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¤U¤@­¶: FIELD ¤W¤@­¶: Unique Factorization Domain «e¤@­¶: Unique factorization domain ªº°ò¥»©Ê½è

Polynomials over unique factorization domain

§Ú­Ì±N§Q¥ÎÃþ¦ü±À¾É $ \mathbb {Z}$[x] ¬O unique factorization domain ªº¤èªk±À¾É·í R ¬O unique factorization domain ®É

R[x] = {anxn + ... + a1x + a0 | ai $\displaystyle \in$ R}

³oºØ¥H R ¬°«Y¼Æªº polynomials ©Ò§Î¦¨ªº polynomial ring ¬O¤@­Ó unique factorization domain.

­Y f (x) $ \in$ R[x] ¥B f (x)$ \ne$ 0, «h§Ú­Ì¥i±N f (x) ¼g¦¨ f (x) = anxn + ... + a1x + a0, ¨ä¤¤ an$ \ne$ 0. ¦p¦P«e­±°Q½× F[x] ªº±¡ªp§Ú­Ì¥i¥H©w¸q deg(f (x)) = n. §Q¥Î©M Lemma 7.2.2 ¦P¼ËªºÃÒ©ú§Ú­Ì¥i¥H±o¨ì: ­Y f (x), g(x) $ \in$ R[x] ¥B¬Ò¤£¬° 0, «h

deg(f (x) . g(x)) = deg(f (x)) + deg(g(x)).

¥D­nªº­ì¦]¬O Lemma 7.2.2 ªºµý©ú¶È¥Î¨ì¨â­Ó«D 0 ¤¸¯À¬Û­¼¤£¬° 0 (§Y integral domain) ªº©Ê½è, ¨Ã¨S¦³¥Î¨ì field ªº©Ê½è. §Q¥Î degree ªº³o­Ó¯S©Ê§Ú­Ì°¨¤W¦³¥H¤Uªº©Ê½è.

Lemma 8.4.3   ¥O R ¬O¤@­Ó integral domain.
  1. R[x] ¤]¬O¤@­Ó integral domain.
  2. R[x] ¤¤ªº unit ´N¬O R ¤¤ªº unit.
  3. ­Y a $ \in$ R ¬O R ¤¤ªº irreducible element «h a ¬Ý¦¨¬O R[x] ¤¤ªº¤¸¯À (§Y±`¼Æ¦h¶µ¦¡) ®É¤]¬O irreducible.

µý ©ú. (1) ­Y f (x)$ \ne$ 0 ¥B g(x)$ \ne$ 0, °²³] f (x) ªº³Ì°ª¦¸¶µ«Y¼Æ¬O an ¥B g(x) ªº³Ì°ª¦¸¶µ«Y¼Æ¬O bm, «h f (x) . g(x) ªº³Ì°ª¦¸¶µ«Y¼Æ¬O an . bm. ¥Ñ©ó an, bm $ \in$ R, ¥B an$ \ne$, bm$ \ne$ 0 §Q¥Î R ¬O integral domain ª¾ an . bm$ \ne$ 0. ¤]´N¬O»¡ f (x) . g(x) ¤£¥i¯à¬° 0 ¦h¶µ¦¡.

(2) ­Y f (x) $ \in$ R[x] ¬O R[x] ¤¤ªº unit, «h§Q¥Î¦s¦b g(x) $ \in$ R[x] º¡¨¬ f (x) . g(x) = 1 ª¾ deg(f (x)) + deg(g(x)) = 0 (ª`·N 1 ¬O±`¼Æ¦h¶µ¦¡¬G degree ¬° 0). ¬G±o deg(f (x)) = deg(g(x)) = 0. ´«¥y¸Ü»¡ f (x), g(x) ³£¬O±`¼Æ¦h¶µ¦¡, ¤]´N¬O»¡ f (x), g(x) $ \in$ R. µM¦Ó¥Ñ°²³] f (x) . g(x) = 1 ª¾ f (x) ¬O R ¤¤ªº unit.

(3) °²³] a $ \in$ R ¬O R ¤¤ªº irreducible element. ª`·N¥Ñ degree ªº©Ê½èª¾­Y g(x) ¬O f (x) ªº divisor (¥Ñ©ó¦s¦b h(x) $ \in$ R[x] º¡¨¬ g(x) . h(x) = f (x)), «h deg(g(x))$ \le$deg(f (x)). ²­Y±N a ¬Ý¦¨¬O±`¼Æ¦h¶µ¦¡, ¥Ñ©ó deg(a) = 0, ¬Gª¾¦b R[x] ¤¤ a ªº divisor ¨ä degree ¤]¬O 0. ´«¥y¸Ü»¡¦b R[x] ¤¤ a ªº divisor ³£¬O R ªº¤¸¯À. ¬G§Q¥Î a ¦b R ¤¤¬O irreducible ª¾³o¨Ç divisor ­n¤£¬O R ¤¤ªº unit ´N¬O©M a associates. µM¦Ó¥Ñ (2) ª¾ R ¤¤ªº unit ·íµM¤]¬O R[x] ¤¤ªº unit, ¬Gª¾ a ¦b R[x] ¨ÌµM¬O irreducible. $ \qedsymbol$

·í R ¬O¤@­Ó unique factorization domain ®É, ¥O F ¬° R ªº quotient field. ±µ¤U¨Ó§Ú­Ì·Q§Q¥Î R ©M F[x] ³£¬O unique factorization domain (Theorem 7.2.14) ÃÒ©ú R[x] ¬O¤@­Ó unique factorization domain.

