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¤U¤@­¶: ¦³Ãö¥»¤å¥ó ... ¤W¤@­¶: ¤¤¯Å Field ªº©Ê½è «e¤@­¶: Roots of Polynomials

Finite Fields

¦b³o­ÓÁ¿¸qªº³Ì«á¤@¸`, §Ú­Ì­n²³æªº¤¶²Ð finite field ªº¤@¨Ç²³æªº©Ê½è.

¦^ÅU¤@¤U©Ò¿× F ¬O¤@­Ó finite field ´N¬O»¡ F ¬O¤@­Ó field ¥B F ªº¤¸¯À­Ó¼Æ (³q±`§Ú­Ì¥Î $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ ¨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 $ \phi$ : $ \mathbb {Z}$$ \to$F, ¨ä¤¤¹ï¥ô·N n $ \in$ $ \mathbb {N}$ §Ú­Ì©w $ \phi$(n) = n1, ¦Ó $ \phi$(- n) = n(- 1). F ªº characteristic ¬O p ªí¥Ü ker($ \phi$) = $ \bigl($p$ \bigr)$. ¦]¦¹¥Ñ ring ªº 1st Isomorphism Theorem §Ú­Ìª¾ $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ $ \simeq$ im($ \phi$) $ \subseteq$ F. §O§Ñ¤F p ¬O½è¼Æ, ¬Gª¾ $ \bigl($p$ \bigr)$ ·|¬O $ \mathbb {Z}$ ªº¤@­Ó maximal ideal, ¦]¦¹ $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ ¬O¤@­Ó field. ¤S¦] $ \left\vert\vphantom{\mathbb{Z}/\bigl({p}\bigr)}\right.$$ \mathbb {Z}$/$ \bigl($p$ \bigr)$$ \left.\vphantom{\mathbb{Z}/\bigl({p}\bigr)}\right\vert$ = p, §Ú­Ì±o F ¤¤¦s¦b¤@­Ó subfield ©M $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ ³o­Ó p ­Ó¤¸¯Àªº finite field ¬O isomorphic ªº. ¬°¤F¤è«K§Ú­Ì±N³o­Ó p ­Ó¤¸¯Àªº finite field °O¬°: $ \mathbb {F}$p.

¬JµM F ¬O $ \mathbb {F}$p ªº¤@­Ó extension, §Ú­Ì·íµM´N¥i¥H§â F ¬Ý¦¨¬O¤@­Ó vector space over $ \mathbb {F}$p. ¨º»ò F ·|¤£·|¬O finite dimensional over $ \mathbb {F}$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] $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = n §a, ¨º»ò F ¤¤©Ò¦³ªº¤¸¯À·íµM span F over $ \mathbb {F}$p ¤F (¦]¬°¨C­Ó a $ \in$ F ³£¥i¥H¬Ý¦¨¬O a = 1 . a). ©Ò¥H¥Ñ Lemma 9.3.4 (1) ª¾ dim$\scriptstyle \mathbb {F}$p(F)$ \le$n. ·íµM§Ú­Ì³o­Ó¦ô­pªº dimension ¬O«D±`²Ê²¤, ¤£¹L§Ú­Ì¥Ø«eªº¥Øªº¥u¬O­nª¾¹D F ¬O $ \mathbb {F}$p ªº¤@­Ó finite extension. ºî¦X¥H¤Wªºµ²ªG§Ú­Ì¥i¥H±o¨ì¥H¤U finite field ²Ä¤@­Ó­«­nªº©Ê½è.

Theorem 10.4.1   °²³] F ¬O¤@­Ó finite field ¥B char(F) = p, «h F ¤¤¦s¦b¤@­Ó subfield $ \mathbb {F}$p, ¨ä¤¤ $ \left\vert\vphantom{\mathbb{F}_p}\right.$$ \mathbb {F}$p$ \left.\vphantom{\mathbb{F}_p}\right\vert$ = p ¥B©M $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ isomorphic, ¦Ó¥B F ¬O $ \mathbb {F}$p ªº¤@­Ó finite extension. ­Y [F : $ \mathbb {F}$p] = k, «h $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk.

