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¤U¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è ¤W¤@­¶: ¤@¨Ç±`¨£ªº Rings «e¤@­¶: Polynomials over the Integers


Quotient Field of an Integral Domain

§Ú­Ì³£ª¾¹D $ \mathbb {Z}$ ¬O integral domain ¦Ó $ \mathbb {Q}$ ¬O field. ¨Æ¹ê¤W $ \mathbb {Q}$ ¬O¥]§t $ \mathbb {Z}$ ³Ì¤pªº field. §Ú­Ì±N±À¼s±q $ \mathbb {Z}$ «Øºc¥X $ \mathbb {Q}$ ªº¤èªk¨ì¥ô·Nªº integral domain D.

µ¹©w¥ô·Nªº integral domain D, ¥O S = {(a, b) | a, b $ \in$ D,  b$ \ne$0}. ­º¥ý§Ú­Ì±N¦b S ¤¤©w¤@­Ó equivalence relation. ¹ï©ó S ¤¤ªº¨â¤¸¯À (a, b),(c, d ) $ \in$ S, §Ú­Ì¥O

(a, b) $\displaystyle \sim$ (c, d )    ­Y¥B°ß­Y    a . d = c . b.

·|©w¥X³oºØ relation ¨Ã¤£©_©Ç, ¤j®a¥i¥H·Q¹³¦b $ \mathbb {Q}$ ¤¤ªº¥ô·N¤¸¯À­Y¥i¼g¦¨ a/b ¤Î c/d, ¨ä¤¤ a, b, c, d $ \in$ $ \mathbb {Z}$ ¥B b$ \ne$0, d$ \ne$ 0, ¨º»ò¦ÛµM¦³ a . d = c . b ³o¤@­ÓÃö«Y¦¡.

§Ú­Ì­nÅçÃÒ $ \sim$ ³o¤@­Ó relation ¬O¤@­Ó equivalence relation:

(equiv1)
¹ï©Ò¦³ªº (a, b) $ \in$ S, ¥Ñ©ó D ¬O¤@­Ó integral domain ©Ò¥H commutative, ¬Gª¾ a . b = b . a. ©Ò¥H±oÃÒ (a, b) $ \sim$ (a, b).
(equiv2)
­Y¤wª¾ (a, b) $ \sim$ (c, d ), §Ú­Ì·Q­nÃÒ±o (c, d ) $ \sim$ (a, b). ¥Ñ (a, b) $ \sim$ (c, d ) §Ú­Ì¦³ a . d = c . b ³o¤@­ÓÃö«Y¦¡. ¦Ó­nÃÒ±o (c, d ) $ \sim$ (a, b) §Ú­Ì¥²¶·­n¦³ c . b = a . d, ¦ý³o©M°²³]ªºÃö«Y¦¡¬Û¦P, ¬G±o (c, d ) $ \sim$ (a, b).
(equiv3)
­Y¤wª¾ (a, b) $ \sim$ (c, d ) ¥B (c, d ) $ \sim$ (e, f ), §Ú­Ì§Æ±æÃÒ±o (a, b) $ \sim$ (e, f ). ¥Ñ°²³]±ø¥ó§Ú­Ì¦³
a . d = c . b (7.1)
c . f = e . d (7.2)

­n¦p¦ó±q¥H¤W (7.1) ©M (7.2) ¨â­ÓÃö«Y¦¡±o¨ì a . f = e . b ³o­ÓÃö«Y¦¡©O? ­º¥ý±N¦¡¤l (7.1) ªºµ¥¦¡¨âÃä­¼¤W f, ±o (a . d ) . f = (c . b) . f = (c . f ) . b. ¦A§Q¥Î¦¡¤l (7.2) ±o (a . d ) . f = (e . d ) . b, ¤]´N¬O d . (a . f - e . b) = 0. ¦] d$ \ne$ 0, ¥B D ¨S¦³ zero divisor (§O§Ñ¤F D ¬O integral domain), ¬G±o a . f = e . b.

