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¤U¤@­¶: ¤T­Ó Ring Isomorphism ©w²z ¤W¤@­¶: ¤¤¯Å Ring ªº©Ê½è «e¤@­¶: Subring ©M Ideal ªº°ò¥»©Ê½è

Ring Homomorphism ©M Correspondence ©w²z

§Ú­Ì´¿¸g§Q¥Î group homomorphism ¨Ó´yø¨â­Ó group ¤§¶¡ªºÃö«Y. ¦P¼Ëªº ring ¤§¶¡¤]¦³©Ò¿×ªº ring homomorphism, ¦Ó correspondence ©w²z´N§i¶D§Ú­Ì¦p¦ó¥Ñ ring homomorphism ¨Ó´yø¨â­Ó ring ¶¡ ideal ªºÃö«Y.

Definition 6.3.1   ·í R, R' ¬O rings ¦Ó $ \phi$ : R$ \to$R' ¬O±q R ¬M®g¨ì R' ªº¨ç¼Æ. ¦pªG $ \phi$ º¡¨¬¹ï©ó©Ò¦³ a, b $ \in$ R ¬Ò¦³

$\displaystyle \phi$(a + b) = $\displaystyle \phi$(a) + $\displaystyle \phi$(b)    ¥B    $\displaystyle \phi$(a . b) = $\displaystyle \phi$(a) . $\displaystyle \phi$(b),

«hºÙ¦¹¨ç¼Æ $ \phi$ ¬O¤@­Ó ring homomorphism.

­nª`·Nªº¬O: ¦]¬° a, b $ \in$ R, ©Ò¥H³o¸Ì a + ba . b ¬O¦b R ¤¤ªº¥[ªk©M­¼ªk; ¦Ó $ \phi$(a),$ \phi$(b) $ \in$ R', ©Ò¥H $ \phi$(a) + $ \phi$(b), $ \phi$(a) . $ \phi$(b) ¬O¦b R' ¤¤ªº¥[ªk©M­¼ªk. ²³æ¦a»¡: ¤@­Ó±q R ¨ì R' ªº ring homomorphism, ¬O¥[ªkªº group homomorphism ¦A¥[¤W«O«ù­¼ªkªº¹Bºâ. ©Ò¥H¤@¯ë¨Ó»¡¦³Ãö©ó group homomorphism ªº©Ê½è³£¥i¥Hª½±µ®M¥Î¦b ring homomorphism ¤W. ¤ñ¤è»¡¥Ñ Lemma 2.5.2 ª¾ $ \phi$(0) = 0 (¨ä¤¤ $ \phi$ ¸Ì­±ªº 0 ¬O R ªº 0, ¥t¤@­Ó 0 ¬O R' ªº 0) ¥B $ \phi$(- a) = - $ \phi$(a). ¦]¦¹¥H«á­n­pºâ $ \phi$(a - b) ®É¥Ñ©ó

$\displaystyle \phi$(a - b) = $\displaystyle \phi$(a + (- b)) = $\displaystyle \phi$(a) + $\displaystyle \phi$(- b) = $\displaystyle \phi$(a) + (- $\displaystyle \phi$(b)),

§Ú­Ì·|ª½±µ¼g¦¨

$\displaystyle \phi$(a - b) = $\displaystyle \phi$(a) - $\displaystyle \phi$(b).

¦b group homomorphism ¤¤§Ú­Ì¤¶²Ð¤F¨â­Ó­«­nªº¶°¦X image ©M kernel, ¦b ring homomorphism ³o¨â­Ó¶°¦X¤´µM«Ü­«­n. §Ú­Ì¦A¦^ÅU¤@¤U¥¦­Ìªº©w¸q.

Definition 6.3.2   ­Y $ \phi$ : R$ \to$R' ¬O¤@­Ó group homomorphism, «h

im($\displaystyle \phi$) = {$\displaystyle \phi$(a) $\displaystyle \in$ R' | a $\displaystyle \in$ R}

ºÙ¬° $ \phi$ ªº image.

ker($\displaystyle \phi$) = {a $\displaystyle \in$ R | $\displaystyle \phi$(a) = 0},

ºÙ¬° $ \phi$ ªº kernel.

ª`·N³o¸Ì kernel ¤¤ªº 0 ¬O R' ¥[ªkªº identity. ¦b group homomorphism ¤¤ image ©M kernel ¤À§O¬O¹ïÀ³°ìªº subgroup ©M©w¸q°ìªº normal subgroup. ¤j®aÀ³¤£Ãø²q¥X¦b ring homomorphism ¥¦­Ìªº©Ê½è§a!

