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¤U¤@­¶: ½u©Ê¥N¼ÆªºÀ³¥Î ¤W¤@­¶: ªì¯Å Field ªº©Ê½è «e¤@­¶: Field ªº°ò¥»©Ê½è

Field ªº Characteristic

¹ï¤@¯ëªº field F, ­Y a $ \in$ F, ¥Ñ©ó 1 $ \in$ F, ¬G¹ï¥ô·Nªº n $ \in$ $ \mathbb {N}$ §Ú­Ì¦³

$\displaystyle \underbrace{a+\cdots +a}_{n\mbox{ ¦¸}}^{}\,$ = ($\displaystyle \underbrace{1+\cdots +1}_{n\mbox{ ¦¸}}^{}\,$) . a. (9.1)

­nª`·N¦b³o¸Ì 1 ¥[ n ¦¸¨Ã¤£µ¥©ó n, ³o¬O¥Ñ©ó³o¸Ìªº 1 ¬O F ¤¤ªº 1 ¨Ã¤£¬O¦ÛµM¼Æ $ \mathbb {N}$ ¤¤ªº 1. ¨Ò¦p¤W¨Ò¤¤ $ \overline{1}$ ¬O $ \mathbb {Z}$/5$ \mathbb {Z}$ ¤¤ªº 1, ¦ý

$\displaystyle \overline{1}$ + $\displaystyle \overline{1}$ + $\displaystyle \overline{1}$ + $\displaystyle \overline{1}$ + $\displaystyle \overline{1}$ = $\displaystyle \overline{0}$,

¦Ó¦b $ \mathbb {N}$ ¤¤ 5 ¬O¤£µ¥©ó 0 ªº. ©Ò¥H§Ú­Ì¤£¯à§â¦¡¤l (9.1) ¼g¦¨

$\displaystyle \underbrace{a+\cdots +a}_{n\mbox{ ¦¸}}^{}\,$ = n . a.

¤£¹L¬°¤F¤è«K, ¹ï¥ô·N a $ \in$ F ¥B n $ \in$ $ \mathbb {N}$ §Ú­Ì¥Î na ¨Óªí¥Ü a ¦Û¤v¥[¦Û¤v n ¦¸, ¤]´N¬O»¡

$\displaystyle \underbrace{a+\cdots +a}_{n\mbox{ ¦¸}}^{}\,$ = na.

§Æ±æ¤£·|³y¦¨¤j®aªº§xÂZ. ¦]¦¹§Ú­Ì¥i¥H±N¦¡¤l (9.1) ¼g¦¨

na = $\displaystyle \underbrace{a+\cdots +a}_{n\mbox{ ¦¸}}^{}\,$ = ($\displaystyle \underbrace{1+\cdots +1}_{n\mbox{ ¦¸}}^{}\,$) . a = (n1) . a.

Lemma 9.2.1   °²³] F ¬O¤@­Ó field, «h¹ï F ¤U­±¨âºØ±¡ªp¤§£¸·|µo¥Í:
  1. ¹ï¥ô·N n $ \in$ $ \mathbb {N}$ ¥B a $ \in$ F $ \setminus$ {0} ¬Ò¦³ na$ \ne$ 0.
  2. ¦s¦b¤@­Ó prime p $ \in$ $ \mathbb {N}$ ¨Ï±o¹ï¥ô·Nªº a $ \in$ F ¬Ò¦³ pa = 0.

µý ©ú. ¦Ò¼ $ \phi$ : $ \mathbb {Z}$$ \to$F ¨ä¤¤ $ \phi$(0) = 0, ¥B¹ï¥ô·N n $ \in$ $ \mathbb {N}$, $ \phi$(n) = n1, $ \phi$(- n) = n(- 1). §Y

$\displaystyle \phi$(n) = $\displaystyle \underbrace{1+\cdots +1}_{n\mbox{ ¦¸}}^{}\,$    ¥B    $\displaystyle \phi$(- n) = $\displaystyle \underbrace{(-1)+\cdots+(-1)}_{n\mbox{ ¦¸}}^{}\,$.

ª`·N³o¸Ìªº 1 ¬O F ¤¤ªº 1, ¦Ó -1 ¬O F ¤¤ 1 ªº¥[ªk inverse. «Ü®e©öÀˬd $ \phi$ ¬O¤@­Ó±q $ \mathbb {Z}$ ¨ì F ªº ring homomorphism.

²¦Ò¼ $ \phi$ ªº kernel. ¥Ñ ring ªº 1st isomorphism theorem (Theorem 6.4.2) §Ú­Ì¦³

$\displaystyle \mathbb {Z}$/ker($\displaystyle \phi$) $\displaystyle \simeq$ im($\displaystyle \phi$).