¦n¤F, ¬JµM $ \sim$ ¬O S ¤¤ªº¤@­Ó equivalence relation, §Ú­Ì´N¥i¥H±N S ¤¤ªº¤¸¯À§Q¥Î $ \sim$ ¨Ó¤ÀÃþ. ­Y (a, b) $ \in$ S, §Ú­Ì¥O [a, b] ªí¥Ü¦b S ¤¤©Ò¦³©M (a, b) ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X. ¥O $ \widetilde{S}$ ªí¥Ü±N S ¤ÀÃþ¥H«á©Ò¦¨ªº·sªº¶°¦X. ¤]´N¬O»¡ $ \widetilde{S}$ ¤¤ªº¤¸¯À³£¬O [a, b] ³oºØ§Î¦¡, ¨ä¤¤ a, b $ \in$ D ¥B b$ \ne$ 0, ¦Ó¥B­Y (a, b) $ \sim$ (c, d ), «h¦b $ \widetilde{S}$ ¤¤ [a, b] = [c, d].

²¦b§Ú­Ì­n¦b $ \widetilde{S}$ ¤¤©w¸q¥[ªk©M­¼ªk. ­Y [a, b] $ \in$ $ \widetilde{S}$ ¥B [c, d] $ \in$ $ \widetilde{S}$, §Ú­Ì©w:

[a, b] + [c, d] = [a . d + c . b, b . d]    ¥H¤Î    [a, b] . [c, d] = [a . c, b . d].

¬°¤°»ò³o¼Ë©w¥[ªk©M­¼ªk¬Û«H¤j®a«Ü§Öªº¬Ý¥X³o¬O±q¦³²z¼Æ¤Wªº¥[ªk©M­¼ªk­l¥Í¥X¨Ó. ¤]¬Û«H¤j®aª¾¹D¤U¤@¨B´N¬O­nÀËÅç³o¼Ë©wªº¥[ªk©M­¼ªk¬O well-defined. ­º¥ý­nÀˬdªº¬O³o¼Ë©wªº [a, b] + [c, d] ©M [a, b] . [c, d] ·|¸¨¦b $ \widetilde{S}$ ¤¤, ¤]´N¬O»¡ b . d$ \ne$ 0. ¥Ñ b$ \ne$ 0 ¥B d$ \ne$ 0 ¥H¤Î D ¬O integral domain, ·íµM¥i±o b . d$ \ne$ 0. ±µ¤U¨Ó­nÀˬdªº¬O­Y [a, b] = [a', b'] ¥B [c, d] = [c', d'], «h [a, b] + [c, d] = [a', b'] + [c', d'] ¥H¤Î [a, b] . [c, d] = [a', b'] . [c', d']. ±q©w¸qª¾­nÀËÅç [a, b] + [c, d] = [a', b'] + [c', d'] µ¥©ó­nÅçÃÒ

(a . d + c . b) . (b' . d') = (a' . d' + c' . b') . (b . d ).

µM¦Ó§Q¥Î a . b' = a' . b ¥H¤Î c . d' = c' . d ±o
(a . d + c . b) . (b' . d') = (a . b') . (d . d') + (c . d') . (b' . b)  
  = (a' . b) . (d . d') + (c' . d ) . (b' . b)  
  = (a' . d' + c' . b') . (b . d ).  

¦P²z, ­nÀˬd [a, b] . [c, d] = [a', b'] . [c', d'] µ¥©ó­nÅçÃÒ (a . c) . (b' . d') = (a' . c') . (b . d ). µM¦Ó§Q¥Î a . b' = a' . b ¥H¤Î c . d' = c' . d ±o

(a . c) . (b' . d') = (a . b') . (c . d') = (a' . b) . (c' . d )= (a' . c') . (b . d ).

¬JµM¦b $ \widetilde{S}$ ¤¤¥i©w¸q¥[ªk©M­¼ªk, §Ú­Ì¦ÛµM·|°Ý $ \widetilde{S}$ ¬O§_¬O¤@­Ó ring, ¤]´N¬O­nÀˬd (R1)-(R8). ³o¤@³s¦êªºÀˬdÁöµM¤£Ãø, ¦ý¬O«ÜÁc½Æ§Ú­Ì´N²¤¹L. ¨Æ¹ê¤W $ \widetilde{S}$ ¬O¤@­Ó commutative ring with 1. ¨ä¤¤ $ \widetilde{S}$ ªº 0 ¬O [0, 1] ¦Ó 1 ¬O [1, 1]. ³o¥i¥H¥Î $ \forall$ [a, b] $ \in$ $ \widetilde{S}$ «h [a, b] + [0, 1] = [a, b] ¥H¤Î [a, b] . [1, 1] = [a, b] ÃÒ±o. ¦Ü©ó $ \widetilde{S}$ ¬O commutative ¥i¥Ñ D ¬O integral domain ªº°²³]ª¾ D ¬O commutative ¬G±o [a, b] . [c, d] = [a . c, b . d] = [c, d] . [a, b].

