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¤U¤@­¶: Unique Factorization Domain ¤W¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è «e¤@­¶: Euclidean Domain

Principle Ideal Domain

³o¤@¸`¤¤§Ú­Ì±N±´°Q principle ideal domain ªº°ò¥»©Ê½è. ¥Ñ©ó¤wª¾¤@­Ó Euclidean domain ¤@©w¬O principle ideal domain, ©Ò¥H³o¤@¸`©Ò½Íªº©Ê½è·íµM¾A¥Î©ó Euclidean Domain.

«e­±´£¹L¹ï¤@¯ëªº integral domain ¥ôµ¹¨â­Ó«D 0 ¤¸¯À¨ä greatest common divisor ¤£¤@©w¦s¦b. ¤£¹L¹ï©ó principle ideal domain, ¥ô·N¨â­Ó«D 0 ¤¸¯À¤§ greatest common divisor ´N¤@©w¦s¦b¤F!

Proposition 8.3.1   °²³] R ¬O¤@­Ó principle ideal domain. ¹ï¥ô·N a, b $ \in$ R ¥B a, b$ \ne$ 0 ¨ä greatest common divisor ¦s¦b. ¦Ó¥B, ­Y d ¬O a, b ªº¤@­Ó greatest common divisor, «h¦s¦b r, s $ \in$ R ¨Ï±o d = r . a + s . b.

µý ©ú. ­º¥ý¦Ò¼ $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$ ³o¤@­Ó ideal. ¥Ñ©ó R ¬O principle ideal domain, ¬G¦s¦b d $ \in$ R º¡¨¬ $ \bigl($d$ \bigr)$ = $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$. §Ú­Ì·Q­nÃÒ©ú d ´N¬O a, b ªº greatest common divisor.

­º¥ý¥ýÃÒ©ú d ¬O a, b ªº common divisor. ¥Ñ©ó

$\displaystyle \bigl($a$\displaystyle \bigr)$ $\displaystyle \subseteq$ $\displaystyle \bigl($a$\displaystyle \bigr)$ + $\displaystyle \bigl($b$\displaystyle \bigr)$ = $\displaystyle \bigl($d$\displaystyle \bigr)$,

¬G¥Ñ Lemma 8.1.2 ª¾ d | a. ¦P²z¥iÃÒ d | b, ¬G±o d ¬O a, b ªº¤@­Ó common divisor.

±µ¤U¨ÓÃÒ©ú­Y c ¬O a, b ªº¤@­Ó common divisor, «h c | d. µM¦Ó­Y c | a ¥B c | b, ªí¥Ü $ \bigl($a$ \bigr)$ $ \subseteq$ $ \bigl($c$ \bigr)$ ¥B $ \bigl($b$ \bigr)$ $ \subseteq$ $ \bigl($c$ \bigr)$. ¥Ñ©ó $ \bigl($c$ \bigr)$ ¬O¤@­Ó ideal, ¥¦¦³¥[ªkªº«Ê³¬©Ê, ¬G±o $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$ $ \subseteq$ $ \bigl($c$ \bigr)$. ¤]´N¬O»¡ $ \bigl($d$ \bigr)$ $ \subseteq$ $ \bigl($c$ \bigr)$. ¬G±oÃÒ c | d.

³Ì«á¥Ñ©w¸q, $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$ ¤¤ªº¤¸¯À³£¬O r . a + s . b, ¨ä¤¤ r, s $ \in$ R ³oºØ§Î¦¡. ¬G¥Ñ d $ \in$ $ \bigl($d$ \bigr)$ = $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$ ª¾¤@©w¦s¦b r, s $ \in$ R ¨Ï±o d = r . a + s . b. ³o­Ó¯S©Ê¹ï©ó¥ô·N a, b ªº greatest common divisor ¬Ò¹ï. ³o¬O¦]¬°¥Ñ Lemma 8.1.6 ª¾­Y d' ¬O a, b ¥t¤@­Ó greatest common divisor, «h§Ú­Ì¨ÌµM¦³ $ \bigl($d'$ \bigr)$ = $ \bigl($d$ \bigr)$ = $ \bigl($a$ \bigr)$ + $ \bigl($b$ \bigr)$. $ \qedsymbol$

``­Y d ¬O a, b ªº¤@­Ó greatest common divisor, «h¦s¦b r, s $ \in$ R º¡¨¬ d = r . a + s . r'' ³o¤@­Ó¯S©Ê«D±`¦³¥Î. ¤j®a¥i¥H§Q¥Î³o­Ó¯S©Ê¦A¥é·Ó Proposition 7.1.7 ©Î Proposition 7.2.11 ªºµý©ú¤è¦¡ÃÒ±o¤@­Ó principle ideal domain ¤¤ªº irreducible element ³£¬O prime element. ¤£¹L³o¸Ì§Ú­Ì¤¶²Ð¥t¤@ºØ§Q¥Î ideal ¤èªkªºÃÒ©ú.

