..., ck ¦@¦³ pk Ó¿ï¾Ü. ¤]´N¬O»¡ F ¤¤¦@¦³ pk Ó¤¸¯À.
Theorem 10.4.1 ²³æ¨Ó»¡´N¬O§i¶D§Ų́C¤@Ó finite field ¨ä¤¸¯ÀªºÓ¼ÆÀ³¸Ó¬O pk Ó³oºØ§Î¦¡. ©Ò¥H¤£¥i¯à¦³ finite field ¦³ 6 Ó¤¸¯À; ¤£¹L Theorem 10.4.1 ¤]¨S¦³§i¶D§Ų́쩳¦³¨S¦³ finite field ¦³ 9 Ó¤¸¯À©Î 16 Ó¤¸¯Àµ¥µ¥. °¨¤W§ÚÌ´Nn¦^µª³oÓ°ÝÃD, ¤£¹L¦b¦¹¤§«e§ÚÌ¥ý½Í½Í finite field ªº¼ªkµ²ºc.
°²³] F ¬O¤@Ó finite field, ¦]¬° F ¬O field, ¥Ñ Corollary 9.1.2 ª¾ F* = F {0} ¦b¼ªk¤§¤U¬O¤@Ó abelian group. ¤S¦]¬° F ¥u¦³¦³Ó¤¸¯À, ©Ò¥H§Ú̪¾¹D F* ¬O¤@Ó finite abelian group. ¬JµM F* ¬O¤@Ó finite group, §Q¥Î Lagrange's Theorem §Ú̦³¥H¤U¤§µ²ªG.
nª`·N Lagrange's Theorem ¬O¹ï¤@¯ëªº finite group ³£¹ïªº, ©Ò¥H Proposition 10.4.2 ¨Ã¨S¦³¥Î¨ì F* ¬O abelian ªº©Ê½è. ±µ¤U¨Ó§ÚÌn¥Î¨ì finite abelian group ªº«n©Ê½è¨ÓÃÒ©ú¨Æ¹ê¤W F* ¬O¤@Ó cyclic group. ¦^ÅU¤@¤U finite abelian group ªº fundamental theorem (Theorem 3.3.11) ¬O»¡¥ô·Nªº finite abelian group ³£¥i¼g¦¨¤@¨Ç cyclic groups ªº direct product. ¥t¥ nª`·Nªº¬OY Cn ªí¥Ü¬O¤@Ó cyclic group of order n, «h Cn×Cm ¤£¨£±o·| isomorphic to Cnm, °£«D n ©M m ¬O¤¬½èªº (Proposition 3.2.2).
§Ú̧Q¥Î¤ÏÃÒªk, ¬°¤F¤è«K´N°²³] n1 ©M n2 ¤£¤¬½è¦n¤F (¨ä¥Lªºª¬ªp³£¬O¥Î¬Û¦PªºÃÒ©ú). ³oªí¥Ü¦s¦b¤@½è¼Æ q ¬O n1 ©M n2 ªº¤½¦]¼Æ. ¦]¬° q | n1 ¥B q ¬O½è¼Æ, Cauchy's ©w²z (Theorem 3.3.2 ©Î Theorem 4.2.1) §i¶D§Ú̦s¦b a Cn1 º¡¨¬ ord(a) = q. ¤]´N¬O»¡ a, a2,..., aq - 1, aq = e1 ¬O Cn1 ¤¤ q Ó¬Û²§ªº¤¸¯À (³o¸Ì§Ú̥Πei ¨Óªí¥Ü Cni ªº identity). ¦P²z§Ú̪¾¦b Cn2 ¤¤¦s¦b b Cn2 º¡¨¬ ord(b) = q. ²¦Ò¼
F* ¬O cyclic ªí¥Ü¦s¦b a F* ¨Ï±o©Ò¦³ F* ¤¤ªº¤¸¯À³£¬O ai ³oºØ§Î¦¡, ©Ò¥H§Ú̦³¥H¤U³oÓ«nªº©Ê½è.