µý ©ú. §Ú­Ì«e­±¤wª¾ F ¤¤¦s¦b¤@­Ó subfield $ \mathbb {F}$p º¡¨¬ $ \mathbb {F}$p $ \simeq$ $ \mathbb {Z}$/$ \bigl($p$ \bigr)$, ¦Ó¥B F ¬O $ \mathbb {F}$p ªº finite extension. ©Ò¥H§Ú­Ì¶È³Ñ¤U­nÃÒ: ­Y [F : $ \mathbb {F}$p] = k, «h $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk.

³o§¹¥þ¬O¤@­Ó½u©Ê¥N¼Æªº°ÝÃD. ¥Ñ dim$\scriptstyle \mathbb {F}$p(F) = [F : $ \mathbb {F}$p] = k ªº°²³]ª¾¦s¦b a1,..., ak $ \in$ F ¬O¤@²Õ F over $ \mathbb {F}$p ªº basis. ¥Ñ basis ªº©w¸qª¾¹ï¥ô·Nªº $ \alpha$ $ \in$ F, ¦s¦b¤@²Õ°ß¤@ªº c1,..., ck $ \in$ $ \mathbb {F}$p ¨Ï±o $ \alpha$ = c1 . a1 + ... + ck . ak. (³o¸Ìªº¦s¦b¬O¦]¬° a1,..., ak span F over $ \mathbb {F}$p, ¦Ó°ß¤@¬O¦]¬° a1,..., ak ¬O linearly independent over $ \mathbb {F}$p.) ª`·N³o¸Ìªº a1,..., ak ¬O©T©wªº¤@²Õ basis, ¦Ó c1,..., ck $ \in$ $ \mathbb {F}$p ·|ÀHµÛ $ \alpha$ $ \in$ F ªº§ïÅܦӧïÅÜ. ´«¨¥¤§ F ¤¤ªº¥ô¤@­Ó¤¸¯À³£¥Ñ°ß¤@ªº¤@²Õ c1, ... , ck ©Ò¨M©w. ¦ý¥Ñ©ó³o¨Ç ci ¬Ò¦b $ \mathbb {F}$p ¤¤¦Ó $ \left\vert\vphantom{\mathbb{F}_p}\right.$$ \mathbb {F}$p$ \left.\vphantom{\mathbb{F}_p}\right\vert$ = p, ¦]¦¹¹ï¨C­Ó i $ \in$ {1,..., k}, ci ³£¦³ p ­Ó¿ï¾Ü, ¬Gª¾³o¨Ç c1,..., ck ¦@¦³ pk ­Ó¿ï¾Ü. ¤]´N¬O»¡ F ¤¤¦@¦³ pk ­Ó¤¸¯À. $ \qedsymbol$

Theorem 10.4.1 ²³æ¨Ó»¡´N¬O§i¶D§Ú­Ì¨C¤@­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§Ú­Ì¨ì©³¦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 $ \setminus$ {0} ¦b­¼ªk¤§¤U¬O¤@­Ó abelian group. ¤S¦]¬° F ¥u¦³¦³­­­Ó¤¸¯À, ©Ò¥H§Ú­Ìª¾¹D F* ¬O¤@­Ó finite abelian group. ¬JµM F* ¬O¤@­Ó finite group, §Q¥Î Lagrange's Theorem §Ú­Ì¦³¥H¤U¤§µ²ªG.

Proposition 10.4.2   °²³] F ¬O¤@­Ó finite field ¥B $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk. ¥O f (x) = xpk - x, «h¹ï¥ô·N a $ \in$ F ¬Ò²Å¦X f (a) = 0 ¥B f (x) splits linear factors in F. ¨Æ¹ê¤W§Ú­Ì¦³

xpk - x = $\displaystyle \prod_{a\in F}^{}$(x - a).