Lemma 8.3.2   °²³] R ¬O¤@­Ó principle ideal domain, a $ \in$ R ¥B a$ \ne$ 0. ­Y a ¬O R ªº¤@­Ó irreducible element «h $ \bigl($a$ \bigr)$ ¬O R ªº¤@­Ó maximal ideal. ¤Ï¤§, ­Y $ \bigl($a$ \bigr)$ ¬O R ªº¤@­Ó maximal ideal, «h a ¬O R ªº¤@­Ó irreducible element.

µý ©ú. ¦pªG a ¬O¤@­Ó irreducible element, ¥Ñ Lemma 8.1.9 (1) §Ú­Ìª¾¹D§ä¤£¨ì¤@­Ó nontrivial ªº principle ideal ¤¶©ó $ \bigl($a$ \bigr)$ ©M R ¤§¶¡. ¤£¹L¥Ñ R ¬O principle ideal ªº°²³]ª¾ R ¤¤ªº ideal ³£¬O principle ideal. ´«¥y¸Ü»¡´N¬O§ä¤£¨ì¤@­Ó ideal ¤¶©ó $ \bigl($a$ \bigr)$ ©M R ¤§¶¡. ¬G±o $ \bigl($a$ \bigr)$ ¬O¤@­Ó maximal ideal.

¤Ï¤§, ¦pªG $ \bigl($a$ \bigr)$ ¬O¤@­Ó maximal ideal, ·íµM§ä¤£¨ì nontrivial principle ideal ¥]§t $ \bigl($a$ \bigr)$. ¬G§Q¥Î Lemma 8.1.9 (1) ª¾ a ¬O¤@­Ó irreducible element. $ \qedsymbol$

¦^ÅU¤@¤U Lemma 8.1.9 ªº¥t¤@³¡¤À¬O»¡ a ¬O prime element ­Y¥B°ß­Y $ \bigl($a$ \bigr)$ ¬O¤@­Ó prime ideal. ©Ò¥H§Ú­Ì«Ü§Öªº´N¥i¥H±o¨ì¥H¤U¤§µ²ªG.

Proposition 8.3.3   °²³] R ¬O¤@­Ó principle ideal domain, «h R ¤¤ªº irreducible element ³£¬O prime element. ¤Ï¤§, R ¤¤ªº prime element ³£¬O irreducible element.

µý ©ú. ¦]¬° R ¬O integral domain, Lemma 8.1.8 §i¶D§Ú­Ì R ¤¤ªº prime element ³£¬O irreducible element.

¤Ï¤§, ­Y a ¬O R ¤¤ªº irreducible element, ¥Ñ Lemma 8.3.2 ª¾ $ \bigl($a$ \bigr)$ ¬O R ªº¤@­Ó maximal ideal. µM¦Ó Corollary 6.5.13 §i¶D§Ú­Ì R ¤¤ªº maximal ideal ³£¬O prime ideal, ¬Gª¾ $ \bigl($a$ \bigr)$ ¬O R ªº¤@­Ó prime ideal. ¦]¦¹§Q¥Î Lemma 8.1.9 (2) ±oÃÒ a ¬O¤@­Ó prime element. $ \qedsymbol$

«e­±´£¹L¦b¤@¯ëªº commutative ring with 1 ¤¤ªº maximal ideal ³£¬O prime ideal, ¦ý¬O prime ideal ¥¼¥²¬O maximal ideal. µM¦Ó Lemma 8.3.2 ¥H¤Î Proposition 8.3.3 ±N principle ideal domain ¤¤ªº maximal ideal ©M prime ideal µ¹¤F¤@­Ó­«­nªºÃö³s.

Corollary 8.3.4   °²³] R ¬O¤@­Ó principle ideal domain ¥B I ¬O R ¤¤¤@­Ó«D 0 ªº ideal. «h I ¬O¤@­Ó prime ideal ­Y¥B°ß­Y I ¬O¤@­Ó maximal ideal.

µý ©ú. §Ú­Ì¤wª¾£¸­Ó maximal ideal ¤@©w¬O prime ideal. ©Ò¥H¥u­nÃÒ©ú­Y I ¬O¤@­Ó«D 0 ªº prime ideal, «h I ¬O¤@­Ó maximal ideal.