¥Ñ©ó¤wª¾ F = pk, ¬G§Q¥Î Theorem 10.4.1 ª¾ [p(a) : p] = [F : p] = k. ¦]¦¹¥Ñ Corollary 10.1.7 ª¾ a over p ªº minimal polynomial ªº degree ¬° k, ¬G¥Ñ©w¸qª¾ a over p ªº degree ¬° k.
±µ¤U¨Ó§ÚÌnÃÒ finite field ªº¦s¦b©Ê, §Yµ¹©w¥ô¤@½è¼Æ p ¥H¤Î k , §ÚÌn§ä¨ì¤@Ó finite field F ¨ä¤¸¯ÀÓ¼Æè¦n¬O pk. º¥ýª`·N·í k = 1 ®É /p ´N¬O¤@Ó¤¸¯ÀӼƬ° p ªº finite filed, ¬°¤F¤è«K§Ú̱N¦¹ filed °O¬° p. Theorem 10.4.1 §i¶D§Ṳ́@Ó¤¸¯ÀӼƬ° pk ªº finite filed F Y¦s¦b, «h F ¤@©w·|¬O p ªº¤@Ó extension. ¥t¥ Proposition 10.4.2 §i¶D§Ú̦b¦¹±¡§Î xpk - x ¦b F ¤¤¥²©w splits into linear factors. ¦]¦¹n´M§ä F ¥²¶·±q³o¨âÓÆ[ÂI¥Xµo.
§Ú̺¥ýÃÒ©ú F ¬O¤@Ó filed. §Q¥Î Lemma 9.1.4, §ÚÌ¥unÀˬd¹ï¥ô·N a, b F ¥B b 0 ¬Ò¦³ a - b F ¥H¤Î a/b F §Y¥i. a - b ¥H¤Î a/b ·íµM³£¬O L ªº¤¸¯À, ¦A¥[¤W¥Ñ Lemma 9.2.5 §Ú̦³
±µ¤U¨ÓnÃÒ©ú F = pk. nª`·N¥Ñ°²³] xpk - x splits into linear factors in L, §ÚÌ¥u¯àª¾ F ªº¤¸¯ÀӼƦܦh¦³ pk Ó, °£«D¯àÃÒ±o xpk - x ¨S¦³«®Ú. nÃÒ©ú xpk - x ¨S¦³«®Ú, §ÚÌ¥ý¥ô¨ú a L ¬O xpk - x ªº¤@Ó®Ú, ¥Ñ Lemma 10.3.1 ª¾¦s¦b h(x) L[x] ¨Ï±o xpk - x = (x - a) . h(x). Y±o h(a) 0, «hª¾ a ¤£¬O«®Ú. µM¦Ó§Q¥Î Lemma 9.2.6, §Ú̪¾¹D (x - a)pk - (x - a) = xpk - apk - x + a. ¥Ñ©ó apk = a (¦]°²³] a ¬O xpk - x ªº¤@Ó®Ú), ¬G±o
§Q¥Î finite field ªº¦s¦b©Ê¥H¤ÎCorollary 10.4.4, §ÚÌ°¨¤W¦³¥H¤UªºÀ³¥Î.
±µ¤U¨Ó§Ų́Ӭݦb p[x] ¤¤ªº irreducible element ªº¯S©Ê.
