¤U¤@¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è
¤W¤@¶: ¤@¨Ç±`¨£ªº Rings
«e¤@¶: Polynomials over the Integers
Quotient Field of an Integral Domain
§Ú̳£ª¾¹D
¬O integral domain ¦Ó
¬O field. ¨Æ¹ê¤W
¬O¥]§t
³Ì¤pªº field. §Ú̱N±À¼s±q
«Øºc¥X
ªº¤èªk¨ì¥ô·Nªº integral domain D.
µ¹©w¥ô·Nªº integral domain D, ¥O
S = {(a, b) | a, b D, b0}. º¥ý§Ú̱N¦b S ¤¤©w¤@Ó equivalence relation. ¹ï©ó S
¤¤ªº¨â¤¸¯À
(a, b),(c, d ) S, §ÚÌ¥O
(
a,
b)
(
c,
d ) Y¥B°ßY
a . d =
c . b.
·|©w¥X³oºØ
relation ¨Ã¤£©_©Ç, ¤j®a¥i¥H·Q¹³¦b
¤¤ªº¥ô·N¤¸¯ÀY¥i¼g¦¨ a/b
¤Î c/d, ¨ä¤¤
a, b, c, d ¥B
b0, d 0, ¨º»ò¦ÛµM¦³
a . d = c . b ³o¤@ÓÃö«Y¦¡.
§ÚÌnÅçÃÒ ³o¤@Ó relation ¬O¤@Ó equivalence relation:
- (equiv1)
- ¹ï©Ò¦³ªº
(a, b) S, ¥Ñ©ó D ¬O¤@Ó integral domain ©Ò¥H commutative, ¬Gª¾
a . b = b . a. ©Ò¥H±oÃÒ
(a, b) (a, b).
- (equiv2)
- Y¤wª¾
(a, b) (c, d ), §ÚÌ·QnÃÒ±o
(c, d ) (a, b). ¥Ñ
(a, b) (c, d ) §Ú̦³
a . d = c . b
³o¤@ÓÃö«Y¦¡. ¦ÓnÃÒ±o
(c, d ) (a, b) §ÚÌ¥²¶·n¦³
c . b = a . d, ¦ý³o©M°²³]ªºÃö«Y¦¡¬Û¦P, ¬G±o
(c, d ) (a, b).
- (equiv3)
- Y¤wª¾
(a, b) (c, d ) ¥B
(c, d ) (e, f ),
§Ú̧ƱæÃÒ±o
(a, b) (e, f ). ¥Ñ°²³]±ø¥ó§Ú̦³
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¥¦¡¨âÃ伤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 0, ¥B D ¨S¦³ zero
divisor (§O§Ñ¤F D ¬O integral domain), ¬G±o
a . f = e . b.
¦n¤F, ¬JµM ¬O S ¤¤ªº¤@Ó equivalence relation, §ÚÌ´N¥i¥H±N
S ¤¤ªº¤¸¯À§Q¥Î ¨Ó¤ÀÃþ. Y
(a, b) S, §ÚÌ¥O [a, b]
ªí¥Ü¦b S ¤¤©Ò¦³©M (a, b) ¦PÃþªº¤¸¯À©Ò¦¨ªº¶°¦X. ¥O
ªí¥Ü±N S ¤ÀÃþ¥H«á©Ò¦¨ªº·sªº¶°¦X. ¤]´N¬O»¡
¤¤ªº¤¸¯À³£¬O [a, b] ³oºØ§Î¦¡, ¨ä¤¤ a, b D ¥B b 0, ¦Ó¥BY
(a, b) (c, d ), «h¦b
¤¤
[a, b] = [c, d].
²¦b§ÚÌn¦b
¤¤©w¸q¥[ªk©M¼ªk. Y
[a, b] ¥B
[c, d] , §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
¤¤, ¤]´N¬O»¡
b . d 0. ¥Ñ b 0 ¥B d 0 ¥H¤Î D ¬O integral domain, ·íµM¥i±o
b . d 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
¤¤¥i©w¸q¥[ªk©M¼ªk, §Ú̦۵M·|°Ý
¬O§_¬O¤@Ó ring, ¤]´N¬OnÀˬd (R1)-(R8).
