¤U¤@¶: Ring Homomorphism
¤W¤@¶: ¤¤¯Å Ring ªº©Ê½è
«e¤@¶: Ideals ©M Quotient Rings
Subring ©M Ideal ªº°ò¥»©Ê½è
«e¤@¸`¤¤§ÚÌ¥i¥H¬Ý¥X normal subgroup ©M group
¶¡ªºÃö«Y¬Û·í©ó ideal ©M ring ªºÃö«Y. ©Ò¥H¤@¨Ç¦b group ¤¤¦³Ãö normal subgroup ªº©Ê½è, ¦b ring
¤¤¤]¦³¬Û¹ïÀ³¦³Ãö ideal ªº©Ê½è. ¤£¹Lnª`·Nªº¬O±q«e¦b group §Ú̳£¬O¥Î
. ·í¹Bºâ, ¦ý¦b ring ¤¤ªº group ¹Bºâ¬O¥Î + ¨Óªí¥Ü,
©Ò¤w¬Û¹ïÀ³ªº©Ê½èn±N . §ï¦¨ +.
§Ú̦b Lemma 2.6.3 ¤¤´£¹L: ·í H, H' ¬O G ªº subgroup,
H . H' ³o¤@Ó¶°¦X¥¼¥²¬O G ªº subgroup, °£«D H ©M H'
¤¤¦³¤@Ó¬O G ªº normal subgroup. ¦b ring ¤¤¤]¦³Ãþ¦üªºµ²ªG:
¤@¯ë¨Ó»¡Y S, T ¬O R ªº subring, ¨º»ò
S +
T = {
s +
t |
s S,
t T}
¥¼¥²¬O R ªº subring. ì¦]¬O S + T ¤¤¥ô¿ï¨â¤¸¯À s + t ©M
s' + t', ¨ä¼¿n
(s + t) . (s' + t') ¨Ã¤£¤@©w¥i¥H¼g¦¨¤@Ó S
ªº¤¸¯À¥[¤W¤@Ó T ªº¤¸¯À³oºØ§Î¦¡, ¤]´N¬O»¡·í S ©M T ¥u¬O R ªº
subring ®É, S + T ¤£¤@©w¬O¼ªk«Ê³¬ªº. ¤£¹L·í S, T ¨ä¤¤¤§¤@¬O R
ªº ideal ®É, S + T ´N¼ªk«Ê³¬¤F!
Lemma 6.2.1
¥O
R ¬O¤@Ó ring,
S,
T ¬O
R ªº subring.
- Y S ¬O R ªº ideal, «h S + T ¬O R ªº subring.
- Y S ©M T ³£¬O R ªº ideal, «h S + T ¬O R ªº
ideal.
µý ©ú.
(1) §Q¥Î¥[ªkªº group ©Ê½è, §Ú̪¾Y
a =
s +
t,
b =
s' +
t' S +
T ¨ä¤¤
s,
s' S ¥B
t.
t' T, «h
a -
b = (
s +
t) - (
s' +
t') = (
s -
s') + (
t -
t')
S +
T.
¥t¥
a . b = (s + t) . (s' + t') = s . s' + s . t' + t . s' + t . t'.
¥Ñ©ó
S ©M
T ¬O
R ªº subring, ¬G
s . s' S ¥B
t . t' T. ¤S¦]
S ¬O
R ªº ideal ¥B
t,
t' R, ¬G
s . t' S ¥B
t . s' S. ¦]¦¹ª¾
s . s' +
s . t' +
t . s' S ©Ò¥H
(
s +
t)
. (
s' +
t')
S +
T. ¬G¥Ñ Lemma
5.4.2 ª¾
S +
T ¬O
R ªº subring.
(2) Y S ©M T ¬O R ªº ideal, «h¹ï¥ô·Nªº r R, s S ¤Î
t T §Ú̬Ҧ³
r . s, s . r S ¥B
r . t, t . r T. ¦]¦¹
r . (
s +
t) =
r . s +
r . t S +
T
¥B
(
s +
t)
. r =
s . r +
t . r S +
T.