¦] R ¬O¤@­Ó principle ideal domain, ¬G¦s¦b a$ \ne$ 0 ¨Ï±o I = $ \bigl($a$ \bigr)$. ¦pªG $ \bigl($a$ \bigr)$ ¬O¤@­Ó prime ideal, «h¥Ñ Lemma 8.1.9 ª¾ a ¬O¤@­Ó prime element. ¬G¥Ñ Proposition 8.3.3 (©Î Lemma 8.1.8) ª¾ a ¬O¤@­Ó irreducible element. ¦]¦¹¥Ñ Lemma 8.3.2 ª¾ $ \bigl($a$ \bigr)$ = I ¬O¤@­Ó maximal ideal. $ \qedsymbol$

§Ú­Ì´¿¸g§Q¥Î $ \mathbb {Z}$ ©M F[x] ¤¤ªº irreducible element ©M prime element ¬O¬Û¦PªºÃÒ©ú $ \mathbb {Z}$ ©M F[x] ªº°ß¤@¤À¸Ñ©Ê½è. §Ú­Ì²¦b´X¥G¤w¨ì¹F¥i¥HÃÒ©ú principle ideal domain ªº°ß¤@¤À¸Ñ©Ê½èªº¥Ø¼Ð. ¤£¹L·í®É§Ú­Ì¦b $ \mathbb {Z}$ ©M F[x] ¤¤¬O§Q¥Î¼Æ¾ÇÂk¯Çªk¨ÓÃÒ©ú°ß¤@¤À¸Ñ©Ê½è, ²¦b¦b¤@¯ëªº principle ideal domain §Ú­Ì¨S¿ìªk¨Ï¥Î¼Æ¾ÇÂk¯Çªk. ¤U¤@­Ó Lemma ¥i¥HÀ°§U§Ú­Ì§JªA³o­Ó§xÃø.

Lemma 8.3.5   °²³] R ¬O¤@­Ó principle ideal domain, «hµLªk¦b R ¤¤§ä¨ìµL½a¦h­ÓÄY®æ»¼¼Wªº ideals. ´«¥y¸Ü»¡¦pªG {In}n = 1$\scriptstyle \infty$ ¬O¤@²Õ R ¤¤ªº ideal º¡¨¬

I1 $\displaystyle \subseteq$ I2 $\displaystyle \subseteq$ ... $\displaystyle \subseteq$ In $\displaystyle \subseteq$ ... ,

«h¦s¦b m $ \in$ $ \mathbb {N}$ ¨Ï±o Im = Im + 1 = ... .

µý ©ú. ­º¥ý§Ú­Ì¦Ò¼ I = $ \cup_{n=1}^{\infty}$ In ³o¤@­Ó¶°¦X. §Ú­Ì·Q­nÃÒ©ú I ¬O R ¤¤ªº ideal. (­nª`·N¤@¯ë¨ÓÁ¿­Y J1, J2 ¬O R ªº ideal ¨º»ò J1 $ \cup$ J2 ¤£¤@©w¬O R ªº ideals. ¤£¹L¦b³o¸Ì¥Ñ©ó In ¦³¥]§tªºÃö«Y, §Ú­Ì¥i¥HÃÒ¥X I ¬O¤@­Ó ideal.)

°²³] a, b $ \in$ I, ´«¥y¸Ü»¡¦s¦b i, j $ \in$ $ \mathbb {N}$ ¨Ï±o a $ \in$ Ii ¥B b $ \in$ Ij. °²³] i$ \ge$j, ¥Ñ°²³]ª¾ Ij $ \subseteq$ Ii. ¬G±o a, b $ \in$ Ii. ¦]¦¹¥Ñ Ii ¬O¤@­Ó ideal, §Ú­Ì¦³ a - b $ \in$ Ii. ©Ò¥H±o a - b $ \in$ I. ¥t¥ ­Y a $ \in$ I ¥B r $ \in$ R, ¥Ñ°²³]ª¾¦s¦b i $ \in$ $ \mathbb {N}$ ¨Ï±o a $ \in$ Ii. ¬G±o a . r $ \in$ Ii, ¤]´N¬O»¡ a . r $ \in$ I. ¬G¥Ñ Lemma 6.1.2 ª¾ I ¬O R ¤¤ªº¤@­Ó ideal.