³o¤@³s¦êªºÀˬdÁöµM¤£Ãø, ¦ý¬O«ÜÁc½Æ§ÚÌ´N²¤¹L. ¨Æ¹ê¤W
¬O¤@Ó commutative ring with 1. ¨ä¤¤
ªº 0 ¬O
[0, 1] ¦Ó 1 ¬O [1, 1]. ³o¥i¥H¥Î
[a, b]
«h
[a, b] + [0, 1] = [a, b] ¥H¤Î
[a, b] . [1, 1] = [a, b] ÃÒ±o. ¦Ü©ó
¬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].
§Ú̳̲תº¥ØªºnÃÒ©ú
¬O¤@Ó field, ¤]´N¬O»¡¹ï¥ô·Nªº
[a, b] ¥B
[a, b][0, 1] ¥i¥H§ä¨ì
[c, d] ¨Ï±o
[a, b] . [c, d] = [1, 1]. ¦]¬°
[a, b][0, 1]
¬Gª¾ a 0, ©Ò¥H
[b, a] . «Ü®e©ö±oª¾
[a, b] . [b, a] = [a . b, a . b] = [1, 1]. Á`¤§, ¥ô·N
¤¤«D
0 ªº¤¸¯À³£¬O unit, ©Ò¥H
¬O¤@Ó field, §Ú̺٤§¬°
D ªº quotient field ©Î fraction field.
D ªº quotient field
¦³¤@Ó«nªº©Ê½è, ´N¬O¥¦¬O¥]§t
D ³Ì¤pªº field. ³o¸Ì¦³¥ó¨Æ±¡§Ú̱o»¡©ú¤@¤U.
§ÚÌ´£¹L¦b¥N¼Æ¤¤³q±`±N¨âÓ isomorphic ªºªF¦è¬Ý¦¨¬O¤@¼Ëªº. ¨Æ¹ê¤W
¨Ã¨S¦³¯u¥¿ªº¥]§t D, ÄY®æ¨Ó»¡À³¸Ó¬O
¤¤¦³¤@Ó subring ©M D ¬O isomorphic. ©Ò¥H³o¸Ì©Ò¿×
¬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
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
¬O
D ªº
quotient field, «h¥i§ä¨ì¤@Ó±q
D ¨ì
ªº injective
(¤@¹ï¤@) ring homomorphism .
µý ©ú.
¦Ò¼
:
D ©w¸q¦¨¹ï¥ô·Nªº
a D,
(
a) = [
a, 1]. ¥Ñ©óY
a,
b D «h
(
a +
b) = [
a +
b, 1] = [
a, 1] + [
b, 1] =
(
a) +
(
b)
¥B
(
a . b) = [
a . b, 1] = [
a, 1]
. [
b, 1] =
(
a)
. (
b).
¬Gª¾
¬O¤@Ó±q
D ¨ì
ªº ring homomorphism. ¦Ü©ónÃÒ
¬O¤@¹ï¤@, §ÚÌ¥unÀˬd
ker(
) = {0}. ¥Ñ©ó
(0) = [0, 1] ¬Gª¾
0
ker(
). ²Y
a ker(
), ªí¥Ü
(
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(
) = {0}.
¦^ÅU Theorem 6.4.2 §i¶D§ÚÌ
D/ker() im()
¦Ó Proposition 7.4.1 §i¶D§ÚÌ
ker() = {0} ¦]¦¹±o
D im(). ¦ý¬O
im() ¬O
ªº
subring (Lemma 6.3.3), ¬Gª¾ D ©M D ªº quotient field
¤¤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
¬O
D ªº
quotient field. Y
F ¬O¤@Ó field ¨ä¤¤¥]§t¤@Ó subring ©M
D
isomorphic, «h
F ¤¤¤]¦³¤@Ó subring ©M
isomorphic.
µý ©ú.
¥Ñ°²³]ª¾¦s¦b¤@Ó¤@¹ï¤@ªº ring homomorphism
:
DF.
§ÚÌ·Q§Q¥Î³oÓ
»s³y¥X¥t¤@Ó¤@¹ï¤@ªº ring homomorphism
:
F.
¹ï¥ô·Nªº
[a, b] , §ÚÌ©w
([a, b]) = (a) . (b)-1. ·íµM³o¸Ì§ÚÌnÀˬd
¬O§_ well-defined.