µý ©ú. ­º¥ý§Ú­Ì¦Ò¼ F* ³o­Ó order ¬° pk - 1 ªº finite group. §Q¥Î Lagrange's Theorem (Corollary 2.3.4), §Ú­Ìª¾¹ï¥ô·N a $ \in$ F*, ¬Ò¦³ apk - 1 = 1 (ª`·N 1 ¬O F* ªº identity). µ¥¦¡¨âÃä­¼¤W a ±o apk = a, ¬Gª¾ f (a) = 0. ¥t¥ ·í a = 0 ®É¦ÛµM¦³ f (a) = 0, ©Ò¥H§Ú­Ì±o¨ì¹ï¥ô·Nªº a $ \in$ F ¬Ò²Å¦X f (a) = 0. µM¦Ó¥Ñ Theorem 10.3.3 §Ú­Ìª¾¹D f (x) ¦b F ¤¤³Ì¦h¥u¯à¦³ deg(f (x)) = pk ­Ó®Ú. ©Ò¥H F ¤¤ªº¤¸¯À­è¦n´N¬O f (x) ©Ò¦³ªº®Ú. ¦]¦¹ f (x) ¥i¥H§¹¥þ¤À¸Ñ¦¨

f (x) = $\displaystyle \prod_{a\in F}^{}$(x - a),

¤]´N¬O»¡ f (x) splits linear factors in F. $ \qedsymbol$

­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).

Theorem 10.4.3   °²³] F ¬O¤@­Ó finite field, «h F* = F $ \setminus$ {0} ¬Ý¦¨¬O¤@­Ó­¼ªkªº group ®É¬O¤@­Ó cyclic group.

µý ©ú. ¥Ñ Theorem 3.3.11 ª¾¦s¦b n1,..., nr $ \in$ $ \mathbb {N}$ ¨Ï±o

F* $\displaystyle \simeq$ Cn1× ... ×Cnr,

¨ä¤¤ Cni ¬O¤@­Ó cyclic group of order ni. ­Y§Ú­Ì¯àÃÒ©ú³o¨Ç ni ³£¬O¨â¨â¤¬½èªº, «h­«½Æ¹B¥Î Proposition 3.2.2 ¥i±o

Cn1× ... ×Cnr $\displaystyle \simeq$ Cn1 ... nr,

´«¨¥¤§ F* ¬O cyclic group.

§Ú­Ì§Q¥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§Ú­Ì¦s¦b a $ \in$ Cn1 º¡¨¬ ord(a) = q. ¤]´N¬O»¡ a, a2,..., aq - 1, aq = e1 ¬O Cn1 ¤¤ q ­Ó¬Û²§ªº¤¸¯À (³o¸Ì§Ú­Ì¥Î ei ¨Óªí¥Ü Cni ªº identity). ¦P²z§Ú­Ìª¾¦b Cn2 ¤¤¦s¦b b $ \in$ Cn2 º¡¨¬ ord(b) = q. ²¦Ò¼

$\displaystyle \alpha$ = (a, e2,..., er),$\displaystyle \beta$ = (e1, b,..., er) $\displaystyle \in$ Cn1×Cn2× ... ×Cnr.

·í i, j $ \in$ {1,..., q} ¥B i$ \ne$j ®É, §Ú­Ìª¾

$\displaystyle \alpha^{i}_{}$ = (ai, e2,..., er)    and    $\displaystyle \alpha^{j}_{}$ = (aj, e2,..., er),

¬G¥Ñ©ó ai$ \ne$aj, §Ú­Ìª¾ $ \alpha^{i}_{}$$ \ne$$ \alpha^{j}_{}$. ¦P²z $ \beta^{i}_{}$$ \ne$$ \beta^{j}_{}$. ¥t¥ ¹ï¥ô·Nªº i, j $ \in$ {1,..., q - 1}, ¥Ñ©ó ai$ \ne$e1 ¥B bj$ \ne$e2, §Ú­Ì¤]ª¾

$\displaystyle \alpha^{i}_{}$ = (ai, e2,..., er)$\displaystyle \ne$(e1, bj,..., er) = $\displaystyle \beta^{j}_{}$.

