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

Roots of Polynomials

³o¤@¸`¤¤§Ú­Ì±N°Q½×¤@­Ó polynomial ¦b¤@­Ó field ¤¤¥¦ªº®Úªº©Ê½è.

­º¥ý§Ú­ÌÁÙ¬O¨Ó¬Ý¤j®a³Ì¼ô±xªº¾l¦¡©w²z.

Lemma 10.3.1   °²³] F ¬O¤@­Ó field. ­Y f (x) $ \in$ F[x], ¨ä¤¤ deg(f (x)) = n, ¥B a $ \in$ F º¡¨¬ f (a) = 0, «h¦s¦b h(x) $ \in$ F[x], ¨ä¤¤ deg(h(x)) = n - 1, ¨Ï±o f (x) = (x - a) . h(x).

µý ©ú. ¥Ñ©ó F ¬O¤@­Ó field, ¦Ò¼ f (x) $ \in$ F[x] ¥H¤Î (x - a) $ \in$ F[x], §Q¥Î Euclid's Algorithm (Theorem 7.2.4) ª¾¦s¦b h(x), r(x) $ \in$ F[x] º¡¨¬ f (x) = (x - a) . h(x) + r(x), ¨ä¤¤ r(x) = 0 ©Î deg(r(x)) < deg(x - a) = 1. ¦pªG r(x)$ \ne$ 0 ¥Ñ deg(r(x)) < 1 ª¾ r(x) = c $ \in$ F ¬O¤@­Ó±`¼Æ. ¦ý¥Ñ©ó f (a) = 0 ¬G±N a ¥N¤J f (x) = (x - a) . h(x) + c ±o c = 0, ¦¹©M r(x)$ \ne$ 0 ¬Û¥Ù¬Þ¬Gª¾ r(x) = 0. ¤]´N¬O f (x) = (x - a) . h(x). ¦Ü©ó deg(h(x)) = n - 1, ¥i¥Ñ Lemma 7.2.2 ª½±µ±oª¾. $ \qedsymbol$

¥Ñ©ó deg(x - a) = 1, §Ú­Ìª¾¹D x - a ¬O F[x] ¤¤ªº irreducible element. ¦]¦¹ Lemma 10.3.1 §i¶D§Ú­Ì­Y f (a) = 0, «h x - a ·|¬O f (x) ªº¤@­Ó irreducible divisor. §Q¥Î F[x] ¬O unique factorization domain (Theorem 7.2.14), §Ú­Ìª¾¦s¦b k $ \in$ $ \mathbb {N}$ ¥H¤Î q(x) $ \in$ F[x] ¨Ï±o f (x) = (x - a)k . q(x), ¨ä¤¤ q(a)$ \ne$ 0 (§Y x - a ¤£¬O q(x) ªº divisor). §Ú­Ì¨Ì¦¹¨Ó©w¸q a ¦b f (x) ªº­«®Ú¼Æ.

Definition 10.3.2   °²³] F ¬O¤@­Ó field. ­Y f (x) $ \in$ F[x] ¥B a $ \in$ F º¡¨¬ f (a) = 0, «hºÙ a ¬O¤@­Ó root of f (x). ¤S¦pªG f (x) = (x - a)k . q(x), ¨ä¤¤ q(a)$ \ne$ 0, «hºÙ a ¬O¤@­Ó root of multiplicity k of f (x).

±µ¤U¨Ó¤]¬O¤j®a¼ô±xªº©w²z: ¤@­Ó n ¦¸¦h¶µ¦¡¦b¤@­Ó field ¤¤­pºâ­«®Ú¦b¤º¦Ü¦h¦³ n ­Ó®Ú. ³o¸Ì«üªº­pºâ­«®Ú¦b¤º¬O»¡¦pªG a ¬O k ­«®Ú, «h­nºâ¦¨¬O k ­Ó®Ú.

Theorem 10.3.3   °²³] F ¬O¤@­Ó field. ­Y f (x) $ \in$ F[x] ¥B deg(f (x)) = n$ \ge$1, «h¦b F ¤¤±N multiplicity ­pºâ¦b¤º, f (x) ¦Ü¦h¦³ n ­Ó roots.