¬JµM I ¬O R ªº ideal ¥B R ¬O principle ideal domain, ¬G¦s¦b a $ \in$ R ¨Ï±o $ \bigl($a$ \bigr)$ = I. µM¦Ó§Q¥Î a $ \in$ $ \bigl($a$ \bigr)$ = I ª¾¦s¦b m $ \in$ $ \mathbb {N}$ ¨Ï±o a $ \in$ Im. ¬G§Q¥Î $ \bigl($a$ \bigr)$ ¬O¥]§t a ³Ì¤pªº ideal (Lemma 6.5.1) ª¾ I = $ \bigl($a$ \bigr)$ $ \subseteq$ Im. ´«¥y¸Ü»¡ I = Im, ¦]¦¹§Q¥Î¹ï©Ò¦³ªº i > m ¬Ò¦³ Im $ \subseteq$ Ii ¥H¤Î Ii $ \subseteq$ I ±oÃÒ I = Im = Ii, $ \forall$ i > m. $ \qedsymbol$

§Ú­Ì­nÂǥΠLemma 8.3.5 ªº¥D­n­ì¦]¬O¦pªG d ¬O a ªº¤@­Ó nontrivial divisor (§Y d | a ¦ý d ¤£¬O unit ¥B©M a ¤£ associates), «h $ \bigl($a$ \bigr)$ $ \subsetneq$ $ \bigl($d$ \bigr)$. ¦p¦¹¤@¨Ó, ¥i¥HÃÒ¥X R ¤¤ªº¤¸¯À¥u¯à¼g¦¨¦³­­¦h­Ó irreducible element ªº­¼¿n.

Theorem 8.3.6   °²³] R ¬O¤@­Ó principle ideal domain ¥B a ¬O R ¤¤¤£¬° 0 ¥B¤£¬O unit ªº¤¸¯À, «h a ¥i¥H¼g¦¨¦³­­¦h­Ó R ¤¤ªº irreducible elements ªº­¼¿n, ¦Ó¥B­Y©¿²¤ associates ªºÃö«Y¥H¤Î­¼ªkªº¶¶§Ç, ³o­Ó­¼¿nªº¼gªk°ß¤@. ¤]´N¬O»¡¦pªG
a = p1n1 ... prnr  
  = q1m1 ... qsms  

¨ä¤¤ p1,..., pr ¬O¨â¨â¤£¬Û associates ªº irreducible elements ¥B q1,..., qs ¬O¨â¨â¤£¬Û associates ªº irreducible elements, «h¸g¹L¾A·íªºÅÜ´«¶¶§Ç, §Ú­Ì¦³ r = s, pi $ \sim$ qi ¥H¤Î ni = mi, $ \forall$ i = 1,..., r.

µý ©ú. ­º¥ý§Ú­ÌÃÒ©ú a ¥i¥H¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n. ¦pªG a ¤£¯à¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n, ªí¥Ü a ¥»¨­¤£¬O irreducible, ¦]¦¹ a = a1 . b1, ¨ä¤¤ a1, b1 $ \in$ R ¬O a ªº nontrivial divisors ¥B a1, b1 ¤¤¥²¦³¤@­Ó¤£¯à¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n. °²³]¬O a1, ¦P¤W§Ú­Ìª¾¦s¦b a2, b2 $ \in$ R ¨Ï±o a1 = a2 . b2, ¨ä¤¤a2 ¬O a1 ªº nontrivial divisor ¥B a2 ¤£¯à¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n. ¦p¦¹¤@ª½¤U¥h§Ú­Ì»s³y¤F¤@³s¦êªº ideals ²Å¦X

$\displaystyle \bigl($a$\displaystyle \bigr)$ $\displaystyle \subsetneq$ $\displaystyle \bigl($a1$\displaystyle \bigr)$ $\displaystyle \subsetneq$ $\displaystyle \bigl($a2$\displaystyle \bigr)$ $\displaystyle \subsetneq$ ... $\displaystyle \subsetneq$ $\displaystyle \bigl($an$\displaystyle \bigr)$ $\displaystyle \subsetneq$ ... .

¦¹©M Lemma 8.3.5 ¥Ù¬Þ, ¬Gª¾ a ¤@©w¥i¥H¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n.