¬°¤F±N R[x] ©M F[x] ªºÃö«Y¬Û³sµ², §Ú­ÌÁÙ¬O±o¤¶²Ð©M $ \mathbb {Z}$[x] ¤¤Ãþ¦üªº content ªº·§©À. ­º¥ý¥Ñ Proposition 8.4.1 ª¾­Y f (x) = anxn + ... a1x + a0 $ \in$ R[x], «h an,..., a1, a0 ªº greatest common divisor ¬O¦s¦bªº.

Definition 8.4.4   ­Y f (x) = anxn + ... + a1x + a0 $ \in$ R[x] ¥B an,..., a1, a0 ªº greatest common divisor ¬O R ¤¤ªº unit, «hºÙ f (x) ¬O R[x] ¤¤ªº primitive polynomial.

Lemma 8.4.5   °²³] R ¬O¤@­Ó unique factorization domain, «h¹ï¥ô·N f (x) $ \in$ R[x] ¥B f (x)$ \ne$ 0, ³£¥i§ä¨ì c $ \in$ R ¥B f*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial º¡¨¬

f (x) = c . f*(x).

¤S°²³]

f (x) = c . f*(x)  
  = c' . g(x)  

¨ä¤¤ c, c' $ \in$ R, ¥B f*(x), g(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomials, «h c $ \sim$ c' ¥B f*(x) $ \sim$ g(x).

µý ©ú. ­º¥ýÃÒ©ú¦s¦b©Ê: ­Y f (x) = anxn + ... + a1x + a0, ¥O c ¬° an,..., a1, a0 ªº greatest common divisor. ©Ò¥H¹ï©Ò¦³ªº i = 0, 1,..., n ¬Ò¦³ ai = c . bi, ¨ä¤¤ bi $ \in$ R, ¦Ó¥B b0,..., bn ªº greatest common divisor ¬O R ªº unit. ¬G¥O f*(x) = bnxn + ... + b1x + b0, «h f*(x) ¬O R[x] ªº primitive polynomial ¥B f (x) = c . f*(x). ¬G±oÃÒ¦s¦b©Ê.

±µµÛÃÒ©ú°ß¤@©Ê: ­Y f (x) = c' . g(x), ¨ä¤¤ g(x) ¬O R[x] ªº primitive polynomial. °²³] g(x) = an'xn + ... + a1'x + a0', «h¹ï©Ò¦³ i = 0, 1,..., n, ¬Ò¦³ ai = c' . ai'. ´«¥y¸Ü»¡ c' ¬O an,..., a0 ªº¤@­Ó common divisor. ¦]¦¹¥Ñ c ¬O an,..., a0 ªº greatest common divisor ª¾ c' | c. §Y¦s¦b d $ \in$ R ¨Ï±o c = c' . d. §Q¥Î ai = c . bi = c' . ai', §Ú­Ìª¾¹ï©Ò¦³ªº i = 0, 1,..., n, ¬Ò¦³

c' . (d . bi) = (c' . d ) . bi = c . bi = c' . ai'.

¨Ò¥Î c'$ \ne$ 0 ¥B R ¬O integral domain, ¥i±o¹ï©Ò¦³ªº i = 0, 1,..., n, ¬Ò¦³ ai' = d . bi. ´«¥y¸Ü»¡ d ¬O an',..., a0' ªº¤@­Ó common divisor. µM¦Ó¥Ñ°²³] an',..., a0' ªº greatest common divisor ¬O unit, ¬G±o d ¬O R ªº¤@­Ó unit. ´«¥y¸Ü»¡ c $ \sim$ c'. ¦A§Q¥Î f (x) = c . f*(x) = c' . g(x), ¥H¤Î R[x] ¬O integral domain, ±o d . f*(x) = g(x). ¥Ñ©ó d ¬O R ªº unit ¤]¬O R[x] ªº unit, ¬G±o f*(x) $ \sim$ g(x). $ \qedsymbol$

§Q¥Î Lemma 8.4.5 ªº°ß¤@©Ê, §Ú­Ì¦ÛµM¦³¥H¤Uªº©w¸q.

Definition 8.4.6   °²³] R ¬O¤@­Ó unique factorization domain. ­Y f (x) $ \in$ R[x] ¥i¼g¦¨ f (x) = c . f*(x) ¨ä¤¤ c $ \in$ R ¥B f*(x) ¬O R[x] ªº primitive polynomial, «hºÙ c ¬° f (x) ªº content, ©w¬° c(f ).

­nª`·N¥Ñ Lemma 8.4.5 ªºÃÒ©ú§Ú­Ìª¾¹D f (x) ªº content ¨ä¹ê´N¬O f (x) ©Ò¦³«Y¼Æªº greatest common divisor. ¥t¥ ­nª`·Nªº¬O f (x) ªº content ¨ä¹ê¨Ã¤£¬O¤@­Ó©T©wªº­È, content ¤§¶¡·|®t­Ó associates.