µM¦Ó im($ \phi$) ·|¬O F ªº¤@­Ó subring (Lemma 6.3.3), ¬G¥Ñ F ¬O integral domain (Lemma 9.1.1) ª¾ im($ \phi$) ¤]¬O¤@­Ó integral domain. ´«¥y¸Ü»¡ $ \mathbb {Z}$/ker($ \phi$) ¬O¤@­Ó integral domain. ¥t¤@¤è­± ker($ \phi$) ·|¬O $ \mathbb {Z}$ ªº¤@­Ó ideal (Lemma 6.3.3) ¬G§Q¥Î Theorem 6.5.7 ª¾ ker($ \phi$) ¬O $ \mathbb {Z}$ ªº¤@­Ó prime ideal. ¦] $ \mathbb {Z}$ ¬O¤@­Ó principle ideal domain, ¬G¦s¦b a $ \in$ $ \mathbb {N}$ º¡¨¬ ker($ \phi$) = $ \bigl($a$ \bigr)$. §Q¥Î Lemma 8.1.9 §Ú­Ìª¾ a = 0 ©Î a = p, ¨ä¤¤ p ¬O $ \mathbb {Z}$ ¤¤ªº¤@­Ó prime.

(1) ker($ \phi$) = $ \bigl($0$ \bigr)$ ªº±¡§Î: ¦¹®É¦]¹ï¥ô·Nªº n $ \in$ $ \mathbb {N}$, ¬Ò¦³ n1$ \ne$ 0 (¦] n $ \not\in$ker($ \phi$)), ¬Gª¾¹ï¥ô·Nªº a $ \in$ F ¥B a$ \ne$ 0, ¦] F ¬O integral domain, ¬Ò¦³

na = (n1) . a$\displaystyle \ne$0.

(2) ker($ \phi$) = $ \bigl($p$ \bigr)$ ªº±¡§Î: ¦¹®É¦] p $ \in$ ker($ \phi$), §Ú­Ì¦³ p1 = 0. ¬G±o¹ï¥ô·Nªº a $ \in$ F ¬Ò¦³

pa = (p1) . a = 0.

$ \qedsymbol$

¦b Lemma 9.2.1 ¤¤ªº 0 ©Î p ¹ï field ªº¤ÀÃþ¤W¬O«Ü­«­nªº, ¦]¦¹§Ú­Ì¦³¥H¤U¤§©w¸q.

Definition 9.2.2   °²³] F ¬O¤@­Ó field. ­Y¹ï¥ô·Nªº n $ \in$ $ \mathbb {N}$ ¥B a $ \in$ F $ \setminus$ {0} ¬Ò¦³ na$ \ne$ 0, «hºÙ F ªº characteristic ¬O 0. °O¬° char(F) = 0. ¤Ï¤§­Y¦s¦b p $ \in$ $ \mathbb {N}$ ¬O $ \mathbb {Z}$ ¤¤ªº prime ¨Ï±o¹ï¥ô·Nªº a $ \in$ F ¬Ò¦³ pa = 0, «hºÙ F ªº characteristic ¬O p. °O¬° char(F) = p.

¨Ò¦p¦³²z¼Æ©Ò¦¨ªº field $ \mathbb {Q}$ ªº characteristic ´N¬O 0. ¤S¨Ò¦p¦b Example 9.1.3 ¤¤ªº $ \mathbb {Z}$/5$ \mathbb {Z}$ ´N²Å¦X¹ï¥ô·Nªº a $ \in$ $ \mathbb {Z}$/5$ \mathbb {Z}$ ¬Ò¦³ 5a = 0, ©Ò¥H§Ú­Ì¦³ char($ \mathbb {Z}$/5$ \mathbb {Z}$) = 5.

­nª`·N¥Ñ Lemma 9.2.1 §Ú­Ìª¾­Y F ¬O¤@­Ó field, «h char(F) ­n¤£¬Oµ¥©ó 0 ´N¬Oµ¥©ó¤@­Ó prime p. ¦pªG char(F) = p$ \ne$ 0, «h¦¹ p ¬Oº¡¨¬ pa = 0 ¨ä¤¤ a $ \in$ F $ \setminus$ {0} ªº³Ì¤pªº¥¿¾ã¼Æ. ¦]¬°­Y n $ \in$ $ \mathbb {N}$ ¥B na = 0, «h¥Ñ F ¬O integral domain ¥H¤Î

na = (n1) . a = 0

ª¾ n1 = 0. ¤]´N¬O»¡ n $ \in$ ker($ \phi$) = $ \bigl($p$ \bigr)$. ³o§i¶D§Ú­Ì n$ \ge$p.

­Y F ¬O¤@­Ó field ¥B F ¥u¦³¦³­­¦h­Ó¤¸¯À, «h§Ú­ÌºÙ F ¬°¤@­Ó finite field.