§Ú­Ì³Ì²×ªº¥Øªº­nÃÒ©ú $ \widetilde{S}$ ¬O¤@­Ó field, ¤]´N¬O»¡¹ï¥ô·Nªº [a, b] $ \in$ $ \widetilde{S}$ ¥B [a, b]$ \ne$[0, 1] ¥i¥H§ä¨ì [c, d] $ \in$ $ \widetilde{S}$ ¨Ï±o [a, b] . [c, d] = [1, 1]. ¦]¬° [a, b]$ \ne$[0, 1] ¬Gª¾ a$ \ne$ 0, ©Ò¥H [b, a] $ \in$ $ \widetilde{S}$. «Ü®e©ö±oª¾ [a, b] . [b, a] = [a . b, a . b] = [1, 1]. Á`¤§, ¥ô·N $ \widetilde{S}$ ¤¤«D 0 ªº¤¸¯À³£¬O unit, ©Ò¥H $ \widetilde{S}$ ¬O¤@­Ó field, §Ú­ÌºÙ¤§¬° D ªº quotient field ©Î fraction field.

D ªº quotient field $ \widetilde{S}$ ¦³¤@­Ó­«­nªº©Ê½è, ´N¬O¥¦¬O¥]§t D ³Ì¤pªº field. ³o¸Ì¦³¥ó¨Æ±¡§Ú­Ì±o»¡©ú¤@¤U. §Ú­Ì´£¹L¦b¥N¼Æ¤¤³q±`±N¨â­Ó isomorphic ªºªF¦è¬Ý¦¨¬O¤@¼Ëªº. ¨Æ¹ê¤W $ \widetilde{S}$ ¨Ã¨S¦³¯u¥¿ªº¥]§t D, ÄY®æ¨Ó»¡À³¸Ó¬O $ \widetilde{S}$ ¤¤¦³¤@­Ó subring ©M D ¬O isomorphic. ©Ò¥H³o¸Ì©Ò¿× $ \widetilde{S}$ ¬O¥]§t D ³Ì¤pªº field ªí¥Ü­Y F ¬O¤@­Ó field ¥B¦³¤@­Ó subring ©M D isomorphic, «h F ¤¤¦³¤@­Ó subring ©M $ \widetilde{S}$ isomorphic.

­º¥ý§Ú­Ì´N¨Ó¬Ý D ¥]§t©ó¥¦ªº quotient field.

Proposition 7.4.1   °²³] D ¬O¤@­Ó integral domain, ¥B¥O $ \widetilde{S}$ ¬O D ªº quotient field, «h¥i§ä¨ì¤@­Ó±q D ¨ì $ \widetilde{S}$ ªº injective (¤@¹ï¤@) ring homomorphism .

µý ©ú. ¦Ò¼ $ \phi$ : D$ \to$$ \widetilde{S}$ ©w¸q¦¨¹ï¥ô·Nªº a $ \in$ D, $ \phi$(a) = [a, 1]. ¥Ñ©ó­Y a, b $ \in$ D «h

$\displaystyle \phi$(a + b) = [a + b, 1] = [a, 1] + [b, 1] = $\displaystyle \phi$(a) + $\displaystyle \phi$(b)

¥B

$\displaystyle \phi$(a . b) = [a . b, 1] = [a, 1] . [b, 1] = $\displaystyle \phi$(a) . $\displaystyle \phi$(b).