´«¥y¸Ü»¡

$\displaystyle \alpha$,$\displaystyle \alpha^{2}_{}$,...,$\displaystyle \alpha^{q-1}_{}$,$\displaystyle \beta$,$\displaystyle \beta^{2}_{}$,...,$\displaystyle \beta^{q-1}_{}$

¥H¤Î

$\displaystyle \alpha^{q}_{}$ = $\displaystyle \beta^{q}_{}$ = (e1, e2,..., er)

¬O Cn1× ... ×Cnr ¤¤¬Û²§ªº 2q - 1 ­Ó¤¸¯À. ¥Ñ©ó aq = e1 ¥B bq = e2, ³o 2q - 1 ­Ó¤¸¯À $ \alpha^{i}_{}$ ¥H¤Î $ \beta^{j}_{}$ ³£²Å¦X

($\displaystyle \alpha^{i}_{}$)q = ($\displaystyle \beta^{j}_{}$)q = (e1, e2,..., er). (10.2)

§O§Ñ¤F (e1,..., er) ¬O Cn1× ... ×Cnr ¤¤ªº identity, ©Ò¥H Cn1× ... ×Cnr ©M F* ¶¡ªº isomorphism ·|±N (e1,..., er) °e¨ì F* ªº identity 1. ¦Ó¥B³o­Ó isomorphism (¦]¬°¬O¤@¹ï¤@) ¤]·|±N $ \alpha^{i}_{}$ ©M $ \beta^{j}_{}$ ³o 2q - 1 ­Ó¬Û²§ªº¤¸¯À°e¨ì F* ¤¤ 2q - 1 ­Ó¬Û²§ªº¤¸¯À. ¥Ñ¦¡¤l (10.2) §Ú­Ìª¾³o 2q - 1 ­Ó F* ¤¤ªº¤¸¯À³£²Å¦X xq - 1 = 0. ¦ý¬O Theorem 10.3.3 §i¶D§Ú­Ì xq - 1 ¦b F ¤¤¦Ü¦h¥u¯à¦³ q ­Ó®Ú, ¦]¦¹±o¨ì¥Ù¬Þ. ¤]´N¬O»¡ F* $ \simeq$ Cn1× ... ×Cnr ¤¤ªº n1,..., nr ³£¨â¨â¤¬½è, ¬G±oÃÒ F* ¬O¤@­Ó cyclic group. $ \qedsymbol$

F* ¬O cyclic ªí¥Ü¦s¦b a $ \in$ F* ¨Ï±o©Ò¦³ F* ¤¤ªº¤¸¯À³£¬O ai ³oºØ§Î¦¡, ©Ò¥H§Ú­Ì¦³¥H¤U³o­Ó­«­nªº©Ê½è.

Corollary 10.4.4   °²³] F ¬O¤@­Ó finite field ¥B $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk, «h¦s¦b a $ \in$ F º¡¨¬ $ \mathbb {F}$p(a) = F ¥B a over $ \mathbb {F}$p ªº degree ¬° k.

µý ©ú. ¥O a $ \in$ F* $ \subseteq$ F ²£¥Í F* ³o¤@­Ó cyclic group. ¦^ÅU¤@¤U $ \mathbb {F}$p(a) ¬O F ¤¤¥]§t a ©M $ \mathbb {F}$p ³Ì¤pªº filed, ¦]¦¹§Ú­Ì¦ÛµM¦³ $ \mathbb {F}$p(a) $ \subseteq$ F. ¥t¤@¤è­±¥ô¨ú b $ \in$ F, ¦pªG b = 0, «h¦ÛµM b $ \in$ $ \mathbb {F}$p(a); ¦pªG b$ \ne$ 0, ªí¥Ü b $ \in$ F*, ¬G¦s¦b i $ \in$ $ \mathbb {N}$ ¨Ï±o b = ai. ¥Ñ©ó $ \mathbb {F}$p(a) ¬O¤@­Ó field, ¬G¦¹®É b = ai $ \in$ $ \mathbb {F}$p(a). ¦]¦¹ÃÒ±o F $ \subseteq$ $ \mathbb {F}$p(a), ¬Gª¾ F = $ \mathbb {F}$p(a).