Lemma 9.2.3   ­Y F ¬O¤@­Ó finite field, «h¦s¦b¤@ prime p $ \in$ $ \mathbb {N}$ ¨Ï±o char(F) = p.

µý ©ú. ¥Ñ Lemma 9.2.1 §Ú­Ìª¾ char(F) = 0 ©Î char(F) = p ¨ä¤¤ p ¬O¤@­Ó½è¼Æ. §Ú­Ì­n»¡©ú char(F) ¤£¥i¯à¬O 0. ¨ä¹ê¦pªG char(F) = 0, ªí¥Ü«e­±©wªº¨º­Ó ring homomorphism $ \phi$ : $ \mathbb {Z}$$ \to$F ²Å¦X ker($ \phi$) = $ \bigl($0$ \bigr)$, ¤]´N¬O»¡ $ \phi$ ¬O¤@¹ï¤@ªº. ´«¨¥¤§ $ \mathbb {Z}$ $ \simeq$ im($ \phi$) $ \subseteq$ F. µM¦Ó $ \mathbb {Z}$ ¦³µL½a¦h­Ó¤¸¯À, ¬G±o¨ì F ¤¤¦³¤@­Ó subring ¨ä¤¸¯À¦³µL½a¦h­Ó. ¦¹©M F ¬O finite field ¬Û¥Ù¬Þ, ¬Gª¾ char(F) = p$ \ne$ 0. $ \qedsymbol$

§Q¥Î Proposition 9.1.5 §Ú­Ì¥i±o¥H¤U¦³Ãö©ó characteristic ªº©Ê½è. ¥¦§i¶D§Ú­Ì·í¨â­Ó field ªº characteristic ¤£¬Û¦P®É, ¥¦­Ì¤§¶¡¤£¥i¯à¦s¦b nontrivial ªº ring homomorphism.

Proposition 9.2.4   °²³] F ©M F' ¬O fields ¥B F ©M F' ¤§¶¡¦s¦b nontrivial ªº ring homomorphism, «h char(F) = char(F').

µý ©ú. °²³] $ \psi$ : F$ \to$F' ¤£¬O¤@­Ó trivial ªº ring homomorphism, ¥Ñ Proposition 9.1.5 (1) ª¾ $ \psi$(1F) = 1F'. ¦]¦¹­Y char(F) = p$ \ne$ 0, §Q¥Î

$\displaystyle \psi$(p1F) = $\displaystyle \psi$(0) = 0

¥H¤Î

$\displaystyle \psi$(p1F) = $\displaystyle \psi$($\displaystyle \underbrace{1_F+\cdots 1_F}_{p\mbox{
¦¸}}^{}\,$) = p$\displaystyle \psi$(1F) = p1F',

§Ú­Ì±o

p1F' = 0.

¬Gª¾ char(F')$ \ne$ 0. µM¦Ó­Y char(F') = q$ \ne$p, «h¦] p ©M q ¬Ò¬O½è¼Æ©Ò¥H¤¬½è, ¬G¦s¦b m, n $ \in$ $ \mathbb {Z}$ ¨Ï±o mp + nq = 1. ¦]¦¹¥Ñ p1F' = q1F' = 0 ¥i±o

1F' = (mp + nq)1F' = 0,

³y¦¨¥Ù¬Þ. ¬Gª¾ char(F) = char(F').

¥t¥ ­Y char(F) = 0, ¦¹®É¹ï¥ô·N n $ \in$ $ \mathbb {N}$ ¬Ò¦³ n1F$ \ne$ 0. §Q¥Î Proposition 9.1.5 (2) ª¾ $ \psi$(n1F)$ \ne$ 0. ´«¥y¸Ü»¡

$\displaystyle \psi$(n1F) = n$\displaystyle \psi$(1F) = n1F'$\displaystyle \ne$0,

³oªí¥Ü char(F') = 0. $ \qedsymbol$

³Ì«á§Ú­Ì¨Ó¬Ý·í char(F) = p$ \ne$ 0 ®É, ¦b¹Bºâ¤Wªº¤@­Ó¯S®í©Ê½è.

Lemma 9.2.5   °²³] F ¬O¤@­Ó field ¥B char(F) = p$ \ne$ 0, «h¹ï¥ô·N a, b $ \in$ F, §Ú­Ì¦³

(a + b)pn = apn + bpn    and    (a - b)pn = apn - bpn,    $\displaystyle \forall$ n $\displaystyle \in$ $\displaystyle \mathbb {N}$.