±µ¤U¨Ó§Ú­ÌÃҰߤ@©Ê. ¤@¯ë¨Ó»¡­Y¤wÃÒ±o irreducible element ´N¬O prime element °ß¤@©Ê´N¦Û°Ê¦¨¥ß. ³o¬O¦]¬°¦pªG

a = p1n1 ... prnr = q1m1 ... qsms,

¥ô¨ú pi ¥Ñ©ó

pi | q1m1 ... qsms,

¥B pi ¬O prime (Proposition 8.3.3) ª¾¦s¦b j $ \in$ {1,..., s} ¨Ï±o pi | qj. ´«¨¥¤§ pi ¬O qj ªº¤@­Ó divisor. µM¦Ó qj ¬O irreducible ¥B pi ¤£¬O unit, ¬G±o pi $ \sim$ qj (§Y pi ©M qj associates). ¦]¦¹§Ú­Ìª¾¹D¹ï³o­Ó pi, ¦b {q1,..., qs} ¤¤¥u¯à§ä¨ì°ß¤@ªº qj ¨Ï±o pi $ \sim$ qj. §_«h­Y j$ \ne$j' ¦ý pi | qj', «h ¦P²z¥i±o pi $ \sim$ qj', §Q¥Î associates ¬O­Ó equivalence relation §Ú­Ì±o qj $ \sim$ qj', ³o©M°²³]­Y j$ \ne$j' «h¤£¥i¯à qj $ \sim$ qj' ¬Û¥Ù¬Þ. ¤Ï¤§¹ï¥ô·Nªº qj §Ú­Ì¥i¥H¦b {p1,..., pr} ¤¤§ä¨ì°ß¤@ªº pi ¨Ï±o qj $ \sim$ pi. ¦]¦¹§Ú­Ì¦b {p1,..., pr} ©M {q1,..., qs} ³o¨â­Ó¶°¦X¤¤§ä¨ì¤@¹ï¤@ªº¹ïÀ³. ¤]´N¬O»¡ r = s ¥B¸g¹L¾A·íªº­«±Æ§Ú­Ì¦³ p1 $ \sim$ q1,..., pr $ \sim$ qr. ²°²³]¬Y­Ó ni$ \ne$mi, ¬°¤F¤è«K°_¨£§Ú­Ì´N°²³] n1$ \ne$m1 ¥B n1 > m1 §a! ¥Ñ©ó q1 = u . p1, ¨ä¤¤ u ¬O R ªº¤@­Ó unit, §Ú­Ì¦³

p1m1(p1n1 - m1 . p2n2 ... prnr - um1 . q2m2 ... qrmr) = 0.

§Q¥Î p1m1$ \ne$ 0 ¥B R ¬O integral domain, §Ú­Ì¦³

p1n1 - m1 . p2n2 ... prnr = um1 . q2m2 ... qrmr.

µM¦Ó¥Ñ©ó n1 - m1 > 0, ¥i±o¦b {q2,..., qr} ¤¤¦s¦b qj ¨Ï±o p1 | qj (ª`·N u ¬O unit ¬G¤£¥i¯à p1 | u). ¤]´N¬O»¡ p1 $ \sim$ qj, ¦ý³o©M q1 ¬O {q1,..., qr} ¤¤°ß¤@º¡¨¬©M p1 associates ªº¤¸¯À¬Û¥Ù¬Þ. ±oÃÒ¥»©w²z. $ \qedsymbol$

º¡¨¬ Theorem 8.3.6 ¤¤ªº°ß¤@¤À¸Ñ©Ê½èªº ring «D±`­«­n, §Ú­Ì¤]µ¹¥¦¤@­Ó¯S®íªº¦W¤l.

Definition 8.3.7   °²³] R ¬O¤@­Ó integral domain ¦Ó¥B R ¤¤«D 0 ¥B¤£¬O unit ªº¤¸¯À³£¥i¥H¼g¦¨¦³­­¦h­Ó R ¤¤ªº irreducible elements ªº­¼¿n, ¦Ó¥B­Y©¿²¤ associates ªºÃö«Y¥H¤Î­¼ªkªº¶¶§Ç, ³o­Ó­¼¿nªº¼gªk°ß¤@, «hºÙ R ¬O¤@­Ó unique factorization domain.

Theorem 8.3.6 §i¶D§Ú­Ì¤@­Ó principle ideal domain ¤@©w¬O¤@­Ó unique factorization domain. ¦ý¬O¤@­Ó unique factorization domain ¨Ã¤£¤@©w¬O principle ideal domain. §Ú­Ì´¿¸g¨£¹L $ \mathbb {Z}$[x] ¬O¤@­Ó unique factorization domain (Theorem 7.3.13) ¦ý¨ä¤¤ $ \bigl($2$ \bigr)$ + $ \bigl($x$ \bigr)$ ³o¤@­Ó ideal ¨Ã¤£¬O principle ideal (Example 7.3.1).


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¤U¤@­¶: Unique Factorization Domain ¤W¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è «e¤@­¶: Euclidean Domain
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