µý ©ú. §Ú­Ì§Q¥Î induction. ¦pªG deg(f (x)) = 1, «h f (x) ·íµM¶È¦³ 1 ­Ó®Ú. °²³] degree ¤p©ó n ªº polynomial ©w²z¬Ò¦¨¥ß. ²¦Ò¼ f (x) $ \in$ F[x] ¥B deg(f (x)) = n ªº±¡§Î. ¦pªG f (x) ¦b F ¤¤¨S¦³ root, «h©w²z·íµM¦¨¥ß. ¦pªG a $ \in$ F ¬O f (x) ªº¤@­Ó root of multiplicity k, §Yªí¥Ü¦s¦b q(x) $ \in$ F[x] ¨Ï±o f (x) = (x - a)k . q(x), ¨ä¤¤ q(a)$ \ne$ 0. §Q¥Î degree ªº©Ê½è (Lemma 7.2.2) §Ú­Ì¦³ deg(q(x)) = n - k < n, ¬G§Q¥Î induction ªº°²³]ª¾¦b F ¤¤±N multiplicity ­pºâ¦b¤º, q(x) ¦Ü¦h¦³ n - k ­Ó roots. µM¦Ó­Y b $ \in$ F ¬O f (x) ªº¤@­Ó root, §Ú­Ì¦³

0 = f (b) = (b - a)k . q(b).

§Q¥Î F ¬O integral domain, §Ú­Ìª¾ f (x) ªº roots ­n¤£¬O a ´N¬O q(x) ªº roots. ¦]¦¹¦b F ¤¤ f (x) roots ªº­Ó¼Æ´N¬O k ¥[¤W q(x) ªº roots ªº­Ó¼Æ, ©Ò¥H¦Ü¦h¦³ k + (n - k) = n ­Ó. $ \qedsymbol$

§Ú­Ì­n¬Ý¤@­Ó¤¸¯À a ¬O§_¬O f (x) ªº¤@­Ó®Ú, ¤j®aª½Ä±ªº·Qªk´N¬O±N a ¥N¤J f (x) ¬Ý¬O§_¬° 0. ¨Æ¹ê¤W³o¬O¤£¹ïªº, ­n±N a ¥N¤J f (x) ²o§è¤W a ©M f (x) ªº«Y¼Æ¶¡ªº¥[ªk©M­¼ªk. ´«¨¥¤§¦pªG a ®y¸¨¦b¤@­Ó¥]§t F ©M a ªº field L (¦Ü¤Ö­n¬O ring) ¤¤, ³o¼Ë§Ú­Ì¤ ¥i¥H±N a ©M f (x) ªº«Y¼Æ¦Ò¼¦¨¬O L ªº¤¸¯À¦Ó¥[¥H¹Bºâ. ³o¼Ë f (a) (¬Ý¦¨¬O L ªº¤¸¯À) ¤ ¦³·N¸q. ³o´N¬O¬°¬Æ»ò§Ú­Ì«e­±ªº°Q½×³£·|¥ýµ¹ F ªº¤@­Ó extension L, µM«á¦A½Í½× a $ \in$ L »P F[x] ¤¤ªº polynomials ªºÃö«Y. ©Ò¥H§Ú­Ì¦ÛµM·|°Ý: µ¹©w¥ô¤@«D±`¼Æªº f (x) $ \in$ F[x] ¬O§_¥i¥H§ä¨ì F ªº¤@­Ó extension L ¨Ï±o f (x) ¦b L ¤¤¦³®Ú? µª®×¬OªÖ©wªº. ¥H¤Uªº©w²z´N¬O¦^µª³o­Ó°ÝÃD. §Ú­Ì±N·|«Øºc¤@­Ó F ªº extension field µM«á»¡©ú¦b¨ä¤¤¥i§ä¨ì¤@­Ó®Ú. ³o­Ó©w²zªºÃÒ©ú¦P¾Ç©Î³|ı±o``µêµê''ªº, ¦]¬°¦n¹³¨S¦³¯uªº¦b§ä®Úªº·Pı. ¤£¹L³o´N¬O¼Æ¾Ç¦b½Í¦s¦b©Ê©ÒÃö¤ßªº­«ÂI, §Ú­Ì¥u­nª¾¹DªF¦è¦s¦b¦Ó¤£¥²¯u¥¿§i¶D§AªF¦è¬O¤°»ò.

Theorem 10.3.4   °²³] F ¬O¤@­Ó field ¥B p(x) $ \in$ F[x] ¬O F[x] ¤¤ªº irreducible element, «h¦s¦b¤@­Ó field L ¬O F ªº finite extension, ¨ä¤¤ [L : F] = deg(p(x)) ¥B L ¤¤¦s¦b a $ \in$ L º¡¨¬ p(a) = 0.