Lemma 6.3.3   ­Y $ \phi$ : R$ \to$R' ¬O¤@­Ó ring homomorphism, «h im($ \phi$) ¬O R' ªº subring, ¦Ó ker($ \phi$) ¬O R ªº ideal.

µý ©ú. §Ú­Ì§Q¥Î Lemma 2.5.4 ª½±µª¾ im($ \phi$) ©M ker($ \phi$) ¤À§O¬O R' ©M R ¥[ªk¤§¤Uªº subgroup. ©Ò¥H§Ú­Ì¥u­nÅçÃÒ­¼ªk.

­Y $ \phi$(a),$ \phi$(b) $ \in$ im($ \phi$), ¨ä¤¤ a, b $ \in$ R, «h $ \phi$(a) . $ \phi$(b) = $ \phi$(a . b). ¤S¦] a . b $ \in$ R, ¬G $ \phi$(a) . $ \phi$(b) $ \in$ im($ \phi$). ¦]¦¹¥Ñ Lemma 5.4.2 ª¾ im($ \phi$) ¬O R' ªº subring.

¦Ü©ó ker($ \phi$) ¬O R ªº ideal, §Ú­Ì¥u­nÃÒ: ¹ï¥ô·Nªº r $ \in$ R ©M a $ \in$ ker($ \phi$) ¬Ò¦³ r . a $ \in$ ker($ \phi$) ¤Î a . r $ \in$ ker($ \phi$). µM¦Ó $ \phi$(r . a) = $ \phi$(r) . $ \phi$(a) = $ \phi$(r) . 0, §Q¥Î Lemma 5.2.1 ª¾ $ \phi$(r . a) = 0 ¬G r . a $ \in$ ker($ \phi$). ¦P²z±o a . r $ \in$ ker($ \phi$). ¦]¦¹¥Ñ Lemma 6.1.2 ª¾ ker($ \phi$) ¬O R ªº ideal. $ \qedsymbol$

¦b Lemma 2.5.6 ¤¤§Ú­Ìª¾¹D¥i¥H¥Î kernel ¨Ó§PÂ_¤@­Ó group homomorphism ¬O§_¬°¤@¹ï¤@, ¬JµM ring homomorphism ¦b¥[ªk¤§¤U¬O group homomorphism ©Ò¤U­±ªº Lemma ·íµM¦¨¥ß.

Lemma 6.3.4   ¤wª¾ $ \phi$ : R$ \to$R' ¬O¤@­Ó ring homomorphism, «h $ \phi$ ¬O¤@­Ó monomorphism (§Y¤@¹ï¤@) ­Y¥B°ß­Y ker($ \phi$) = {0}.

ÁA¸Ñ¤F ring homomorphism, ±µ¤U¨Ó§Ú­Ì¨Ó½Í ring homomorphism ªº correspondence ©w²z. ¦^ÅU¤@¤U group homomorphism ¤¤ªº correspondence ©w²z´y­z¤F¨â­Ó group ªº subgroup ©M normal subgroup §Q¥Î group homomorphism ©Ò±o¨ìªº¹ïÀ³Ãö«Y. ¹ï ring homomorphism §Ú­Ì¤]¦³Ãþ¦üª¬ªp.

Theorem 6.3.5 (Correspondence Theorem)   ­Y $ \phi$ : R$ \to$R' ¬O¤@­Ó onto ªº ring homomorphism. ­Y S' ¬O R' ªº subring ¥B¥O

S = {a $\displaystyle \in$ R | $\displaystyle \phi$(a) $\displaystyle \in$ R'},

«h S ¬O R ªº¤@­Ó subring ¥B S $ \supseteq$ ker($ \phi$). ¥t¥ ­Y¥O

$\displaystyle \phi$(S) = {$\displaystyle \phi$(a) | a $\displaystyle \in$ S},

«h $ \phi$(S) = S'.

¦pªG¤S°²³] S' ¬O R' ªº ideal. «h«e­±©Ò©wªº S ¤]·|¬O R ªº ideal.

µý ©ú. ­º¥ý¥ýÃÒ S ¬O R ªº subring. ­Y a, b $ \in$ S, §Ú­Ì­nÃÒ©ú a - b $ \in$ S ¥B a . b $ \in$ S. ¥Ñ©w¸qª¾ a, b $ \in$ S ªí¥Ü $ \phi$(a) $ \in$ S' ¥B $ \phi$(b) $ \in$ S', ¬G $ \phi$(a) - $ \phi$(b) $ \in$ S' ¥B $ \phi$(a) . $ \phi$(b) $ \in$ S'. ¤S¦] $ \phi$ ¬O ring homomorphism, ¬G $ \phi$(a - b) = $ \phi$(a) - $ \phi$(b) ¥B $ \phi$(a . b) = $ \phi$(a) . $ \phi$(b). ¦]¦¹ $ \phi$(a - b) $ \in$ S' ¥B $ \phi$(a . b) $ \in$ S', ¤]´N¬O»¡ a - b $ \in$ S ¥B a . b $ \in$ S. ¬Gª¾ S ¬O R ªº subring. (ª`·N³o­Ó³¡¤ÀªºÃÒ©ú¥u¥Î¨ì $ \phi$ ¬O ring homomorphism, ¨Ã¤£»Ý­n onto.)