µý ©ú. §Ú­Ì¥ý¥Î induction ÃÒ©ú (a + b)pn = apn + bpn. ­º¥ý¦Ò¼ n = 1 ªº±¡ªp. §Ú­Ì¥ýÀˬd (a + b)2 ¬°¦ó? ¥Ñ©ó (a + b)2 = a2 + a . b + b . a + b2, §Q¥Î F ¬O¤@­Ó field ª¾ a . b = b . a, ¦]¦¹§Ú­Ì±o (a + b)2 = a2 + 2(a . b) + b2. ¦A¦¸±j½Õ³o¸Ì 2(a . b) ¬O (a . b) + (a . b) ¦Ó¤£¬O 2 . (a . b). ©Ò¥HÄ Äò¤U¥h§Ú­Ì¥i¥H§Q¥ÎÃþ¦ü¤G¶µ¦¡©w²z±o

(a + b)p = ap + p(ap - 1 . b) + ... + $\displaystyle \binom{p}{i}$(ai . bp - i) + ... + bp.

¥Ñ©ó char(F) = p, ¹ï¥ô·N $ \alpha$ $ \in$ F, $ \alpha$ ¦Û¤v³s¥[¦Û¤v p ¦¸µ¥©ó 0 (§Y p$ \alpha$ = 0). ¤j®a³£ª¾¹D·í p ¬O½è¼Æ¥B·í i = 1,..., p - 1 ®É, $ \binom{p}{i}$ ¬O p ªº­¿¼Æ, ¬Gª¾¦¹®É $ \binom{p}{i}$(ai . bp - i) = 0. ¦]¦¹§Ú­Ì¥i±o

(a + b)p = ap + bp. (9.2)

²§Q¥ÎÂk¯Ç°²³]

(a + b)pn - 1 = apn - 1 + bpn - 1, (9.3)

¬G§Q¥Î¦¡¤l (9.2) ©M (9.3) §Ú­Ìª¾

(a + b)pn = $\displaystyle \bigl($(a + b)pn - 1$\displaystyle \bigr)^{p}_{}$ = (apn - 1 + bpn - 1)p = apn + bpn.    

±µ¤U¨ÓÃÒ©ú (a - b)pn = apn - bpn. ­º¥ýª`·N·í char(F) = 2 ®É, ¹ï¥ô·N $ \alpha$ $ \in$ F §Ú­Ì¦³ $ \alpha$ + $ \alpha$ = 2$ \alpha$ = 0, ¬Gª¾ $ \alpha$ = - $ \alpha$. ¦]¦¹¦b p = 2 ®É§Ú­Ì¦ÛµM¦³

(a - b)pn = (a + b)pn = apn + bpn = apn - bpn.

¦Ó·í p ¬O odd prime number ®É, ¥Ñ©ó¹ï¥ô·N $ \alpha$ ¬Ò¦³ (- $ \alpha$)pn = - $ \alpha^{p^n}_{}$ (Corollary 5.2.4), §Ú­Ì±o

(a - b)pn = $\displaystyle \bigl($a + (- b)$\displaystyle \bigr)^{p^n}_{}$ = apn + (- b)pn = apn - bpn.

$ \qedsymbol$

Lemma 9.2.5 ¤]¥i¥H±À¼s¨ì F[x] ¤Wªº¹Bºâ. ª`·N F[x] ¤Wªº polynomial ªº«Y¼Æ³£¦b F ¤¤, ¦Ó¥B F[x] ¤Wªº¥[ªk¨Ì©w¸q¬O±N¦P¦¸¶µªº«Y¼Æ³£¥[°_¨Ó. ¦]¦¹­Y char(F) = p ®É, ¹ï¥ô·Nªº f (x) = anxn + ... + a0 $ \in$ F[x] §Ú­Ì³£¦³

$\displaystyle \underbrace{f(x)+\cdots+f(x)}_{\mbox{$p$
¦¸}}^{}\,$ = ($\displaystyle \underbrace{a_n+\cdots+a_n}_{\mbox{$p$
¦¸}}^{}\,$)xn + ... + ($\displaystyle \underbrace{a_0+\cdots+a_0}_{\mbox{$p$ ¦¸}}^{}\,$) = 0.

¦]¦¹§Q¥ÎÃþ¦ü Lemma 9.2.5 ªºÃÒ©ú§Ú­Ì¦³¥H¤Uªº©Ê½è:

Lemma 9.2.6   °²³] F ¬O¤@­Ó field ¥B char(F) = p$ \ne$ 0, «h¹ï¥ô·N f (x) = amxm + ... + a0 $ \in$ F[x], §Ú­Ì¦³

(f (x))pn = ampnxmpn + ... + a0pn,    $\displaystyle \forall$ n $\displaystyle \in$ $\displaystyle \mathbb {N}$.

¯S§O·í a $ \in$ F ®É, §Ú­Ì¦³

(x - a)pn = xpn - apn,    $\displaystyle \forall$ n $\displaystyle \in$ $\displaystyle \mathbb {N}$.


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