µý ©ú. ¥O L = F[x]/$ \bigl($p(x)$ \bigr)$. ¥Ñ©ó p(x) ¬O irreducible, §Ú­Ìª¾ $ \bigl($p(x)$ \bigr)$ ¬O F[x] ¤¤ªº maximal ideal, ¬Gª¾ L ¬O¤@­Ó field.

­º¥ý§Ú­Ì­nÅçÃÒ L ¤¤¦s¦b¤@­Ó subfield ©M F ¬O isomorphic ªº, ¦]¦¹§Ú­Ì¥i¥H±N L ¬Ý¦¨¬O F ªº¤@­Ó extension. ¨Æ¹ê¤W¦Ò¼ $ \pi$ : F$ \to$F[x]/$ \bigl($p(x)$ \bigr)$, ©w¸q¦¨ $ \pi$(c) = $ \overline{c}$, «Ü®e©öÅçÃÒ $ \pi$ ¬O¤@­Ó ring homomorphism. ¤]«Ü®e©öÅçÃÒ $ \pi$ ¬O¤@¹ï¤@ªº: ³o¬O¦]¬°¦pªG c $ \in$ ker($ \pi$), ªí¥Ü $ \overline{c}$ = $ \overline{0}$, §Y c $ \in$ $ \bigl($p(x)$ \bigr)$. ¦ý¬O $ \bigl($p(x)$ \bigr)$ ¤¤°£¤F 0 ¥H¥ ¨S¦³¨ä¥Lªº±`¼Æ, ¬G±o c = 0 (¤]¥i®M¥Î Proposition 9.1.5 (2) ±o¨ì $ \pi$ ¬O¤@¹ï¤@). ¦]¦¹±oÃÒ im($ \pi$) ¬O L ªº subfield ¥B©M F ¬O isomorphic ªº.

²¦b­nÃÒ©ú L ¤¤¦s¦b¤@¤¸¯À¬O p(x) ªº®Ú. ¦Ò¼ a = $ \overline{x}$ $ \in$ L, §Ú­Ì­n»¡©ú p($ \overline{x}$) = $ \overline{0}$ (ª`·N $ \overline{0}$ ¬O L = F[x]/$ \bigl($p(x)$ \bigr)$ ªº 0). °²³] p(x) = anxn + ... + a1x + a0, ¨ä¤¤ ai $ \in$ F. ¨º»ò p(a) ·|¬O¤°»ò©O? §O§Ñ¤F§Ú­Ì´£¹L³o¸Ì¥N¤J a ¥²¶·¥Î¨ìªº¬O L ¤¤ªº¹Bºâ, ¦Ó¦b L ¤¤ c $ \in$ F ¬O»Ý¸g¹L $ \pi$ °e¨ì L ªº, ´«¥y¸Ü»¡§Ú­Ì¥²¶·¦Ò¼ªº¬O $ \overline{c}$. ¦]¦¹¦³

p(a) = p($\displaystyle \overline{x}$)  
  = an . $\displaystyle \overline{x}^{n}_{}$ + ... + a1 . $\displaystyle \overline{x}$ + a0  
  = $\displaystyle \overline{a_n}$ . $\displaystyle \overline{x}^{n}_{}$ + ... + $\displaystyle \overline{a_1}$ . $\displaystyle \overline{x}$ + $\displaystyle \overline{a_0}$    ($\displaystyle \mbox{¨Ì $L$\ ªº¹Bºâ©w¸q}$)  
  = $\displaystyle \overline{a_nx^n+\cdots+a_1x+a_0}$  
  = $\displaystyle \overline{p(x)}$ = $\displaystyle \overline{0}$  

©Ò¥H L ¤¤¯uªº¦s¦b¤@­Ó¤¸¯À¥N¤J p(x) µ¥©ó L ¤¤ªº 0.

³Ì«á¥Ñ Lemma 9.3.6 ª¾ [L : F] = dimF(L) = dimF(F[x]/$ \bigl($p(x)$ \bigr)$) = deg(p(x)). $ \qedsymbol$

¥Ñ Theorem 10.3.4 §Ú­Ì«Ü®e©ö±o¨ì¥H¤U¤@¯ëªºª¬ªp.