§Ú­Ì¥i¥H±N content ªº©w¸q±À¼s¨ì F[x]. §O§Ñ¤F F ¬O R ªº quotient field, ©Ò¥H F ¤¤¨C­Ó¤¸¯À³£¥i¥H¼g¦¨ a/b ªº§Î¦¡, ¨ä¤¤ a, b $ \in$ R ¥B b$ \ne$ 0. ²¹ï¥ô·Nªº f (x) = rnxn + ... + r1x + r0 $ \in$ F[x], ¥Ñ©ó¹ï¥ô·Nªº i = 0, 1,..., n, ¬Ò¦³ ri = ai/bi, ¨ä¤¤ ai, bi $ \in$ R, §Ú­Ì¥i§ä¨ì d $ \in$ R ¥B d$ \ne$ 0 ¨Ï±o d . f (x) $ \in$ R[x] (¤ñ¤è»¡¥O d = bn ... b0). ¦]¦¹§Q¥Î Lemma 8.4.5 ª¾¦s¦b c $ \in$ R ¥H¤Î f*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial ¨Ï±o d . f (x) = c . f*(x). ¥Ñ©ó d$ \ne$ 0, §Ú­Ì¥i±N f (x) ¼g¦¨

f (x) = $\displaystyle {\frac{c}{d}}$ . f*(x).

´«¥y¸Ü»¡¥ô·N F[x] ¤¤«D 0 ªº polynomial f (x) ¬Ò¥i¼g¦¨ f (x) = r . f*(x), ¨ä¤¤ r $ \in$ F ¥B f*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial. §Ú­Ì¨ÌµMºÙ¦¹ r ¬O f (x) ªº content ¥B¤´°O§@ c(f ).

Corollary 8.4.7   °²³] R ¬O¤@­Ó unique factorization domain, ¥B F ¬O R ªº quotient field. «h¹ï¥ô·N f (x) $ \in$ F[x] ¥B f (x)$ \ne$ 0, ³£¥i§ä¨ì c $ \in$ F ¥B f*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial º¡¨¬

f (x) = c . f*(x).

¤S°²³]

f (x) = c . f*(x)  
  = c' . g(x)  

¨ä¤¤ c, c' $ \in$ F, ¥B f*(x), g(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomials, «h¦s¦b u $ \in$ R ¬O R ªº unit ¨Ï±o c = u . c' ¥B u . f*(x) = g(x).

µý ©ú. «e­±¤wÃÒ¦s¦b©Ê, §Ú­Ì¶ÈÃҰߤ@©Ê. §Ú­Ì±N c ©M c' ¤À§O¼g¦¨ c = a/b ¥B c' = a'/b', ¨ä¤¤ a, a', b, b' $ \in$ R ¥B b$ \ne$ 0, b'$ \ne$ 0. ±N f (x) ­¼¤W b . b', §Ú­Ì¦³ (b . b') . f (x) $ \in$ R[x] ¥B
(b . b') . f (x) = (a . b') . f*(x)  
  = (a' . b) . g(x).  

¬JµM (b . b') . f (x) $ \in$ R[x] §Ú­Ì¥i¥H±N Lemma 8.4.5 ®M¥Î¦b (b . b') . f (x) ¤W, ¬Gª¾¦s¦b u $ \in$ R ¬O R ¤¤ªº unit º¡¨¬ a . b' = u . (a' . b). ¤]´N¬O»¡ c = u . c'. ¦A§Q¥Î c'$ \ne$ 0 ¤Î F[x] ¬O integral domain ±o u . f*(x) = g(x). $ \qedsymbol$

©M $ \mathbb {Z}$[x] ¤@¼Ëªºª¬ªp, §Ú­Ì¦³¥H¤Uªº Gauss Lemma ¨ÓÀ°§U§Ú­Ì­pºâ¨â­Ó polynomials ¬Û­¼«á¤§ content.

Lemma 8.4.8 (Gauss)   °²³] R ¬O¤@­Ó unique factorization domain. ­Y f (x), g(x) $ \in$ R[x] ¬O R[x] ¤¤ªº primitive polynomials, «h f (x) . g(x) ¨ÌµM¬O R[x] ¤¤ªº primitive polynomial.

µý ©ú. §Ú­Ì§Q¥Î©M Lemma 7.3.5 ¬Û¦Pªºµý©ú, ©Ò¥H¥uµ¹¤j²¤ªºµý©ú. °²³] f (x) . g(x) ¤£¬O primitive polynomial, ªí¥Ü f (x) . g(x) ©Ò¦³«Y¼Æªº greatest common divisor ¤£¬O R ¤¤ªº unit. ¦]¦¹§Q¥Î R ¬O unique factorization domain ª¾¦s¦b p $ \in$ R ¬O R ¤¤ªº¤@­Ó irreducible (¤]¬O prime) element ¬O f (x) . g(x) ©Ò¦³«Y¼Æªº common divisor. µM¦Ó f (x) ©M g(x) ¬Ò¬O primitive polynomials, p ¤£¥i¯à¾ã°£©Ò¦³ f (x) ªº«Y¼Æ¤]¤£¥i¯à¾ã°£©Ò¦³ g(x) ªº«Y¼Æ. ©Ò¥H­Y i ¬O³Ì¤pªº¼Æ¨Ï±o f (x) ªº xi ¶µ«Y¼Æ¤£¯à³Q p ¾ã°£, ¦Ó j ¬O³Ì¤pªº¼Æ¨Ï±o g(x) ªº xj ¶µ«Y¼Æ¤£¯à³Q p ¾ã°£, «h«Ü®e©ö¬Ý¥X f (x) . g(x) ªº xi + j ¶µ«Y¼Æ¤£¥i¯à³Q p ¾ã°£. ³o©M p ¬O f (x) . g(x) ¦U¶µ«Y¼Æªº common divisor ¥Ù¬Þ, ¬G±oÃÒ f (x) . g(x) ¬O R[x] ªº primitive polynomial. $ \qedsymbol$

Primitive polynomial ¦b R[x] ¤¤¬O©M F[x] ·¾³qªº¾ô¼Ù, ¨Æ¹ê¤W¦b R[x] ¤¤¤£¬O±`¼Æªº irreducible element ³£¬O primitive polynomial.