¬Gª¾ $ \phi$ ¬O¤@­Ó±q D ¨ì $ \widetilde{S}$ ªº ring homomorphism. ¦Ü©ó­nÃÒ $ \phi$ ¬O¤@¹ï¤@, §Ú­Ì¥u­nÀˬd ker($ \phi$) = {0}. ¥Ñ©ó $ \phi$(0) = [0, 1] ¬Gª¾ 0 $ \in$ ker($ \phi$). ²­Y a $ \in$ ker($ \phi$), ªí¥Ü $ \phi$(a) = [a, 1] = [0, 1]. §Q¥Î©w¸q, [a, 1] = [0, 1] ªí¥Ü a . 1 = 0 . 1, ¬G±o a = 0. ¦]¦¹±oÃÒ ker($ \phi$) = {0}. $ \qedsymbol$

¦^ÅU Theorem 6.4.2 §i¶D§Ú­Ì D/ker($ \phi$) $ \simeq$ im($ \phi$) ¦Ó Proposition 7.4.1 §i¶D§Ú­Ì ker($ \phi$) = {0} ¦]¦¹±o D $ \simeq$ im($ \phi$). ¦ý¬O im($ \phi$) ¬O $ \widetilde{S}$ ªº subring (Lemma 6.3.3), ¬Gª¾ D ©M D ªº quotient field $ \widetilde{S}$ ¤¤ªº¤@­Ó subring ¬O isomorphic. ±µ¤U¨Ó§Ú­Ì­nÃÒ©ú D ªº quotient field ¬O¦³³o­Ó¯S©Ê¤§³Ì¤pªº field.

Proposition 7.4.2   °²³] D ¬O¤@­Ó integral domain, ¥B¥O $ \widetilde{S}$ ¬O D ªº quotient field. ­Y F ¬O¤@­Ó field ¨ä¤¤¥]§t¤@­Ó subring ©M D isomorphic, «h F ¤¤¤]¦³¤@­Ó subring ©M $ \widetilde{S}$ isomorphic.

µý ©ú. ¥Ñ°²³]ª¾¦s¦b¤@­Ó¤@¹ï¤@ªº ring homomorphism $ \phi$ : D$ \to$F. §Ú­Ì·Q§Q¥Î³o­Ó $ \phi$ »s³y¥X¥t¤@­Ó¤@¹ï¤@ªº ring homomorphism $ \psi$ : $ \widetilde{S}$$ \to$F.

¹ï¥ô·Nªº [a, b] $ \in$ $ \widetilde{S}$, §Ú­Ì©w $ \psi$([a, b]) = $ \phi$(a) . $ \phi$(b)-1. ·íµM³o¸Ì§Ú­Ì­nÀˬd $ \psi$ ¬O§_ well-defined.

­º¥ý§Ú­ÌÀˬd $ \psi$([a, b]) ¬O§_¬O F ¤¤ªº¤¸¯À. ¥Ñ©ó [a, b] $ \in$ $ \widetilde{S}$, ª¾ b$ \ne$ 0, ¦]¦¹¥Ñ $ \phi$ ¬O¤@¹ï¤@ª¾ $ \phi$(b) ¬O F ¤¤ªº¤@­Ó¤£µ¥©ó 0 ªº¤¸¯À. ©Ò¥H¥Ñ F ¬O field ªº°²³]ª¾ $ \phi$(b)-1 $ \in$ F. ¬G±oÃÒ $ \psi$([a, b]) = $ \phi$(a) . $ \phi$(b)-1 $ \in$ F. ±µµÛ­nÀˬd¬O§_­Y [a, b] = [c, d] «h $ \psi$([a, b]) = $ \psi$([c, d]). (¦A¦¸´£¿ô: ·í§Ú­Ì«Øºc¤@­Ó¨ç¼Æ®É¦pªG©w¸q°ì¸Ìªº¤¸¯Àªºªí¥Üªk¤£°ß¤@, §Ú­Ì¤@©w­nÀˬd¬O§_¦P¤@¤¸¯À¨ä¤£¦Pªºªí¥Üªk·|³Q¬M®g¨ì¬Û¦Pªº­È, ¥H§Kµo¥Í¤@¹ï¦hªº±¡ªp.) ¤]´N¬O»¡­Y a . d = c . b, ­nÀˬd¬O§_

$\displaystyle \phi$(a) . $\displaystyle \phi$(b)-1 = $\displaystyle \phi$(c) . $\displaystyle \phi$(d )-1.