Corollary 10.3.5   °²³] F ¬O¤@­Ó field ¥B f (x) $ \in$ F[x], ¨ä¤¤ deg(f (x)) = n$ \ge$1, «h¦s¦b¤@­Ó field L ¬O F ªº finite extension, ¨ä¤¤ [L : F]$ \le$n ¥B L ¤¤¦s¦b a $ \in$ L º¡¨¬ f (a) = 0.

µý ©ú. ¥Ñ©ó f (x) $ \in$ F[x] ¦Ó¥B deg(f (x))$ \ge$1, ©Ò¥H f (x) ¤£¬O F[x] ¤¤ªº unit. §Q¥Î F[x] ¬O unique factorization domain, §Ú­Ìª¾¦s¦b p(x) $ \in$ F[x] ¬O F[x] ¤¤ªº irreducible element º¡¨¬ p(x) | f (x). ª`·N¦pªG p(a) = 0, «h·íµM±o f (a) = 0. ¦]¦¹¥Ñ Theorem 10.3.4 ª¾¦s¦b L, ¨ä¤¤ [L : F] = deg(p(x))$ \le$deg(f (x)) ¥B a $ \in$ L, º¡¨¬ f (a) = p(a) = 0. $ \qedsymbol$

§Ú­Ì¥i¥H¤@ª½®M¥Î Corollary 10.3.5 §ä¨ì¤@­Ó F ªº finite extension L' ¨Ï±o f (x) ¦b L' ¤¤¥i¥H§¹¥þ¤À¸Ñ. ³o¸Ì©Ò¿×ªº f (x) ¦b L' §¹¥þ¤À¸Ñ´N¬O»¡: ¦pªG deg(f (x)) = n, «h f (x) ¦b L'[x] ¤¤¥i¥H¼g¦¨ f (x) = c . (x - a1) ... (x - an), ¨ä¤¤ ai $ \in$ L'. ¦¹®É§Ú­Ì³q±`ºÙ f (x) splits into linear factors in L'.

Theorem 10.3.6   °²³] F ¬O¤@­Ó field ¥B f (x) $ \in$ F[x], ¨ä¤¤ deg(f (x)) = n$ \ge$1, «h¦s¦b¤@­Ó field L' ¬O F ªº finite extension, ¨ä¤¤ [L' : F]$ \le$n!, ¨Ï±o f (x) splits into linear factors in L'.

µý ©ú. §Q¥Î Corollary 10.3.5 ª¾¦s¦b L1 ¬O F ªº¤@­Ó extension º¡¨¬ [L1 : F]$ \le$n ¥B a1 $ \in$ L1 ¨Ï±o f (a1) = 0. ¬G¥Ñ Lemma 10.3.1 ª¾¦s¦b f1(x) $ \in$ L1[x] ¥B deg(f1(x)) = n - 1 ¨Ï±o f (x) = (x - a1) . f1(x). ¹ï f1(x) ¦A®M¥Î¤@¦¸ Corollary 10.3.5 ª¾¦s¦b L2 ¬O L1 ªº¤@­Ó extension º¡¨¬ [L2 : L1]$ \le$n - 1 ¥B a2 $ \in$ L2 ¨Ï±o f1(a2) = 0. ª`·N¦¹®É

[L2 : F] = [L2 : L1][L1 : F]$\displaystyle \le$n(n - 1),

¥B¦s¦b f2(x) $ \in$ L2[x] ¨Ï±o

f (x) = (x - a1) . (x - a2) . f2(x).

©Ò¥H³o¼Ë¤@ª½§@¤U¥h (©Î¬O¹ï degree §@ induction) §Ú­Ì±oÃÒ¥»©w²z. $ \qedsymbol$

³Ì«á§Ú­Ì±j½Õ¤@¤U Theorem 10.3.6 ¸Ìªº L' ·íµM·|¦] f (x) ¤£¦P¦Ó¤£¦P, ¤£¹L¨Æ¹ê¤W§Ú­Ì¥i¥H§ä¨ì¤@­Ó F ªº extension $ \tilde{F}$ ¨Ï±o F[x] ¤¤ªº©Ò¦³ polynomial ¦b $ \tilde{F}$ ¤¤³£¥i¥H splits into linear factors (·íµM¦¹®É $ \tilde{F}$ ¦³¥i¯à¤£¬O F ªº finite extension). ¤£¹L¥Ñ©ó³o­Ó©w²zªºÃÒ©ú»Ý¥Î¨ì©Ò¿×ªº Zorn's Lemma, §Ú­Ì´N²¤¥h¤£ÃÒ¤F.


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