Lemma 8.4.9   °²³] R ¬O¤@­Ó unique factorization domain. ­Y f (x) $ \in$ R[x] ¬O R[x] ªº irreducible element ¥B deg(f (x))$ \ge$1, «h f (x) ¬O R[x] ¤¤ªº primitive polynomial.

µý ©ú. ­Y f (x) ¬O R[x] ¤¤ªº irreducible element, ¥Ñ©ó f (x) ¥i¼g¦¨ f (x) = c(f ) . f*(x) ¨ä¤¤ c(f ) $ \in$ R $ \subseteq$ R[x] ¥B f*(x) $ \in$ R[x], ¬Gª¾ c(f ) ¬O f (x) ªº¤@­Ó divisor. ¥Ñ f (x) ¬O irreducible element ªº°²³]ª¾ c(f ) ¬O R ¤¤ªº unit (f (x) ¤£¥i¯à©M c(f ) associates ¦] deg(f (x))$ \ge$1 ¦ý deg(c(f )) = 0), ¬Gª¾ f (x) ¬O primitive polynomial. $ \qedsymbol$

­Y f (x), g(x) $ \in$ R[x], ¥Ñ©ó R $ \subseteq$ F, f (x) ©M g(x) ¥i¦P®É¬Ý¦¨¬O R[x] ªº polynomials ¤]¥i¥H¬Ý¦¨¬O F[x] ªº polynomials. ¦]¦¹³o¨â­Ó polynomials ¶¡Ãö«Y¬Ý¦¨¬O R[x] ©Î F[x] ¤¤ªº±¡ªp´N·|¤£¦P. ¨Ò¦p­Y g(x) = f (x) . h(x), ¨ä¤¤ h(x) $ \in$ R[x] §Ú­Ì´N»¡ f (x) | g(x) in R[x]. µM¦Ó­Y h(x) $ \in$ F[x], §Ú­Ì´N»¡ f (x) | g(x) in F[x]. ¥Ñ©ó R[x] $ \subseteq$ F[x], «Ü¦ÛµMªº§Ú­Ìª¾¹D­Y f (x) | g(x) in R[x] «h f (x) | g(x) in F[x]. µM¦Ó¤@¯ë¨Ó»¡ f (x) | g(x) in F[x] ¤£¨£±o·|¦³ f (x) | g(x) in R[x]. ¤£¹L·í f (x) ¬O R[x] ªº primitive polynomial ®É, ´N¹ï¤F.

Lemma 8.4.10   °²³] R ¬O¤@­Ó unique factorization domain ¥B F ¬O R ªº quotient field. °²³] f (x), g(x) $ \in$ R[x] ¥B f (x) ¬O R[x] ªº¤@­Ó primitive polynomial, «h f (x) | g(x) in F[x] ­Y¥B°ß­Y f (x) | g(x) in R[x].

µý ©ú. §Ú­Ì¥u­nÃÒ©ú: ­Y f (x) | g(x) in F[x] «h f (x) | g(x) in R[x]. ¥Ñ°²³]ª¾¦s¦b h(x) $ \in$ F[x] ¨Ï±o g(x) = f (x) . h(x). §Q¥Î content, §Ú­Ì±o

c(g) . g*(x) = (c(f ) . c(h)) . (f*(x) . h*(x)).

¨ä¤¤ c(g), c(f ) $ \in$ R ¬O g(x), f (x) ªº content, ¦Ó c(h) $ \in$ F ¬O h(x) ªº content, ¥B g*(x), f*(x) ¥H¤Î h*(x) ³£¬O R[x] ªº primitive polynomials. §Q¥Î Lemma 8.4.8 ª¾ f*(x) . h*(x) ¬O R[x] ªº primitive polynomial. ¦A§Q¥Î Corollary 8.4.7 ª¾¦s¦b u $ \in$ R ¬O R ªº unit º¡¨¬ u . c(g) = c(f ) . c(h). µM¦Ó¥Ñ f (x) ¬O R[x] ªº primitive polynomial, ª¾ c(f ) ¬O R ªº unit. ¤S¥Ñ°²³] g(x) $ \in$ R[x] ª¾ c(g) $ \in$ R. ¬G±o

c(h) = c(f )-1 . u . c(g) $\displaystyle \in$ R.

µM¦Ó h(x) = c(h) . h*(x), ¬G¥Ñ c(h) $ \in$ R ¥H¤Î h*(x) $ \in$ R[x] ¥i±o h(x) $ \in$ R[x]. ´«¥y¸Ü»¡ f (x) | g(x) in R[x]. $ \qedsymbol$

§Q¥Î Lemma 8.4.10 §Ú­Ì¥i¥H±o¨ì R[x] ©M F[x] ¤¤ prime element ªºÃö«Y.