¥Ñ©ó¤wª¾ $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk, ¬G§Q¥Î Theorem 10.4.1 ª¾ [$ \mathbb {F}$p(a) : $ \mathbb {F}$p] = [F : $ \mathbb {F}$p] = k. ¦]¦¹¥Ñ Corollary 10.1.7 ª¾ a over $ \mathbb {F}$p ªº minimal polynomial ªº degree ¬° k, ¬G¥Ñ©w¸qª¾ a over $ \mathbb {F}$p ªº degree ¬° k. $ \qedsymbol$

±µ¤U¨Ó§Ú­Ì­nÃÒ finite field ªº¦s¦b©Ê, §Yµ¹©w¥ô¤@½è¼Æ p ¥H¤Î k $ \in$ $ \mathbb {N}$, §Ú­Ì­n§ä¨ì¤@­Ó finite field F ¨ä¤¸¯À­Ó¼Æ­è¦n¬O pk. ­º¥ýª`·N·í k = 1 ®É $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ ´N¬O¤@­Ó¤¸¯À­Ó¼Æ¬° p ªº finite filed, ¬°¤F¤è«K§Ú­Ì±N¦¹ filed °O¬° $ \mathbb {F}$p. Theorem 10.4.1 §i¶D§Ú­Ì¤@­Ó¤¸¯À­Ó¼Æ¬° pk ªº finite filed F ­Y¦s¦b, «h F ¤@©w·|¬O $ \mathbb {F}$p ªº¤@­Ó extension. ¥t¥  Proposition 10.4.2 §i¶D§Ú­Ì¦b¦¹±¡§Î xpk - x ¦b F ¤¤¥²©w splits into linear factors. ¦]¦¹­n´M§ä F ¥²¶·±q³o¨â­ÓÆ[ÂI¥Xµo.

Theorem 10.4.5   µ¹©w¥ô¤@½è¼Æ p ¥H¤Î k $ \in$ $ \mathbb {N}$, ¤@©w¦s¦b¤@­Ó finite field F º¡¨¬ $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk.

µý ©ú. ¦Ò¼ xpk - x $ \in$ $ \mathbb {F}$p[x], Theorem 10.3.6 §i¶D§Ú­Ì¦s¦b¤@­Ó filed L ¬O $ \mathbb {F}$p ªº¤@­Ó finite extension ¨Ï±o xpk - x ¦b L ¤¤ splits into linear factors. ²¦b¦Ò¼

F = {a $\displaystyle \in$ L | apk = a},

¤]´N¬O»¡ F ¬O L ¤¤ xpk - x ©Ò¦³ªº®Ú©Ò¦¨ªº¶°¦X.

§Ú­Ì­º¥ýÃÒ©ú F ¬O¤@­Ó filed. §Q¥Î Lemma 9.1.4, §Ú­Ì¥u­nÀˬd¹ï¥ô·N a, b $ \in$ F ¥B b$ \ne$ 0 ¬Ò¦³ a - b $ \in$ F ¥H¤Î a/b $ \in$ F §Y¥i. a - b ¥H¤Î a/b ·íµM³£¬O L ªº¤¸¯À, ¦A¥[¤W¥Ñ Lemma 9.2.5 §Ú­Ì¦³

(a - b)pk = apk - bpk    and    (a/b)pk = apk/bpk,

¬G¦] a, b $ \in$ F (§Y apk = a ¥B bpk = b) ±oª¾ (a - b)pk = a - b ¥H¤Î (a/b)pk = a/b. ¤]´N¬O»¡ a - b ¥H¤Î a/b ³£¬O F ªº¤¸¯À.

