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¤U¤@­¶: Principle Ideal Domain ¤W¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è «e¤@­¶: Divisor

Euclidean Domain

§Ú­Ìª¾¹D $ \mathbb {Z}$ ©M F[x] ¦³©Ò¿×ªº Euclid's Algorithm (¾l¼Æ¤Î¾l¦¡©w²z). ¦b³o¤@¸`¤¤, §Ú­Ì±N§Q¥Î³o­Ó©Ê½èªº¯S©Ê©w¸q¤@ºØ¯S®íªº ring ºÙ¬° Euclidean domain. ­nª`·N§Ú­Ìªº©w¸q¤ñ¤@¯ë®Ñ¤Wªº©w¸q²¤Æ, ¥D­nªº­ì¦]¬O§Ú­Ì¥u­«µø¥Ø«e¦³¥Îªº¯S©Ê. ¤£¹L¨Æ¹ê¤W§Ú­Ì©w¸qªº Euclidean domain ©M¤@¯ë®Ñ¤W©w¸qªº Euclidean domain ¥i¥HÃÒ©ú¬O¬Û¦Pªº.

¦^ÅU¤@¤U $ \mathbb {Z}$ ¤¤ªº Euclid's Algorithm ¥i¥H»¡¬O¥ô¨ú a, b $ \in$ $ \mathbb {Z}$, ¨ä¤¤ b$ \ne$ 0, «h¦s¦b h, r $ \in$ $ \mathbb {Z}$, ¨ä¤¤ r ²Å¦X r = 0 ©Î $ \left\vert\vphantom{ r}\right.$r$ \left.\vphantom{ r}\right\vert$ < $ \left\vert\vphantom{ b}\right.$b$ \left.\vphantom{ b}\right\vert$ ¨Ï±o a = b . h + r. ¦Ó¦b F[x] ¤¤ªº Euclid's Algorithm ¬O»¡¥ô¨ú f (x), g(x) $ \in$ F[x] ¨ä¤¤ g(x)$ \ne$ 0, «h¦s¦b h(x), r(x) $ \in$ F[x], ¨ä¤¤ r(x) ²Å¦X r(x) = 0 ©Î deg(r(x)) < deg(g(x)) ¨Ï±o f (x) = g(x) . h(x) + r(x). ³o¸Ì­«­nªº¬O¦b $ \mathbb {Z}$ ¤¤¦³¤@­Óµ´¹ï­È¨ç¼Æ±N $ \mathbb {Z}$ ¤¤ªº«D 0 ¤¸¯À°e¨ì«D­tªº¾ã¼Æ, ¦Ó¦b F[x] ¤¤¦³¤@­Ó degree ¨ç¼Æ±N F[x] ¤¤ªº«D 0 ¤¸¯À°e¨ì«D­tªº¾ã¼Æ. §Ú­Ì´N¬O­nÂ^¨ú³o¼Ëªº¨ç¼Æªº¯S©Ê.

Definition 8.2.1   ³] R ¬O¤@­Ó integral domain. ¦pªG¦s¦b¤@¨ç¼Æ

$\displaystyle \Phi$ : R $\displaystyle \setminus$ {0}$\displaystyle \to$$\displaystyle \mathbb {N}$ $\displaystyle \cup$ {0}

¨Ï±o¹ï¥ô·Nªº a, b $ \in$ R ¨ä¤¤ b$ \ne$ 0 ³£¥i¥H§ä¨ì h, r $ \in$ R, ¨ä¤¤ r ²Å¦X r = 0 ©Î $ \Phi$(r) < $ \Phi$(b), º¡¨¬ a = b . h + r, «hºÙ R ¬°¤@­Ó Euclidean domain.

°£¤F $ \mathbb {Z}$ ©M F[x] ¥ ÁÙ¦³³hªº Euclidean domain. ¨Ò¦p $ \mathbb {Z}$[i] = {a + bi | a, b $ \in$ $ \mathbb {Z}$} ³o¤@­Ó integral domain §Q¥Î $ \Phi$(a + bi) = a2 + b2 ³o­Ó¨ç¼Æ´N¥i±o $ \mathbb {Z}$[i] ¬O¤@­Ó Euclidean domain (¦b¦¹§Ú­Ì²¤¥hÃÒ©ú, ­Y¦³¿³½ìªº¦P¾Ç¥i¨ìºô¯¸ http://math.ntnu.edu.tw/$ \sim$li/note ¤U¸üÁ¿¸q ``Factorization of Commutative Rings'' ¦³¸Ô²ÓÃÒ©ú).