­Y a $ \in$ ker($ \phi$), «h $ \phi$(a) = 0. ¦] 0 $ \in$ S' ¬G a $ \in$ S. ©Ò¥H ker($ \phi$) $ \subseteq$ S. (³o³¡¤ÀªºÃÒ©ú¤]¤£»Ý onto.)

²¦bÃÒ $ \phi$(S) = S'. ­º¥ýÃÒ©ú $ \phi$(S) $ \subseteq$ S' ³o³¡¥÷¬O®e©öªº. ¥D­n¬O¦] $ \phi$(S) ªº¤¸¯À³£¬O $ \phi$(a) ³oºØ§Î¦¡, ¨ä¤¤ a $ \in$ S. ¥Ñ©w¸q a $ \in$ S, ªí¥Ü $ \phi$(a) $ \in$ S'. ¬G $ \phi$(S) ªº¤¸¯À³£¸¨¦b S' ¤¤. «Ü¦h¦P¾Ç³£·|»¬° S' ªº¤¸¯À¤]·|¦b $ \phi$(S) ¤¤; ¤@¯ë³o¬O¤£¤@©w¹ïªº. ¦]¬°¦b¤@¯ëªº±¡ªp b $ \in$ S' ¤£¥Nªí¦³¤¸¯À a $ \in$ R ¨Ï±o $ \phi$(a) = b. ³o¸Ì§Ú­Ì´N­n¥Î¨ì onto ªº©Ê½è¤F. ¦]¬° $ \phi$ ¬O onto ¬G¹ï¥ô·N b $ \in$ S' $ \subseteq$ R' ³£¥i§ä¨ì a $ \in$ R ¨Ï±o $ \phi$(a) = b. ¬JµM $ \phi$(a) = b $ \in$ S', ³o¤@­Ó a ¤]´N¦b S ¤¤¤F. ©Ò¥H b = $ \phi$(a) $ \in$ $ \phi$(S), ¤]´N¬O»¡ S' $ \subseteq$ $ \phi$(S). ¥Ñ¦¹±oÃÒ S' = $ \phi$(S).

³Ì«á§Ú­Ì­nÃÒ©ú­Y S' ¬O R' ªº ideal, «h S ¤]¬O R ªº ideal. ¹ï¥ô·Nªº r $ \in$ R, a $ \in$ S ¬Ò¦³ $ \phi$(r . a) = $ \phi$(r) . $ \phi$(a). ¥Ñ©ó $ \phi$(r) $ \in$ R' ¥B $ \phi$(a) $ \in$ S' ¤Î S' ¬O R' ªº ideal, §Ú­Ì¦³ $ \phi$(r) . $ \phi$(a) $ \in$ S'. ¬G r . a $ \in$ S, ¦P²z±o a . r $ \in$ S. ©Ò¥H S ¬O R ªº ideal. $ \qedsymbol$

¦A¦¸±j½Õ³o­Ó©w²z¤¤°£¤F $ \phi$(S) = S' »Ý¥Î¨ì $ \phi$ ¬O onto ¥ , ¨ä¥L©Ê½è¨Ã¤£»Ý onto ªº°²³].