Corollary 8.4.11   °²³] R ¬O¤@­Ó unique factorization domain ¥B F ¬O R ªº quotient field ¥B°²³] p(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial. ­Y p(x) ¬O F[x] ¤¤ªº prime element «h p(x) ¬O R[x] ¤¤ªº prime element.

µý ©ú. °²³] p(x) ¬O F[x] ¤¤ªº prime element. ­nÃÒ©ú p(x) ¬O R[x] ¤¤ªº prime element, §Ú­Ì¥²¶·ÃÒ©ú­Y p(x) | f (x) . g(x) in R[x], ¨ä¤¤ f (x), g(x) $ \in$ R[x], «h p(x) | f (x) in R[x] ©Î p(x) | g(x) in R[x]. ¦] p(x) ¬O R[x] ¤¤ªº primitive polynomial, ¥Ñ Lemma 8.4.10 §Ú­Ì¦³ p(x) | f (x) . g(x) in F[x]. ¬G§Q¥Î p(x) ¬O F[x] ªº prime element, §Ú­Ìª¾ p(x) | f (x) in F[x] ©Î p(x) | g(x) in F[x]. ¦A¤@¦¸§Q¥Î Lemma 8.4.10, §Ú­Ìª¾ p(x) | f (x) in R[x] ©Î p(x) | g(x) in R[x], ¬G±oÃÒ p(x) ¬O R[x] ªº prime element. $ \qedsymbol$

¥t¥ ¦b R[x] ©M F[x] ¤¤­n°Ï¤À²M·¡ªº¬O¤@­Ó R[x] ¤¤ªº polynomial ¦b R[x] ©M F[x] ¤¤¥i§_¤À¸Ñ (§Y¬O§_ irreducible) ªºÃöÁp©Ê.

Lemma 8.4.12   °²³] R ¬O¤@­Ó unique factorization domain ¥B F ¬O R ªº quotient field ¥B°²³] f (x) $ \in$ R[x] ¤Î deg(f (x))$ \ge$1. ­Y¦s¦b g(x), h(x) $ \in$ F[x] º¡¨¬ deg(g(x))$ \ge$1 ¥B deg(h(x))$ \ge$1, ¨Ï±o f (x) = g(x) . h(x), «h¦s¦b m(x), n(x) $ \in$ R[x] º¡¨¬ deg(g(x)) = deg(m(x)) ¥B deg(h(x)) = deg(n(x)) ¨Ï±o f (x) = m(x) . n(x).

µý ©ú. §Q¥Î content §Ú­Ì±N f (x) = g(x) . h(x) ¼g¦¨:

c(f ) . f*(x) = (c(g) . c(h)) . (g*(x) . h*(x)),

¨ä¤¤ c(f ) $ \in$ R, c(g), c(h) $ \in$ F, ¦Ó f*(x), g*(x) ©M h*(x) ³£¬O R[x] ªº primitive polynomial. §Q¥Î Lemma 8.4.8 ª¾ g*(x) . h*(x) ¬O R[x] ªº primitive polynomial, ¬G¥Ñ Lemma 8.4.5 ª¾¦s¦b u $ \in$ R ¬O R ªº unit ¨Ï±o c(g) . c(h) = c(f ) . u. ´«¨¥¤§, c(g) . c(h) $ \in$ R. ¬G­Y¥O m(x) = (c(g) . c(h)) . g*(x) $ \in$ R[x], n(x) = h*(x), «h m(x), n(x) ²Å¦X©w²z©Ò­n¨D. $ \qedsymbol$

¥Ñ Lemma 8.4.12 §Ú­Ì¥i±o R[x] ©M F[x] ¶¡ irreducible element ªºÃö«Y.

Corollary 8.4.13   °²³] R ¬O¤@­Ó unique factorization domain ¥B F ¬O R ªº quotient field. ­Y p(x) $ \in$ R[x] º¡¨¬ deg(p(x))$ \ge$1 ¬O R[x] ªº primitive polynomial, «h p(x) ¬O R[x] ªº irreducible element ­Y¥B°ß­Y p(x) ¬O F[x] ªº irreducible element.

µý ©ú. ­º¥ý°²³] p(x) ¬O R[x] ªº irreducible element, ­nÃÒ©ú p(x) ¤]¬O F[x] ªº irreducible element. °²¦p p(x) ¦b F[x] ¤£¬O irreducible element, «h¦s¦b g(x), h(x) $ \in$ F[x] º¡¨¬ deg(g(x))$ \ge$1 ¥B deg(h(x))$ \ge$1 ¨Ï±o p(x) = g(x) . h(x). ¬G¥Ñ Lemma 8.4.12 ª¾¦s¦b m(x), n(x) $ \in$ R[x] º¡¨¬ deg(m(x))$ \ge$1 ¥B deg(n(x))$ \ge$1 ¨Ï±o p(x) = m(x) . n(x). ´«¥y¸Ü»¡¥Ñ 1$ \le$deg(m(x)) < deg(p(x)) ª¾, m(x) ¬O p(x) ¦b R[x] ªº¤@­Ó divisor ¥B¬J¤£¬O unit ¤]¤£©M p(x) associates. ¬Gª¾ p(x) ¤£¬O R[x] ªº irreducible element. ¦¹©M°²³]¥Ù¬Þ, ¬Gª¾ p(x) ¬O F[x] ªº irreducible element.