¤@¯ë¦Ó¨¥­nÅçÃÒ¤@­Ó integral domain ¬O§_¬°¤@­Ó Euclidean domain ¬O«Ü§xÃøªº. ¦b¦¹§Ú­Ì¨Ã¤£°Q½×³oÃþªº°ÝÃD. §Ú­Ì¶È¦C¥X Euclidean domain ªº­«­n©Ê½è. ¦^ÅU§Ú­Ì´¿§Q¥Î Euclid's Algorithm ÃÒ¥X¦b $ \mathbb {Z}$ ©M F[x] ¤¤©Ò¦³ªº ideal ³£¬O principle ideal. ³o¤@®MÃÒ©ú¥i¥H§¹§¹¾ã¾ã·h¨ì Euclidean domain ¤W.

Theorem 8.2.2   ­Y R ¬O¤@­Ó Euclidean domain «h R ¤¤ªº ideal ³£¬O principle ideal.

µý ©ú. ­Y I ¬O R ¤¤ªº¤@­Ó ideal. ¦Ò¼ T = {$ \Phi$(a) | a $ \in$ I $ \setminus$ {0}} ³o¤@­Ó¶°¦X. ¥Ñ©ó $ \Phi$ ªº­È°ì¦b $ \mathbb {N}$ $ \cup$ {0} ©Ò¥H T ¬O $ \mathbb {N}$ $ \cup$ {0} ªº¤@­Ó¤l¶°¦X. ¦]¦¹ T ¥²¦s¦b³Ì¤pªº¤¸¯À. ´«¥y¸Ü»¡¦s¦b d $ \in$ I $ \setminus$ {0} ¨Ï±o¹ï¥ô·Nªº a $ \in$ I $ \setminus$ {0} ¬Ò¦³ $ \Phi$(d )$ \le$$ \Phi$(a). §Ú­Ì±ýÃÒ I = $ \bigl($d$ \bigr)$.

¥Ñ©ó d $ \in$ I, ¦ÛµM±o $ \bigl($d$ \bigr)$ $ \subseteq$ I. ¥t¥ ¹ï¥ô·N a $ \in$ I, ¥Ñ Euclidean domain ªº°²³]ª¾¦s¦b h, r $ \in$ R º¡¨¬ a = d . h + r ¥B r = 0 ©Î $ \Phi$(r) < $ \phi$(d ). ¦pªG r$ \ne$ 0, ¥Ñ r = a - d . h ¥B a, d $ \in$ I ¥iª¾ r $ \in$ I. ¤]´N¬O»¡ r $ \in$ I $ \setminus$ {0} ¥B $ \Phi$(r) < $ \Phi$(d ). ³o©M $ \Phi$(d ) ¬O T ¤¤³Ì¤pªº°²³]¬Û¥Ù¬Þ, ¬Gª¾ r = 0. ´«¨¥¤§ a = d . h, §Y a $ \in$ $ \bigl($d$ \bigr)$. ¬G±oÃÒ I $ \subseteq$ $ \bigl($d$ \bigr)$. $ \qedsymbol$

¥Ñ©ó¤@­Ó integral domain ªº ideal ³£¬O principle ideal ³o¼Ëªº ring «D±`¯S§O, §Ú­Ì¤]µ¹¥¦¤@­Ó¯S§Oªº¦WºÙ.

Definition 8.2.3   ¦pªG R ¬O¤@­Ó integral domain ¥B R ¤¤ªº ideal ³£¬O principle ideal, «hºÙ R ¬°¤@­Ó principle ideal domain.

Theorem 8.2.2 §i¶D§Ú­Ì¤@­Ó Euclidean domain ¤@©w¬O¤@­Ó principle ideal domain. ­nª`·N, £¸­Ó principle ideal domain ¥¼¥²·|¬O¤@­Ó Euclidean domain. ¦³¿³½ìªº¦P¾Ç¥i¥H°Ñ¦Ò§ÚªºÁ¿¸q ``Factorization of Commutative Rings'' ¨ä¤¤¦³µ¹¤@­Ó principle ideal domain ¦ý¤£¬O Euclidean domain ªº¨Ò¤l.


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¤U¤@­¶: Principle Ideal Domain ¤W¤@­¶: Integral Domain ¤Wªº¤À¸Ñ©Ê½è «e¤@­¶: Divisor
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