±µ¤U¨Ó­nÃÒ©ú $ \left\vert\vphantom{ F}\right.$F$ \left.\vphantom{ F}\right\vert$ = pk. ­nª`·N¥Ñ°²³] xpk - x splits into linear factors in L, §Ú­Ì¥u¯àª¾ F ªº¤¸¯À­Ó¼Æ¦Ü¦h¦³ pk ­Ó, °£«D¯àÃÒ±o xpk - x ¨S¦³­«®Ú. ­nÃÒ©ú xpk - x ¨S¦³­«®Ú, §Ú­Ì¥ý¥ô¨ú a $ \in$ L ¬O xpk - x ªº¤@­Ó®Ú, ¥Ñ Lemma 10.3.1 ª¾¦s¦b h(x) $ \in$ L[x] ¨Ï±o xpk - x = (x - a) . h(x). ­Y±o h(a)$ \ne$ 0, «hª¾ a ¤£¬O­«®Ú. µM¦Ó§Q¥Î Lemma 9.2.6, §Ú­Ìª¾¹D (x - a)pk - (x - a) = xpk - apk - x + a. ¥Ñ©ó apk = a (¦]°²³] a ¬O xpk - x ªº¤@­Ó®Ú), ¬G±o

xpk - x = (x - a)pk - (x - a) = (x - a) . h(x),

¨ä¤¤ h(x) = (x - a)pk - 1 - 1. ¦]¬° h(a) = - 1$ \ne$ 0, ¬Gª¾¥ô·N xpk - x ªº®Ú³£¤£¬O­«®Ú. ¦]¦¹±oÃÒ F ¬O¤@­Ó¦³ pk ­Ó¤¸¯Àªº finite field. $ \qedsymbol$

§Q¥Î finite field ªº¦s¦b©Ê¥H¤ÎCorollary 10.4.4, §Ú­Ì°¨¤W¦³¥H¤UªºÀ³¥Î.

Corollary 10.4.6   °²³] $ \mathbb {F}$p ¬O¤@­Ó¦³ p ­Ó¤¸¯Àªº finite field, «h¹ï¥ô·N k $ \in$ $ \mathbb {N}$, ¬Ò¦s¦b g(x) $ \in$ $ \mathbb {F}$p[x] ¦b $ \mathbb {F}$p[x] ¤¤¬O irreducible ¥B deg(g(x)) = k.

µý ©ú. §Q¥Î Theorem 10.4.5 ª¾¦s¦b¤@­Ó finite field F º¡¨¬ [F : $ \mathbb {F}$p] = k. ¬G¥Ñ Corollary 10.4.4 ª¾¦s¦b a $ \in$ F ¨Ï±o F = $ \mathbb {F}$p(a), ¥B¥Ñ©ó [$ \mathbb {F}$p(a) : $ \mathbb {F}$p] = k ª¾ a over $ \mathbb {F}$p ªº minimal polynomial ªº degree ¬° k. ¥Ñ©ó minimal polynomial ¤@©w¬O irreducible (Lemma 10.1.1), ¬G±oÃÒ¥»©w²z. $ \qedsymbol$

±µ¤U¨Ó§Ú­Ì¨Ó¬Ý¦b $ \mathbb {F}$p[x] ¤¤ªº irreducible element ªº¯S©Ê.

Lemma 10.4.7   °²³] $ \mathbb {F}$p ¬O¤@­Ó¦³ p ­Ó¤¸¯Àªº finite field ¥B g(x) $ \in$ $ \mathbb {F}$p[x] ¦b $ \mathbb {F}$p[x] ¤¤¬O irreducible. ­Y deg(g(x)) = k, «h¦b $ \mathbb {F}$p[x] ¤¤ g(x) | xpk - x.