¤Ï¤§, °²³] p(x) ¬O F(x) ªº irreducible element. ¦pªG p(x) ¦b R[x] ¤¤¤£¬O irreducible, §Y¦s¦b l (x), m(x) $ \in$ R[x] º¡¨¬ p(x) = l (x) . m(x), ¨ä¤¤ l (x) ©M m(x) ³£¤£¬O R[x] ¤¤ªº unit. ¦ý l (x), m(x) $ \in$ R[x] $ \subseteq$ F[x], ¬G§Q¥Î p(x) ¬O F[x] ¤¤ªº irreducible element ª¾ l (x) ©M m(x) ¤¤¥²¦³¤@­Ó¬O F[x] ¤¤ªº unit (§Y±`¼Æ¦h¶µ¦¡). ´N°²³]¬O l (x) = a $ \in$ R §a! ¥Ñ°²³] a ¤£¯à¬O R ªº unit, §_«h l (x) = a ¬O R[x] ªº unit (Lemma 8.4.3). µM¦Ó¥Ñ f (x) = l (x) . m(x) = a . m(x) ¥B m(x) $ \in$ R[x] ª¾ a ¬O f (x) ¦U¶µ«Y¼Æ¤§ common divisor, §Y a | c(f ) in R. ¦ý¥Ñ°²³] f (x) ¬O primitive polynomial ª¾ c(f ) ¬O R ¤¤ªº unit, ¬G¥Ñ a | c(f ) in R ª¾ a ¬O R ªº unit; ¦¹©M a ¤£¬O R ªº unit ¬Û¥Ù¬Þ. ¬Gª¾ f (x) ¦b R[x] ¤¤¬O irreducible. $ \qedsymbol$

±µµÛ§Ú­Ì¨Ó¬ÝÃÒ©ú R[x] ¬O unique factorization domain ³ÌÃöÁ䪺©Ê½è.

Proposition 8.4.14   °²³] R ¬O¤@­Ó unique factorization domain, «h R[x] ¤¤ªº irreducible element ©M prime element ¬O¬Û¦Pªº.

µý ©ú. ¥Ñ©ó R[x] ¬O integral domain, §Ú­Ìª¾ R[x] ªº prime element ´N¬O irreducible element (Lemma 8.1.8). ¦]¦¹¥u­nÃÒ©ú­Y f (x) $ \in$ R[x] ¬O¤@­Ó irreducible element, «h f (x) ¬O¤@­Ó prime element. §Ú­Ì·QÂÇ¥Ñ F[x] (³o¸Ì F ¬O R ªº quotient field) ¤¤ªº irreducible element ¬O prime element (Proposition 7.2.11) ¨ÓÃÒ©ú.

­º¥ý¦Ò¼ deg(f (x)) = 0 (§Y f (x) = a $ \in$ R ¬O±`¼Æ) ªº±¡§Î. ¦] a $ \in$ R ¬O irreducible ¥B R ¬O unique factorization domain, ¥Ñ Proposition 8.4.2 ª¾ a ¬O R ªº prime element. §Ú­Ì­nÃÒ©ú a ¤]¬O R[x] ¤¤ªº prime element. °²³] g(x), h(x) $ \in$ R[x] º¡¨¬ a | g(x) . h(x) in R[x], §Y¦s¦b l (x) $ \in$ R[x] ¨Ï±o a . l (x) = g(x) . h(x). §Q¥Î content ±o

(a . c(l )) . l*(x) = (c(g) . c(h)) . (g*(x) . h*(x)),

¨ä¤¤ c(l ), c(g), c(h) $ \in$ R ¥B l*(x), g*(x), h*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomials. ¥Ñ Lemma 8.4.8 ª¾ g*(x) . h*(x) ¨ÌµM¬O primitive polynomial, ¬G¥Ñ Lemma 8.4.5 ª¾¦s¦b u $ \in$ R ¬O R ªº unit º¡¨¬

u . a . c(l )= c(g) . c(h),

´«¥y¸Ü»¡ a | c(g) . c(h) in R. §Q¥Î a ¬O R ªº prime element ¤§°²³]±o a | c(g) ©Î a | c(h). µM¦Ó g(x) = c(g) . g*(x), ¬G­Y a | c(g) «h a | g(x). ¦P²z­Y a | c(h), «h a | h(x). ¬Gª¾ a = f (x) ¬O R[x] ¤¤ªº prime element.

²¦Ò¼ deg(f (x))$ \ge$1 ªº±¡§Î. ¥O F ¬O R ªº quotient field. ¦]¬° f (x) ¬O R[x] ªº irreducible element ¥Ñ Corollary 8.4.13 ª¾ f (x) ¬O F[x] ªº irreducible element. µM¦Ó Proposition 7.2.11 §i¶D§Ú­Ì¦¹®É f (x) ¤]¬O F[x] ¤¤ªº prime element. ¥Ñ©ó Lemma 8.4.9 §i¶D§Ú­Ì f (x) ¬O R[x] ªº primitive polynomial, ¬G¥i®M¥Î Corollary 8.4.11 ±oÃÒ f (x) ¤]¬O R[x] ¤¤ªº prime element. $ \qedsymbol$

²¦b§Ú­Ì¦³¨¬°÷ªº©Ê½è¨ÓÀ°§U§Ú­ÌÃÒ©ú R[x] ¤]¬O¤@­Ó unique factorization domain. ¤j®a¥i¥Hªu¥ÎÃÒ©ú $ \mathbb {Z}$[x] ¬O unique factorization domain (Theorem 7.3.13) ªº¤èªk¨Ó³B²z. ³o¸Ì§Ú­Ì·QÂÇ¥Ñ F[x] ¬O unique factorization domain (Theorem 7.2.14) ³o­Ó¨Æ¹ê¨Ó±À¾É. ³o­ÓÃÒ©ú¤£¨£ªº¤ñ¸û²©ú, ¤£¹L¥i¥HÀ°§U§Ú­Ì¦h¤F¸Ñ R[x] ©M F[x] ¶¡ªºÃöÁp.