³Ì«á§Ų́Ӭݦ³Ãö finite field ªº°ß¤@©Ê. §Ú̱NÃÒ©úY K ©M L ³£¬O finite field ¥B K = L «h K L. º¥ýn±j½Õªº¬O³o¸Ìªº isomorphic «üªº¬O ring ªº isomorphism. ¤j®a©Î³Ù°O±o¦b½u©Ê¥N¼Æ¤¤¨âÓ vector space Y dimension ¬Û¦P, «h¥¦Ì¤§¶¡¬O isomorphic. ¤£¹L³o¸Ìªº isomorphic ¬O«ü vector space ¶¡ªº isomorphism, n¨Dªº¨ç¼Æ¬O linear transformation, ¶È«O«ù¥[ªkªºµ²ºc. ¥t¥ K* ©M L* ¬O¤¸¯ÀӼƬۦPªº cyclic group, ±q Theorem 3.1.1 ª¾ K* ©M L* ¤]¬O isomorphic. ¤£¹L³o¸Ìªº isomorphic «üªº¬O group ªº isomorphism, ¶È«O«ù¼ªkªºµ²ºc. ³o¨âºØ isomorphic ³£¤£¯à«OÃÒ K ©M L ¶¡¦s¦bµÛ ring isomorphism. §Ú̪ºÃÒ©ú¤£¬O¯uªº§ä¨ì K ªº L ªº ring isomorphism. ¦Ó¬O·Q§ä¨ì¤@Ó field F º¡¨¬ K F ¥B F L, «h§Q¥Î isomorphism ªº transitivity ©Ê½è±oÃÒ K L.
²¦b¬Ý¤@¯ë K = L = pk ªº±¡§Î. ¥Ñ©ó©Ò¦³¤¸¯ÀӼƬ° p ªº finite field ¬Ò isomorphic, ©Ò¥H§ÚÌ¥i¥H°²³] K ©M L ³£¬O p ªº extension, ¨ä¤¤ p ´N¬O¤¸¯ÀӼƬ° p ªº finite field. ¥Ñ©ó K = pk, §Q¥Î Corollary 10.4.4 ª¾¦s¦b a K ¨Ï±o p(a) = K ¥B a over p ªº minimal polynomial g(x) ªº degree ¬O k. ¦]¦¹¥Ñ Corollary 10.1.7 ±o
©Î³P¾ÇÌ·|·Q¹ï L ¦pªkªw»s±o¨ì L p[x]/g(x). ¨Æ¹ê¤W³o¬O¤£¦æªº, ¦]¬°ÁöµM Corollary 10.4.4 §i¶D§Ú̦s¦b a' L ¨Ï±o L = p(a'), ¤£¹L a' over p ªº minimal polynomial ¤£¨£±o´N¬O g(x). n§JªA³oÓ§xÃø§Ú̱o§Q¥Î Lemma 10.4.7. º¥ý, ¥Ñ©ó L = pk, Proposition 10.4.2 §i¶D§ÚÌ xpk - x splits into linear factors in L. ¤£¹L¥Ñ©ó g(x) ¦b p[x] ¤¤¬O irreducible (Lemma 10.1.1), ¦]¦¹¥Ñ Lemma 10.4.7 ±oª¾ g(x) | xpk - x. ©Ò¥H g(x) ¤] splits into linear factors in L. ´«¨¥¤§¦b L ¤¤¦s¦b b L º¡¨¬ g(b) = 0. ·íµM¤F g(x) ¬O b over p ªº minimal polynomial. ì¦]¬O b over p ªº minimal polynomial ¤@©w¬O g(x) ªº divisor (Lemma 10.1.1) ¦ý g(x) ¬O irreducible ¥B¨âªÌ¬Ò¬° monic polynomial, ¬G±oÃÒ g(x) ¬O b over p ªº minimal polynomial. ¦]¦¹¥Ñ Corollary 10.1.7 ª¾
Ãö©ó¤j¾Ç°ò¦¥N¼Æ¤¤ field ªº©Ê½è, §ÚÌ´N¤¶²Ð¦Ü¦¹. §Ų́èS¦³Ä²¤Î©Ò¿×ªº Galois Theory, ¤£¹L¤w¦³¨¬°÷ªº¹w³Æª¾ÃÑ. Y¦P¾Ç̹糧Á¿¸q¤¤ªº field ²z½×«Ü²M·¡¤F, À³¸Ó¥i¥H§ó¶i¤@¨Bªº¥h¤F¸Ñ Galois Theory.