µý ©ú. ¥Ñ©ó deg(g(x)) = k, §Q¥Î Theorem 10.3.4 ª¾¦s¦b¤@­Ó $ \mathbb {F}$p ªº extension L º¡¨¬ [L : $ \mathbb {F}$p] = k ¥B a $ \in$ L º¡¨¬ g(a) = 0. ´«¨¥¤§, L ¬O¤@­Ó finite field ¥B $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$ = pk. µM¦Ó Proposition 10.4.2 §i¶D§Ú­Ì L ¤¤ªº¤¸¯À³£·|¬O f (x) = xpk - x ªº®Ú, ¦]¦¹¥Ñ a $ \in$ L ª¾ f (a) = 0. ­nª`·N¨Æ¹ê¤W g(x) ·|©M a over $ \mathbb {F}$p ªº minimal polynomial h(x) associates. ³o¬O¦]¬° g(a) = 0 ¬G§Q¥Î Lemma 10.1.1 (1) ª¾ h(x) | g(x), ¦ý g(x) ¤S°²³]¬O irreducible, ¬G±o h(x) ©M g(x) associates (ª`·N h(x) ¤£¥i¯à¬O unit). ¤S¥Ñ©ó f (a) = 0, ¦A§Q¥Î¤@¦¸ Lemma 10.1.1 (1) ª¾ h(x) | f (x) (§Y f (x) $ \in$ $ \bigl($h(x)$ \bigr)$). ¬G¥Ñ g(x) ©M h(x) associates ª¾ $ \bigl($g(x)$ \bigr)$ = $ \bigl($h(x)$ \bigr)$, ¦]¦¹±oÃÒ f (x) $ \in$ $ \bigl($g(x)$ \bigr)$ §Y g(x) | f (x). $ \qedsymbol$

³Ì«á§Ú­Ì¨Ó¬Ý¦³Ãö finite field ªº°ß¤@©Ê. §Ú­Ì±NÃÒ©ú­Y K ©M L ³£¬O finite field ¥B $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$ «h K $ \simeq$ 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¤¸¯À­Ó¼Æ¬Û¦Pªº 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. §Ú­ÌªºÃÒ©ú¤£¬O¯uªº§ä¨ì K ªº L ªº ring isomorphism. ¦Ó¬O·Q§ä¨ì¤@­Ó field F º¡¨¬ K $ \simeq$ F ¥B F $ \simeq$ L, «h§Q¥Î isomorphism ªº transitivity ©Ê½è±oÃÒ K $ \simeq$ L.

Theorem 10.4.8   °²³] K ©M L ³£¬O finite field ¥B $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$, «h K ©M L ¤§¶¡¦s¦b¤@­Ó ring isomorphism. ¤]´N¬O»¡ K $ \simeq$ L as rings.

µý ©ú. ­º¥ýÆ[¹î·í $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$ = p ®É, ¥Ñ Theorem 10.4.1 ª¾ K ¦s¦b¤@­Ó subfield ©M $ \mathbb {Z}$/$ \bigl($p$ \bigr)$ isomorphic. ¤£¹L¥Ñ©ó $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = $ \left\vert\vphantom{\mathbb{Z}/\bigl({p}\bigr)}\right.$$ \mathbb {Z}$/$ \bigl($p$ \bigr)$$ \left.\vphantom{\mathbb{Z}/\bigl({p}\bigr)}\right\vert$ = p, ¬G±oª¾ K $ \simeq$ $ \mathbb {Z}$/$ \bigl($p$ \bigr)$. ¦P²z±o L $ \simeq$ $ \mathbb {Z}$/$ \bigl($p$ \bigr)$, ¬Gª¾ K $ \simeq$ L.