Theorem 8.4.15   °²³] R ¬O¤@­Ó unique factorization domain, «h R[x] ¤]¬O¤@­Ó unique factorization domain.

µý ©ú. ¥O F ¬O R ªº quotient field.

­º¥ýÃÒ©ú¦s¦b©Ê: §Y¥ô¤@ R[x] ¤¤«D 0 ¥B¤£¬O unit ªº¤¸¯À f (x) ¥i¼g¦¨¦³­­¦h­Ó R[x] ªº irreducible elements ªº­¼¿n. ­º¥ý±N f (x) ¼g¦¨ f (x) = c(f ) . f*(x), ¨ä¤¤ c(f ) $ \in$ R ¥B f*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial. ­Y c(f ) ¤£¬O unit, «h§Q¥Î R ¬O unique factorization domain §Ú­Ì¥i¥H±N c(f ) ¼g¦¨¦³­­¦h­Ó R ¤¤ªº irreducible elements ªº­¼¿n. §Q¥Î Lemma 8.4.3 (3) ª¾¹D c(f ) ¥i¥H¼g¦¨¦³­­¦h­Ó R[x] ¤¤ªº irreducible elements ªº­¼¿n. ©Ò¥H§Ú­Ì¥u­nÃÒ©ú f*(x) ¥i¥H¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n. ²±N f*(x) ¬Ý¦¨¬O F[x] ¤¤ªº¤¸¯À, «h§Q¥Î F[x] ¬O unique factorization domain, ª¾¹D f*(x) = p1(x) ... pm(x), ¨ä¤¤ p1(x),..., pm(x) $ \in$ F[x] ¬O F[x] ¤¤ªº irreducible elements. ¦A§Q¥Î content, ª¾¨C­Ó pi(x) ³£¥i¼g¦¨ pi(x) = c(pi) . pi*(x), ¨ä¤¤ pi*(x) $ \in$ R[x] ¬O R[x] ªº primitive polynomial. ´«¥y¸Ü»¡

f*(x) = (c(p1) ... c(pm)) . p1*(x) ... pm*(x).

§Q¥Î Lemma 8.4.8 ª¾ p1*(x) ... pm*(x) ¬O R[x] ªº primitive polynomial, ¬G¥Ñ f*(x) ¬O R[x] ªº primitive polynomial ¥H¤Î Lemma 8.4.5 ª¾ c(p1) ... c(pm) = u ¬O R ¤¤ªº unit, ¥Ñ Lemma 8.4.3 ª¾ u ¤]¬O R[x] ªº unit. ¦]¦¹§Ú­Ì¥u­nÃÒ©ú p1*(x),...pm*(x) ¬O R[x] ¤¤ªº irreducible elements ´N¥i. ¦p¦¹¤@¨Ó

f*(x) = (u . p1*(x)) . p2*(x) ... pm*(x),

©Ò¥H f*(x) ¥i¥H¼g¦¨¦³­­¦h­Ó irreducible elements ªº­¼¿n (ª`·N u . p1*(x) ©M p1*(x) associates, ©Ò¥H¤]¬O R[x] ¤¤ªº irreducible element). µM¦Ó¦] pi(x) = c(pi) . pi*(x), ¥Ñ pi(x) ¦b F[x] ¤¤ irreducible ª¾ pi*(x) ¤]¬O F[x] ªº irreducible element. ¥Ñ©ó pi*(x) ¬O R[x] ªº primitive polynomial, ®M¥Î Corollary 8.4.13 ª¾ pi*(x) ¤]¬O R[x] ªº irreducible element.

±µµÛÃÒ©ú¤À¸Ñªº°ß¤@©Ê: ¨ä¹ê§Ú­Ì¥i¥H§Q¥Î Proposition 8.4.14 ª½±µÃÒ©ú°ß¤@©Ê, ¤£¹L³o¸Ì§Ú­Ì¨ÌµM§Q¥Î F[x] ©M R ¬O unique factorization domain ¨ÓÃÒ©ú. ­º¥ý°²³]

f (x) = (a1n1 ... arnr) . p1nr + 1(x) ... pvnr + v(x)  
  = (b1m1 ... bsms) . q1ms + 1(x) ... qwms + w(x),  