¬G¥Ñ Lemma
6.1.2 ª¾
S +
T ¬O
R ªº ideal.
§Ú̦b°Q½× group ®É´¿½Í¹L¨âÓ subgroup ªº¥æ¶°¨ÌµM¬O subgroup, ¦Ó¨âÓ
normal subgroup ªº¥æ¶°¤]¬O normal subgroup. ¦b ring
ªº±¡ªp§Ṳ́]¦³Ãþ¦ü±¡§Î.
Lemma 6.2.2
¥O
R ¬O¤@Ó ring,
S,
T ¬O
R ªº subring.
- S T ¬O R ªº subring.
- Y S ©M T ³£¬O R ªº ideal, «h S T ¬O R ªº
ideal.
ª`·NY S ©M T Y¶È¦³¤@Ó¬° R ªº ideal, «h S T ·íµMÁÙ¬O
R ªº subring. ¤£¹L´N¤£¨£±o¬O R ªº ideal ¤F! ¥t¥ ¦b group
®É§Ú̪¾¹D¨âÓ subgroup ªºÁp¶°¤£¤@©w¬O subgroup, ¦P²z¦pªG S ©M T
¬O R ªº subring, S T ¤]¤£¤@©w¬O R ªº subring.
¬JµM ring ¤¤¦³¼ªk, ¦pªG S, T ¬O R ªº subring ¨º»ò¦Ò¼
{s . t | s S, t T} ³o¼Ëªº¶°¦X·|¤£·|¤]¬O R ªº
subring ©O? ¨Æ¹ê¤WY s, s' S, t, t' T, «h
(s . t) . (s' . t') ¤£¨£±o¥i¥H¼g¦¨
s'' . t'', ¨ä¤¤ s'' S,
t'' T ³o¼Ëªº§Î¦¡ (°£«D R ¬O commutative). ¤£¹L§Y¨Ï R ¬O
commutative,
s . t + s' . t' ¤]¤£¨£±o¥i¥H¼g¦¨
s'' . t'',
¨ä¤¤ s'' S, t'' T. ©Ò¥H¦pªG¦Ò¼
{s . t | s S, t T} ³o¼Ëªº¶°¦X¬OµLªk¹F¨ì¥[ªk«Ê³¬ªºn¨D. §ÚÌÀ³¦Ò¼¥H¤U¤§¶°¦X
{
si . ti |
si S,
ti T,
for some
n }.
¤@¯ë§ÚÌ·|±N¥H¤Wªº¶°¦X°O§@ S . T.
²³æ¨Ó»¡, ¨C¤@Ó S . T ªº¤¸¯À³£¥i¼g¦¨¦³¦h¶µªº S ¤¤¤¸¯À¼¤W
T ¤¤¤¸¯Àªº©M.
Lemma 6.2.3
¥O
R ¬O¤@Ó ring,
S ©M
T ³£¬O
R ªº ideal, «h
S . T ¬O
R ªº ideal.
µý ©ú.
Y
a =
s1 . t1 +
... +
sn . tn ©M
b =
s'1 . t1' +
... +
sm'
. tm' ¬O
S . T ¤¤¥ô·Nªº¨â¤¸¯À, «h
a - b = s1 . t1 + ... + sn . tn + (- s'1) . t1' + ... + (- sm') . tm'
¤´¥i¼g¦¨¦³¦h¶µªº
S ¤¤¤¸¯À¼¤W
T ¤¤¤¸¯Àªº©M. ¬G
a -
b S . T.
¥t¥ ¹ï¥ô·Nªº r R,
r . a =
r . si . ti =
(
r . si)
. ti.
¥Ñ©ó
si S ¥B
S ¬O
R ªº ideal, ©Ò¥H
r . si S. ¦]¦¹
r . a
¤´¥i¼g¦¨¦³¦h¶µªº
S ¤¤¤¸¯À¼¤W
T ¤¤¤¸¯Àªº©M. ¬G
r . a S . T. ¦P²zª¾
a . r S . T. ¬G¥Ñ Lemma
6.1.2 ª¾
S . T ¬O
R ªº ideal.