º¥ý§ÚÌÀˬd
([a, b]) ¬O§_¬O F ¤¤ªº¤¸¯À. ¥Ñ©ó
[a, b] , ª¾ b 0, ¦]¦¹¥Ñ ¬O¤@¹ï¤@ª¾
(b) ¬O F ¤¤ªº¤@Ó¤£µ¥©ó 0 ªº¤¸¯À. ©Ò¥H¥Ñ F ¬O field
ªº°²³]ª¾
(b)-1 F. ¬G±oÃÒ
([a, b]) = (a) . (b)-1 F. ±µµÛnÀˬd¬O§_Y
[a, b] = [c, d] «h
([a, b]) = ([c, d]). (¦A¦¸´£¿ô:
·í§Ú̫غc¤@Ó¨ç¼Æ®É¦pªG©w¸q°ì¸Ìªº¤¸¯Àªºªí¥Üªk¤£°ß¤@,
§Ṳ́@©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§_
(
a)
. (
b)
-1 =
(
c)
. (
d )
-1.
µM¦Ó§Q¥Î
¬O ring homomorphism ª¾
(
a . d )=
(
a)
. (
d )
¥B
(
c . b) =
(
c)
. (
b). ¬G¥Ñ
a . d =
c . b
¥i±o
(
a . d )=
(
c . b) ¤]´N¬O»¡
(
a)
. (
d )=
(
c)
. (
b). ¤W¦¡¨âÃä¦U¼¤W
(
d )
-1 . (
b)
-1 (§O§Ñ¤F
(
b) ©M
(
d )
¬Ò¤£µ¥©ó 0) ¥i±o
(
a)
. (
b)
-1 =
(
c)
. (
d )
-1. ¦]¦¹
¬O¤@Ó well-defined ªº¨ç¼Æ.
±µ¤U¨Ó§ÚÌÃÒ ¬O¤@Ó ring homomorphism. ¹ï¥ô·Nªº
[a, b],[c, d] , ¨Ì ªº©w¸q§Ú̦³
([
a,
b] + [
c,
d]) =
([
a . d +
c . b,
b . d]) =
(
a . d +
c . b)
. (
b . d )
-1
¥B
([
a,
b]) +
([
c,
d]) =
(
a)
. (
b)
-1 +
(
c)
. (
d )
-1.
µM¦Ó§Q¥Î
¬O ring homomorphism, ¼¤W
(
b . d )=
(
b)
. (
d ) §ÚÌ«Ü®e©öÀËÅç
(
a . d +
c . b)
. (
b . d )
-1 =
(
a)
. (
b)
-1 +
(
c)
. (
d )
-1.
¬Gª¾
([
a,
b] + [
c,
d]) =
([
a,
b]) +
([
c,
d]).
¦P²z¥iÃÒ
([
a,
b]
. [
c,
d]) =
([
a . c,
b . d]) =
(
a . c)
. (
b . d )
-1 =
([
a,
b])
. ([
c,
d]),
¬Gª¾
¬O¤@Ó
ring homomorphism.
³Ì«á§ÚÌÅçÃÒ ¬O¤@¹ï¤@ªº, ¤]´N¬OÅçÃÒ
ker() = {[0, 1]}.
°²³]
[a, b] ker(), §Y
([a, b]) = (a) . (b)-1 = 0. ¼¤W (b) °¨¤W¥i±o
(a) = 0. ¦ý¥Ñ©ó ¬O¤@¹ï¤@, ¬G¥Ñ
a ker() = {0}, ±o a = 0. ´«¥y¸Ü»¡
[a, b] = [0, 1]. ©Ò¥H±oÃÒ
¬O¤@¹ï¤@.
±q¤µ¥H«á, Y
¬° D ªº quotient field,
§Ú̱Nª½±µ¬Ý¦¨ D ¥]§t©ó
, ¤]´N¬O±N [a, 1] ¼g¦¨
a. ¥t¥ §Ú̱N
[a, b] ª½±µ¼g¦¨ a/b.
¤U¤@¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è
¤W¤@¶: ¤@¨Ç±`¨£ªº Rings
«e¤@¶: Polynomials over the Integers
Administrator
2005-06-18