¨ä¤¤ a1,...ar $ \in$ R (§Y deg(ai) = 0) ¬O R[x] ¤¤¨â¨â¤£ associates ªº irreducible elements ¦Ó p1(x),..., pv(x) $ \in$ R[x] ¬O R[x] ¤¤¨â¨â¤£ associates ¥B degree ¤j©ó 0 ªº irreducible elements, ¹ï©ó b1,..., bs $ \in$ R ¥H¤Î q1(x),..., qw(x) $ \in$ R[x] ¤]¬O¦P¼Ëªº°²³]. ­º¥ýª`·N¥Ñ©ó³o¨Ç pi(x) ©M qj(x) ³£¬O R[x] ¤¤ªº irreducible elements ¥B deg(pi(x))$ \ge$1 ¥H¤Î deg(qj(x))$ \ge$1, ¥Ñ Lemma 8.4.9 ª¾³o¨Ç pi(x) ©M qj(x) ³£¬O primitive polynomial, ¬G¥Ñ Lemma 8.4.8 ¥H¤Î Lemma 8.4.5 ª¾¦s¦b R ¤¤ªº unit u º¡¨¬

a1n1 ... arnr = u . b1m1 ... bsms,

¬G§Q¥Î R ¬O unique factorization domain ªº©Ê½èª¾¸g¹L¾A·í¶¶§Ç±¼´«§Ú­Ì¦³ r = s, ai $ \sim$ bi ¥B ni = mi, $ \forall$ i = 1,..., r. ©Ò¥H³Ì«á§Ú­Ì¥u­n¦Ò¼
f0(x) = u . p1nr + 1(x) ... pvnr + v(x)  
  = q1ms + 1(x) ... qwms + w(x)  

³o¤@³¡¤Àªº°ß¤@©Ê. ¥Ñ©ó f0(x) $ \in$ R[x] $ \subseteq$ F[x], ¥B pi(x), qi(x) ¬O R[x] ¤¤ªº irreducible elements ©Ò¥H¤]¬O F[x] ¤¤ªº irreducible elements (Corollary 8.4.13), ¬G§Q¥Î F[x] ¬O unique factorization domain ª¾¸g¹L­«±Æ«á v = w, pi(x) = ki . qi(x) ¥B ni = mi, $ \forall$ i = r + 1,...r + v, ¨ä¤¤ ki $ \in$ F. µM¦Ó pi(x) ©M qi(x) ³£¬O R[x] ªº primitive polynomial, ¬Gª¾ ki ¬O R ªº unit. ´«¨¥¤§, ¹ï©Ò¦³ªº i = r + 1,..., r + v, ¬Ò¦³ pi(x) $ \sim$ qi(x). ¬G±oÃҰߤ@©Ê. $ \qedsymbol$

³Ì«á§Ú­Ì¨Ó¬Ý Theorem 8.4.15 ¤@­Ó­«­nªºÀ³¥Î. ­Y R ¬O¤@­Ó unique factorization domain, ¥Ñ Theorem 8.4.15 ª¾ R' = R[x] ¤]¬O¤@­Ó unique factorization domain. ²¦Ò¼ R'[y] ³o¤@­Ó¥H y ¬°ÅÜ¼Æ R' ªº¤¸¯À¬°«Y¼Æªº polynomial ring, ¤]´N¬O R'[y] ªº¤¸¯À³£¬O

fn(x)yn + fn - 1(x)yn - 1 + ... + f1(x)y + f0(x),

¨ä¤¤¹ï©Ò¦³ªº i = 0, 1,..., n, fi(x) $ \in$ R' = R[x] ¬O«Y¼Æ¦b R ªº x ªº¦h¶µ¦¡. «Ü®e©ö¬Ý¥X R'[y] = R[x][y] = R[x, y] ´N¬O¥H R ªº¤¸¯À¬°«Y¼Æ x, y ¬°Åܼƪº¨â­ÓÅܼƪº¦h¶µ¦¡©Ò¦¨ªº¶°¦X, ¬G¦A¦¸¥Ñ Theorem 8.4.15 ª¾ R[x, y] ¬O unique factorization domain. §Ú­Ì¥i¥H±N¥H¤Wªº½×­z±À¼s¨ì R[x1,..., xn] ³o­Ó¥H R ªº¤¸¯À¬°«Y¼Æ x1,..., xn ¬°Åܼƪº n ­ÓÅܼƪº polynomial ring:

Theorem 8.4.16   °²³] R ¬O¤@­Ó unique factorization domain, «h R[x1,..., xn] ³o­Ó n ­ÓÅܼƪº polynomial ring ¤]¬O¤@­Ó unique factorization domain.

µý ©ú. §Q¥Î¼Æ¾ÇÂk¯Çªk, ·í n = 1 ®É Theorem 8.4.15 §i¶D§Ú­Ì R[x1] ¬O¤@­Ó integral domain. °²³] n - 1 ®É, R' = R[x1,..., xn - 1] ¬O unique factorization domain. ¦A¥Ñ Theorem 8.4.15 ª¾ R'[xn] = R[x1,..., xn] ¤]¬O unique factorization domain. $ \qedsymbol$

Theorem 8.4.16 ¬O¤@­Ó¥N¼Æ¤W«Ü­«­nªº©w²z, ³Ì±`¨£ªºª¬ªp¬O·í F ¬O¤@­Ó field ®É¦] F[x1] ¬O¤@­Ó unique factorization domain, ¬Gª¾ F[x1,..., xn] ¤]¬O¤@­Ó unique factorization domain.


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¤U¤@­¶: FIELD ¤W¤@­¶: Unique Factorization Domain «e¤@­¶: Unique factorization domain ªº°ò¥»©Ê½è
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