²¦b¬Ý¤@¯ë $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$ = pk ªº±¡§Î. ¥Ñ©ó©Ò¦³¤¸¯À­Ó¼Æ¬° p ªº finite field ¬Ò isomorphic, ©Ò¥H§Ú­Ì¥i¥H°²³] K ©M L ³£¬O $ \mathbb {F}$p ªº extension, ¨ä¤¤ $ \mathbb {F}$p ´N¬O¤¸¯À­Ó¼Æ¬° p ªº finite field. ¥Ñ©ó $ \left\vert\vphantom{ K}\right.$K$ \left.\vphantom{ K}\right\vert$ = pk, §Q¥Î Corollary 10.4.4 ª¾¦s¦b a $ \in$ K ¨Ï±o $ \mathbb {F}$p(a) = K ¥B a over $ \mathbb {F}$p ªº minimal polynomial g(x) ªº degree ¬O k. ¦]¦¹¥Ñ Corollary 10.1.7 ±o

K = $\displaystyle \mathbb {F}$p(a) $\displaystyle \simeq$ $\displaystyle \mathbb {F}$p[x]/$\displaystyle \bigl($g(x)$\displaystyle \bigr)$.

©Î³P¾Ç­Ì·|·Q¹ï L ¦pªkªw»s±o¨ì L $ \simeq$ $ \mathbb {F}$p[x]/$ \bigl($g(x)$ \bigr)$. ¨Æ¹ê¤W³o¬O¤£¦æªº, ¦]¬°ÁöµM Corollary 10.4.4 §i¶D§Ú­Ì¦s¦b a' $ \in$ L ¨Ï±o L = $ \mathbb {F}$p(a'), ¤£¹L a' over $ \mathbb {F}$p ªº minimal polynomial ¤£¨£±o´N¬O g(x). ­n§JªA³o­Ó§xÃø§Ú­Ì±o§Q¥Î Lemma 10.4.7. ­º¥ý, ¥Ñ©ó $ \left\vert\vphantom{ L}\right.$L$ \left.\vphantom{ L}\right\vert$ = pk, Proposition 10.4.2 §i¶D§Ú­Ì xpk - x splits into linear factors in L. ¤£¹L¥Ñ©ó g(x) ¦b $ \mathbb {F}$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 $ \in$ L º¡¨¬ g(b) = 0. ·íµM¤F g(x) ¬O b over $ \mathbb {F}$p ªº minimal polynomial. ­ì¦]¬O b over $ \mathbb {F}$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 $ \mathbb {F}$p ªº minimal polynomial. ¦]¦¹¥Ñ Corollary 10.1.7 ª¾

$\displaystyle \mathbb {F}$p[x]/$\displaystyle \bigl($g(x)$\displaystyle \bigr)$ $\displaystyle \simeq$ $\displaystyle \mathbb {F}$p(b).

¤£¹L¥Ñ©ó $ \mathbb {F}$p(b) $ \subseteq$ L ¥B [L : $ \mathbb {F}$p] = [$ \mathbb {F}$p(b) : $ \mathbb {F}$p] = k, §Ú­Ì±oÃÒ L = $ \mathbb {F}$p(b). ¬Gª¾

L $\displaystyle \simeq$ $\displaystyle \mathbb {F}$p[x]/$\displaystyle \bigl($g(x)$\displaystyle \bigr)$ $\displaystyle \simeq$ K.

$ \qedsymbol$

Ãö©ó¤j¾Ç°ò¦¥N¼Æ¤¤ field ªº©Ê½è, §Ú­Ì´N¤¶²Ð¦Ü¦¹. §Ú­Ì¨Ã¨S¦³Ä²¤Î©Ò¿×ªº Galois Theory, ¤£¹L¤w¦³¨¬°÷ªº¹w³Æª¾ÃÑ. ­Y¦P¾Ç­Ì¹ï¥»Á¿¸q¤¤ªº field ²z½×«Ü²M·¡¤F, À³¸Ó¥i¥H§ó¶i¤@¨Bªº¥h¤F¸Ñ Galois Theory.


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