Remark 6.3.6   Correspondence Theorem §i¶D§Ú­Ì»¡­Y $ \phi$ : R$ \to$R' ¬O¤@­Ó onto ªº ring homomorphism, «h¦b R' ¤¤¥ô¿ï¤@­Ó subring S' ³£¥i¦b R ¤¤§ä¨ì¤@­Ó subring S ¨Ï±o $ \phi$(S) = S', ¦Ó¥B ker($ \phi$) $ \subseteq$ S. ¨ä¹ê¦b R ¤¤²Å¦X $ \phi$(S) = S' ¤Î ker($ \phi$) $ \subseteq$ S ªº subring ¬O°ß¤@ªº. °²³] R ¤¤¦³¥t¤@­Ó subring T ²Å¦X $ \phi$(T) = S' ¥B ker($ \phi$) $ \subseteq$ T. «h¹ï©ó©Ò¦³ a $ \in$ T, ¦] $ \phi$(a) $ \in$ $ \phi$(T) = S', ¬G¥Ñ°²³] $ \phi$(S) = S' ª¾¦b S ¤¤¥²¦s¦b¤@¤¸¯À b ¨Ï±o $ \phi$(b) = $ \phi$(a). ´«¥y¸Ü»¡ $ \phi$(a) - $ \phi$(b) = 0. ¥Ñ¦¹±o $ \phi$(a - b) = 0. ¤]´N¬O»¡ a - b $ \in$ ker($ \phi$). §O§Ñ¤F ker($ \phi$) $ \subseteq$ S ¥B b $ \in$ S ¬G a $ \in$ S, ¤]´N¬O»¡ T $ \subseteq$ S. ¥Î¦P¼Ëªº¤èªk¥i±o S $ \subseteq$ T. ©Ò¥H T = S. ´«¥y¸Ü»¡: ¹ï©ó R' ¤¤¥ô¤@ subring S', ¦b R ¤¤¬Ò`¦s¦b' ``°ß¤@'' ªº subring S º¡¨¬ $ \phi$(S) = S' ¥B ker($ \phi$) $ \subseteq$ S.

Correspondence Theorem ³Ì±`¥Îªº±¡ªp¬O·í I ¬O R ªº¤@­Ó ideal, ¦Ó $ \phi$ ¬O R ¨ì R/I ªº ring homomorphism ¨ä¤¤¹ï¥ô·Nªº a $ \in$ R, ©w¸q $ \phi$(a) = $ \overline{a}$.

Corollary 6.3.7   °²³] R ¬O¤@­Ó ring ¥B I ¬O R ªº¤@­Ó ideal. «h¹ï¥ô·N R/I ¤¤ªº subring S' ³£¥i¦b R ¤¤§ä¨ì subring S ²Å¦X I $ \subseteq$ S ¥B S/I = S'.

·í S' ¬O R/I ªº ideal ®É, «h S ¤]·|¬O R ªº ideal.

µý ©ú. $ \phi$ ¬O ring homomorphism ¬O¦]¬°

$\displaystyle \phi$(a - b) = $\displaystyle \overline{a-
b}$ = $\displaystyle \overline{a}$ - $\displaystyle \overline{b}$ = $\displaystyle \phi$(a) - $\displaystyle \phi$(b)

¥B

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

¦AÃÒ©ú $ \phi$ ¬O onto ªº, ¨Æ¹ê¤W¹ï©Ò¦³ y $ \in$ R/I ³£¬O y = $ \overline{a}$, ¨ä¤¤ a $ \in$ R ³oºØ§Î¦¡. ¬G¿ï a $ \in$ R ±a¤J $ \phi$ ±o $ \phi$(a) = $ \overline{a}$ = y. ±oÃÒ $ \phi$ ¬O onto.

ker($ \phi$) ¬O¬Æ»ò©O? ­Y a $ \in$ ker($ \phi$) «h $ \phi$(a) = $ \overline{0}$, ¦ý¥Ñ $ \phi$ ªº©w¸q $ \phi$(a) = $ \overline{a}$. ¬G¥Ñ $ \overline{a}$ = $ \overline{0}$, ±o a $ \in$ I. ¤Ï¤§­Y a $ \in$ I, «h $ \phi$(a) = $ \overline{a}$ = $ \overline{0}$, ¬G a $ \in$ ker($ \phi$). ¥Ñ¦¹±o ker($ \phi$) = I.

²¦b Correspondence Theorem ¤¤ªº±ø¥ó³£§ä¨ì¤F, ©Ò¥H§Q¥Î Theorem 6.3.5 ª¾¥ô¨ú R/I ¤¤ªº¤@­Ó subring (©Î idealS'), ¦b R ¤¤³£¥i¥H§ä¨ì¤@­Ó subring (©Î ideal) S ²Å¦X I = ker($ \phi$) $ \subseteq$ S ¥B $ \phi$(S) = S/I = S'. $ \qedsymbol$

¦³³h®Ñ¤]ºÙ Corollary 6.3.7 ¬° Correspondence Theorem. ¥¦§i¶D§Ú­Ì R/I ¤¤ªº subring (©Î ideal) ³£¬Oªø S/I ³oºØ§Î¦¡, ¨ä¤¤ S ¬O R ªº subring (©Î ideal) ¥B I $ \subseteq$ S.


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¤U¤@­¶: ¤T­Ó Ring Isomorphism ©w²z ¤W¤@­¶: ¤¤¯Å Ring ªº©Ê½è «e¤@­¶: Subring ©M Ideal ªº°ò¥»©Ê½è
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