next up previous
¤U¤@­¶: Subring ©M Ideal ªº°ò¥»©Ê½è ¤W¤@­¶: ¤¤¯Å Ring ªº©Ê½è «e¤@­¶: ¤¤¯Å Ring ªº©Ê½è

Ideals ©M Quotient Rings

§Ú­Ì¦b¾Ç²ß group ®Éª¾¹D¤@­Ó group ªº subgroup ¤¤¦³¤@ºØ¯S§Oªº subgroup ¦b³B²z group ªº°ÝÃD®É¯S§O¦n¥Î, ´N¬O normal subgroup. ¦P¼Ëªº¦b¤@­Ó ring ¤¤ªº subring ¸Ì, ¤]¦³¤@ºØ«Ü¯S§Oªº subring, §Ú­ÌºÙ¤§¬° ideal.

§Ú­Ì¦^¾Ð¤@¤U, normal subgroup ¤§©Ò¥H¤ñ¤@¯ëªº subgroup ¦n¥Î¦b©ó¥i¥H§Q¥Î¥¦±o¨ì¤@­Ó·sªº group ºÙ¤§¬° quotient group. ¤]´N¬O»¡¹ï©Ò¦³ G ªº subgroup H, §Ú­Ì¥i¥H±N G ¥Î H ¨Ó¤ÀÃþ, µM«á±N¦PÃþªº¤¸¯À¬Ý¦¨¤@­Ó·sªº¤¸¯À. ¤£¹L³o¨Ç·sªº¤¸¯À¶¡¤@¯ë§Ú­ÌµLªk©w¸q¤@­Ó¹BºâÅý¥¦¦¨¬°¤@­Ó group, °£«D H ¬O G ªº¤@­Ó normal subgroup. ²¦b, ­Y R ¬O¤@­Ó ring ¥B S ¬O R ªº subring, ¥Ñ©ó R ¦b¥[ªk¤§¤U¬O¤@­Ó abelian group, ¦Ó S ¦b¥[ªk¤§¤U¬O R ªº¤@­Ó subgroup, §Q¥Î abelian group ªº subgroup ³£¬O normal subgroup, §Ú­Ì·íµM¦³ R/S ³o¤@­Ó¥[ªk¤§¤Uªº quotient group. §Ú­Ì·íµMÁ٧Ʊæ R/S ¤¤¤]¦³­¼ªk, ³o¼Ë´N¥i¯à±o¨ì¤@­Ó·sªº ring ¤F. ­n«ç¼Ë¦b R/S ¤¤©w¤@­Ó©M R ªº­¼ªk¬ÛÃöªº­¼ªk©O? §Ú­Ì¥i¥H¾Ç 2.4 ¸`ªº¤èªk¨Ó³B²z.

­º¥ý¥²¶·¤F¸Ñ R/S ¤¤ªº¤¸¯Àªø¤°»ò¼Ë¤l. ¥ô¨ú R/S ¤¤ªº¤@­Ó¤¸¯À³£¥i¥H¥Î $ \overline{a}$ ¨Óªí¥Ü, ¨ä¤¤ a $ \in$ R ¦Ó $ \overline{a}$ ¬O±N R ¤¤©Ò¦³©M a ¦PÃþªº¤¸¯À¬Ý¦¨¬O¤@­Ó¤¸¯À. «ç¼Ëªº¤¸¯À·|©M a ¦PÃþ©O? §O§Ñ¤F³o¸Ì§Ú­Ì¬O¥Î¥[ªk©Ò¥H¨Ì©w¸q a ©M a' ¦PÃþ­Y¥B°ß­Y a - a' $ \in$ S. ²¦b­Y $ \overline{a}$,$ \overline{b}$ $ \in$ R/S, ¦] S ¦b¥[ªk¤§¤U¬O R ªº normal subgroup, ¥Ñ«e­±ª¾§Ú­Ì¦ÛµM¥i©w

$\displaystyle \overline{a}$ + $\displaystyle \overline{b}$ = $\displaystyle \overline{a+b}$.

§Ú­Ì·íµM§Æ±æ©wªº­¼ªk¬O

$\displaystyle \overline{a}$ . $\displaystyle \overline{b}$ = $\displaystyle \overline{a\cdot b}$.

