§Ú̦^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§Ú̵Lªk©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 ¨Óªí¥Ü, ¨ä¤¤ a R ¦Ó ¬O±N R ¤¤©Ò¦³©M a ¦PÃþªº¤¸¯À¬Ý¦¨¬O¤@Ó¤¸¯À. «ç¼Ëªº¤¸¯À·|©M a ¦PÃþ©O? §O§Ñ¤F³o¸Ì§Ú̬O¥Î¥[ªk©Ò¥H¨Ì©w¸q a ©M a' ¦PÃþY¥B°ßY a - a' S. ²¦bY , R/S, ¦] S ¦b¥[ªk¤§¤U¬O R ªº normal subgroup, ¥Ñ«e±ª¾§Ú̦۵M¥i©w
S n¦³«ç¼Ëªº©Ê½è R/S ¤W©wªº¼ªk¤ ¤£·|¦³°ÝÃD©O? ¤]´N¬O¥ô¨ú r, r' R ¥H¤Î s, s' S §Ú̦³ = ¥B = ¦]¦¹ = ªí¥Ü r . r' ©M (r + s) . (r' + s') ¦b S ªº¤ÀÃþ¤§¤U¬O¬Û¦Pªº. ´«¥y¸Ü»¡: §ÚÌn¨D
¥Ñ©ó S ¬O¤@Ó subring, ·íµM±o s . s' S, ¦]¦¹¦¡¤l (6.1) µ¥¦P©ón¨D¹ï¥ô·Nªº r, r' R ¤Î s, s' S ¬Ò»Ý²Å¦X
ÁöµM¤@Ó ring ªº ideal ¥²¶·¬O¤@Ó ring, ´N¦p¦P subring ªº±¡ªp§Ṳ́£¥²Àˬd ring ªº©Ò¦³±ø¥ó, §Q¥Î Lemma 5.4.2 §Ú̦³¥H¤U§PÂ_ ideal ªº¤èªk.
²¦b¦^¨ì§Ú̦Ҽ 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§Ú̧Q¥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 ¬O»P a ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X, ¬O»P b ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X, «h§ÚÌ©w
º¥ý§Q¥Î§Ú̪¾¹Dªº 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.