§Ṳ́w¬Ý¨ì³h¦³Ãö ideal ©M subring ªº®t²§, ¤@¯ë¨Ó»¡ subring
¦]¨ä±ø¥ó¸û¤Ö©Ò¥H¸ûÃø±±¨î. ¨Ò¦p¤@Ó subring ¥i¯à§t¦³ì¥» ring ¤¤ªº
unit (
¬O
ªº subring, ¥B
1 ), ¦ý¹ï ideal ¨Ó»¡
³o´Nµ´¤£¥i¯àµo¥Í¤F!
Lemma 6.2.4
³]
R ¬O¤@Ó ring with 1, ¥B
I ¬°
R ªº¤@Ó ideal. Y¦b
I
¤¤¦s¦b
u I ¬O
R ªº¤@Ó unit, «h
I =
R. ¤×¨ä·í
R ¬O¤@Ó
division ring ®É,
R ªº ideal ´N¥u¦³ {0} ©M
R ¥»¨.
µý ©ú.
¦]
I ¬O
R ªº ideal, §Ú̦۵M¦³
I R. ²¥ô¨ú
r R,
¦]
u ¬O
R ªº¤@Ó unit, ¥Ñ Lemma
5.3.7 ª¾¦s¦b
r' R º¡¨¬
r' . u =
r. µM¦Ó
u I, ¥Ñ ideal ªº©Ê½èª¾
r' . u =
r I.
¦]¦¹ª¾
R I, ¬G±o
R =
I.
²¦bY R ¬O¤@Ó division ring, ¨Ì©w¸q, ¥ô·N R ¤¤ªº«D 0
¤¸¯À³£¬O unit. ¬GY I ¬O R ¤¤¤@Ó¤£¬° {0} ªº ideal, §Y I
¤¤¦s¦b«D 0 ªº¤¸¯À, ¬G¥Ñ«e±ªºµ²ªGª¾ R = I.
³q±`¨ÌºD¨Ò, §ÚÌ·|ºÙ R ©M {0} ¬O R ªº trivial ideals,
°£¦¹¥H¥ ªº ideal ´NºÙ¬° nontrivial proper ideal. Lemma
6.2.4 §i¶D§Ṳ́@Ó division ring ¤¤¨S¦³ nontrivial proper
ideal (¤£¹L·íµM¦³¥i¯à¦³ proper subring).
³Ì«á§Ú̦^ÅU¤@¤U¦b Remark 2.4.2 ¤¤§ÚÌ´¿´£¨ì subgroup ©M normal
subgroup ¬Û¤¬¤§¶¡nª`·Nªº¨Æ¶µ, ¦P¼Ëªº¹ï©ó subring ©M ideal
§Ṳ́]nª`·N¥H¤U¨Æ¶µ:
°²³] R ¬O¤@Ó ring ¥B
T S R.
(1) ¦pªG¤wª¾ S ¬O R ªº subring ¥B T ¬O S ªº subring, ¨º»ò
T ¬O R ªº subring.
(2) ¦pªG¤wª¾ S ¬O R ªº subring ¥B T ¬O R ªº ideal , ¨º»ò T
¤]·|¬O S ªº ideal.
(3) ¦pªG¤wª¾ S ¬O R ªº subring ¦Ó T ¬O S ªº ideal, ¨º»ò T
¤£¤@©w¬O R ªº ideal.
(4) ¦pªG¤wª¾ S ¦b R ªº ideal ¥B T ¦b S ªº ideal, ¨º»ò T
¤£¤@©w¬O R ªº ideal.
¤U¤@¶: Ring Homomorphism
¤W¤@¶: ¤¤¯Å Ring ªº©Ê½è
«e¤@¶: Ideals ©M Quotient Rings
Administrator
2005-06-18