¤£¹L³o¼Ë©wªº­¼ªk¥i¯à·|¦³°ÝÃD. °ÝÃDµo¥Í©ó $ \overline{a}$ ¦b R/S ¤¤ªí¥Üªk¨Ã¤£°ß¤@, ¤]´N¬O»¡¦s¦b a' $ \in$ R ¥B a'$ \ne$a º¡¨¬ $ \overline{a}$ = $ \overline{a'}$ (¥u­n a - a' $ \in$ S ´N¥i). ¦]¦¹§Ú­Ì­n°Ýªº¬O: ¦pªG $ \overline{a}$ = $ \overline{a'}$ ¥B $ \overline{b}$ = $ \overline{b'}$ ·|¤£·|µo¥Í $ \overline{a\cdot b}$$ \ne$$ \overline{a'\cdot b'}$ ªº²¶H? ¸U¤@µo¥Í¤F§Ú­Ì©wªº­¼ªk´N¦³°ÝÃD.

S ­n¦³«ç¼Ëªº©Ê½è R/S ¤W©wªº­¼ªk¤ ¤£·|¦³°ÝÃD©O? ¤]´N¬O¥ô¨ú r, r' $ \in$ R ¥H¤Î s, s' $ \in$ S §Ú­Ì¦³ $ \overline{r}$ = $ \overline{r+s}$ ¥B $ \overline{r'}$ = $ \overline{r'+s'}$ ¦]¦¹ $ \overline{r\cdot r'}$ = $ \overline{(r+s)\cdot(r'+s')}$ ªí¥Ü r . r' ©M (r + s) . (r' + s') ¦b S ªº¤ÀÃþ¤§¤U¬O¬Û¦Pªº. ´«¥y¸Ü»¡: §Ú­Ì­n¨D

(r + s) . (r' + s') - r . r' = r . s' + s . r' + s . s' $\displaystyle \in$ S. (6.1)

¥Ñ©ó S ¬O¤@­Ó subring, ·íµM±o s . s' $ \in$ S, ¦]¦¹¦¡¤l (6.1) µ¥¦P©ó­n¨D¹ï¥ô·Nªº r, r' $ \in$ R ¤Î s, s' $ \in$ S ¬Ò»Ý²Å¦X

r . s' + s . r $\displaystyle \in$ S (6.2)

¤À§O¥N s = 0 ¤Î s' = 0 ªº±¡ªp©ó¦¡¤l (6.2), §Ú­Ìª¾³oµ¥¦P©ó­n¨D¹ï¥ô·Nªº r $ \in$ R ¤Î s $ \in$ S ¬Ò»Ý²Å¦X

r . s $\displaystyle \in$ S    ¥B    s . r $\displaystyle \in$ S.

¦]¦¹§Ú­Ì¦ÛµM¦³¥H¤U¤§©w¸q:

Definition 6.1.1   ­Y I ¬O R ªº¤@­Ó subring ¥B²Å¦X¹ï¥ô·Nªº r $ \in$ R ¤Î a $ \in$ I ¬Ò¦³

r . a $\displaystyle \in$ I    ¥B    a . r $\displaystyle \in$ I,

«hºÙ I ¬° R ªº¤@­Ó ideal.

ÁöµM¤@­Ó ring ªº ideal ¥²¶·¬O¤@­Ó ring, ´N¦p¦P subring ªº±¡ªp§Ú­Ì¤£¥²Àˬd ring ªº©Ò¦³±ø¥ó, §Q¥Î Lemma 5.4.2 §Ú­Ì¦³¥H¤U§PÂ_ ideal ªº¤èªk.

Lemma 6.1.2   ¥O R ¬O¤@­Ó ring, I $ \subseteq$ R. ­Y I ²Å¦X¥H¤U¨âÂI, «h I ¬O R ªº ideal:

  1. ¹ï©ó©Ò¦³ªº a, b $ \in$ I ¬Ò¦³ a - b $ \in$ I.
  2. ¹ï¥ô·Nªº a $ \in$ I, r $ \in$ R ¬Ò¦³ r . a $ \in$ I ¥B a . r $ \in$ I.