µM¦Ó§Q¥Î $ \phi$ ¬O ring homomorphism ª¾ $ \phi$(a . d )= $ \phi$(a) . $ \phi$(d ) ¥B $ \phi$(c . b) = $ \phi$(c) . $ \phi$(b). ¬G¥Ñ a . d = c . b ¥i±o $ \phi$(a . d )= $ \phi$(c . b) ¤]´N¬O»¡ $ \phi$(a) . $ \phi$(d )= $ \phi$(c) . $ \phi$(b). ¤W¦¡¨âÃä¦U­¼¤W $ \phi$(d )-1 . $ \phi$(b)-1 (§O§Ñ¤F $ \phi$(b) ©M $ \phi$(d ) ¬Ò¤£µ¥©ó 0) ¥i±o $ \phi$(a) . $ \phi$(b)-1 = $ \phi$(c) . $ \phi$(d )-1. ¦]¦¹ $ \psi$ ¬O¤@­Ó well-defined ªº¨ç¼Æ.

±µ¤U¨Ó§Ú­ÌÃÒ $ \psi$ ¬O¤@­Ó ring homomorphism. ¹ï¥ô·Nªº [a, b],[c, d] $ \in$ $ \widetilde{S}$, ¨Ì $ \psi$ ªº©w¸q§Ú­Ì¦³

$\displaystyle \psi$([a, b] + [c, d]) = $\displaystyle \psi$([a . d + c . b, b . d]) = $\displaystyle \phi$(a . d + c . b) . $\displaystyle \phi$(b . d )-1

¥B

$\displaystyle \psi$([a, b]) + $\displaystyle \psi$([c, d]) = $\displaystyle \phi$(a) . $\displaystyle \phi$(b)-1 + $\displaystyle \phi$(c) . $\displaystyle \phi$(d )-1.

µM¦Ó§Q¥Î $ \phi$ ¬O ring homomorphism, ­¼¤W $ \phi$(b . d )= $ \phi$(b) . $ \phi$(d ) §Ú­Ì«Ü®e©öÀËÅç

$\displaystyle \phi$(a . d + c . b) . $\displaystyle \phi$(b . d )-1 = $\displaystyle \phi$(a) . $\displaystyle \phi$(b)-1 + $\displaystyle \phi$(c) . $\displaystyle \phi$(d )-1.

¬Gª¾

$\displaystyle \psi$([a, b] + [c, d]) = $\displaystyle \psi$([a, b]) + $\displaystyle \psi$([c, d]).

¦P²z¥iÃÒ

$\displaystyle \psi$([a, b] . [c, d]) = $\displaystyle \psi$([a . c, b . d]) = $\displaystyle \phi$(a . c) . $\displaystyle \phi$(b . d )-1 = $\displaystyle \psi$([a, b]) . $\displaystyle \psi$([c, d]),

¬Gª¾ $ \psi$ ¬O¤@­Ó ring homomorphism.

³Ì«á§Ú­ÌÅçÃÒ $ \psi$ ¬O¤@¹ï¤@ªº, ¤]´N¬OÅçÃÒ ker($ \psi$) = {[0, 1]}. °²³] [a, b] $ \in$ ker($ \psi$), §Y $ \psi$([a, b]) = $ \phi$(a) . $ \phi$(b)-1 = 0. ­¼¤W $ \phi$(b) °¨¤W¥i±o $ \phi$(a) = 0. ¦ý¥Ñ©ó $ \phi$ ¬O¤@¹ï¤@, ¬G¥Ñ a $ \in$ ker($ \phi$) = {0}, ±o a = 0. ´«¥y¸Ü»¡ [a, b] = [0, 1]. ©Ò¥H±oÃÒ $ \psi$ ¬O¤@¹ï¤@. $ \qedsymbol$

±q¤µ¥H«á, ­Y $ \widetilde{S}$ ¬° D ªº quotient field, §Ú­Ì±Nª½±µ¬Ý¦¨ D ¥]§t©ó $ \widetilde{S}$, ¤]´N¬O±N [a, 1] ¼g¦¨ a. ¥t¥ §Ú­Ì±N [a, b] $ \in$ $ \widetilde{S}$ ª½±µ¼g¦¨ a/b.


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¤U¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è ¤W¤@­¶: ¤@¨Ç±`¨£ªº Rings «e¤@­¶: Polynomials over the Integers
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