µý ©ú. ­Y a, b $ \in$ I, «h·íµM b $ \in$ R, ¬G±ø¥ó (2) §i¶D§Ú­Ì¹ï©Ò¦³ªº a, b $ \in$ I ¬Ò¦³ a . b $ \in$ I. µ²¦X±ø¥ó (1), §Q¥Î Lemma 5.4.2 ª¾ I ¬O R ªº¤@­Ó subring. ¦]¦¹¦A¥Ñ±ø¥ó (2) ±o I ¬O R ªº ideal. $ \qedsymbol$

²¦b¦^¨ì§Ú­Ì¦Ò¼ ideal ªº¯u¥¿¥Øªº. ­Y I ¬O R ³o­Ó ring ªº ideal, §Ú­Ì·Q§Q¥Î R ªº ring ªº©Ê½è¨Ó³Ð³y¥t¤@­Ó ring. ­º¥ý§Ú­Ì§Q¥Î R ¦b¥[ªk¤§¤U¬O abelian group ¥B I ¬O¨ä normal subgroup, ¥Î I ±N R ¤ÀÃþ, µM«á±N¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X¬Ý¦¨¤@­Ó·sªº¤¸¯À. ¦p¦¹¤@¨Ó³o¤@­Ó¤ÀÃþ«áªº¶°¦X R/I ¥i©w¥X¤@­Ó¥[ªk, ¦Ó¥B¬O abelian group. µM«á¦A¥Î I ¬O ideal ªº©Ê½è, µ¹ R/I ­¼ªkªºµ²ºc. ¤]´N¬O»¡­Y $ \overline{a}$ ¬O»P a ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X, $ \overline{b}$ ¬O»P b ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X, «h§Ú­Ì©w

$\displaystyle \overline{a}$ + $\displaystyle \overline{b}$ = $\displaystyle \overline{a+b}$    ¥B    $\displaystyle \overline{a}$ . $\displaystyle \overline{b}$ = $\displaystyle \overline{a\cdot b}$.

¥H¤U§Ú­Ì±N»¡©ú R/I ¦b¦¹ + ©M . ¤§¤U¬O¤@­Ó ring.

­º¥ý§Q¥Î§Ú­Ìª¾¹Dªº group ²z½×, R/I ¦b + ¤§¤U¬O¤@­Ó abelian group, ¤]´N¬O»¡ R/I ²Å¦X (R1) ¨ì (R5) ³o 5 ¶µ ring ªº±ø¥ó. §Ú­Ì¥u­nÀˬd (R6), (R7) ©M (R8) §Y¥i.

(R6)
­Y $ \overline{a}$$ \overline{b}$ $ \in$ R/I, «h¥Ñ©ó a . b $ \in$ R ¬G $ \overline{a\cdot b}$ $ \in$ R/I. ¤]´N¬O»¡ $ \overline{a}$ . $ \overline{b}$ $ \in$ R/I.

(R7)
§Ú­Ì­nÃÒ©ú ($ \overline{a}$ . $ \overline{b}$) . $ \overline{c}$ = $ \overline{a}$ . ($ \overline{b}$ . $ \overline{c}$). µM¦Ó

($\displaystyle \overline{a}$ . $\displaystyle \overline{b}$) . $\displaystyle \overline{c}$ = $\displaystyle \overline{a\cdot b}$ . $\displaystyle \overline{c}$ = $\displaystyle \overline{(a\cdot
b)\cdot c}$,

¥B

$\displaystyle \overline{a}$ . ($\displaystyle \overline{b}$ . $\displaystyle \overline{c}$) = $\displaystyle \overline{a}$ . $\displaystyle \overline{b\cdot c}$ = $\displaystyle \overline{a\cdot
(b\cdot c)}$

¦A¥[¤W (a . b) . c = a . (b . c) ©Ò¥Hµ¥¦¡¦¨¥ß.

(R8)
¦P«e­±ªºÃÒ©ú, ¥Ñ©ó a . (b + c) = a . b + a . c ·íµM¥i±o

$\displaystyle \overline{a}$ . ($\displaystyle \overline{b}$ + $\displaystyle \overline{c}$) = $\displaystyle \overline{a}$ . $\displaystyle \overline{b}$ + $\displaystyle \overline{a}$ . $\displaystyle \overline{c}$.

¦P²zª¾

($\displaystyle \overline{b}$ + $\displaystyle \overline{c}$) . $\displaystyle \overline{a}$ = $\displaystyle \overline{b}$ . $\displaystyle \overline{a}$ + $\displaystyle \overline{c}$ . $\displaystyle \overline{a}$.

§Ú­ÌºÙ R/I ¬O R ªº¤@­Ó quotient ring.


next up previous
¤U¤@­¶: Subring ©M Ideal ªº°ò¥»©Ê½è ¤W¤@­¶: ¤¤¯Å Ring ªº©Ê½è «e¤@­¶: ¤¤¯Å Ring ªº©Ê½è
Administrator 2005-06-18