2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)

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2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)


잠깐 λ§₯락을 κ³λ“€μ΄μžλ©΄, β€œGauss’s Lemmaβ€λŠ” μ•„λž˜μ˜ λͺ…μ œλ₯Ό 증λͺ…ν•˜λŠ” κ³Όμ •μ—μ„œ λ§Œλ‚˜λŠ” 쀑간단계이닀.

"If $D$ is a UFD, then $D[x]$ is a UFD."


μ•„λž˜μ— 이와 λΉ„μŠ·ν•œ λŠλ‚Œμ˜ 정리듀을 μ’€ λͺ¨μ•„λ΄€λ‹€.

  • $R$ is a ring $\implies$ $R[x]$ is also a ring.
  • $D$ is an integral domain $\implies$ $D[x]$ is also an integral domain.
  • $F$ is a field $\iff$ $F[x]$ is a PID. (Thm 27.24)
    • μ΄λ•Œ λͺ¨λ“  FieldλŠ” PIDμ΄λ―€λ‘œ μœ„ λͺ…μ œμ—μ„œ β€˜field’λ₯Ό β€˜PIDβ€™λ‘œ 바꿔도 μ„±λ¦½ν•œλ‹€.
  • $D$ is an integral domain with an irreducible elt $\implies$ $D[x]$ is not a PID.
  • $F$ is a field $\implies$ $F[x_1, \dots, x_n]$ is not a PID for $n \ge 2$.



μš°λ¦¬λŠ” μ§€κΈˆκΉŒμ§€ μžμ—°μˆ˜μ—μ„œμ˜ GCDλ₯Ό μ ‘ν–ˆλ‹€. ν•˜μ§€λ§Œ μ•žμ—μ„œ μ‚΄νŽ΄λ³Έ β€˜μ‚°μˆ μ˜ κΈ°λ³Έμ •λ¦¬β€˜μ— λ”°λ₯΄λ©΄ μžμ—°μˆ˜ 집합도 결ꡭ은 UFD의 ν•œ μ˜ˆμ— λΆˆκ³Όν–ˆλ‹€. μžμ—°μˆ˜ μ§‘ν•©μ—μ„œ μ •μ˜ν•œ GCDλ₯Ό UFD둜 ν™•μž₯ν•˜μ—¬ λ‹€μ‹œ μ •μ˜ν•΄λ³΄μž.

Definition. GCD in UFD

Let $D$ be a UFD, and $a_1$, $a_2$, …, $a_n$ be non-zero elts of $D$.

$d$ is a GCD of all of $a_i$,

if $d \mid a_i$ for $i=1, …, n$ and all $d’ \in D$ that divides all the $a_i$ also divides $d$.

μžμ—°μˆ˜ μ§‘ν•©μ—μ„œλŠ” GCDκ°€ μœ μΌν•˜κ²Œ μ‘΄μž¬ν–ˆμ§€λ§Œ, μžμ—°μˆ˜ μ§‘ν•©λ₯Ό ν¬κ΄„ν•˜λŠ” κ°œλ…μΈ UFDμ—μ„œ GCDλŠ” 더이상 μœ μΌν•˜μ§€ μ•Šλ‹€.

UFDμ—μ„œ 두 개의 GCD $d$, $d’$κ°€ μ‘΄μž¬ν•œλ‹€κ³  κ°€μ •ν•΄λ³΄μž. 그러면, GCD의 μ •μ˜μ— μ˜ν•΄ $d \mid d’$, $d’ \mid d$κ°€ μ„±λ¦½ν•œλ‹€.

즉, μ„œλ‘œ λ‹€λ₯Έ 두 GCD $d$, $d’$κ°€ associate ν•˜λ‹€.


Simple Example.

μ„œλ‘œ λ‹€λ₯Έ 두 GCD에 λŒ€ν•œ μ˜ˆλŠ” 생각보닀 κ°„λ‹¨νžˆ 찾을 수 μžˆλ‹€. 두 μ •μˆ˜ $6$와 $-8$에 λŒ€ν•œ GCDλŠ” $2$와 $-2$이닀.

즉, μ •μˆ˜ $\mathbb{Z}$λŠ” UFDμ΄λ©΄μ„œ μ„œλ‘œ λ‹€λ₯Έ 두 GCDλ₯Ό 찾을 수 μžˆλ‹€. λ°˜λ©΄μ— μžμ—°μˆ˜ $\mathbb{N}$λŠ” UFDμ΄μ§€λ§Œ, GCDκ°€ μœ μΌν•˜κ²Œ κ²°μ •λœλ‹€.



Primitive polynomial

Definition. content & primitive part

The content of (polynomial with integer coeffi-.) = $\gcd$ of it coeffi-.

The primitive part of polynomial = quotient of polynomial by its content.

λ”°λΌμ„œ (polynomial) = (content) x (primitive part)


Definition. Primitive Polynomial

A polynomial is primitive, if its content equals 1.



Gauss’s Lemma

Lemma 45.23

If $D$ is a UFD,

then for every non-constant $f(x) \in D[x]$, we have $f(x) = (c)g(x)$, where $c \in D$ and primitive $g(x) \in D[x]$.

(즉, UFDμ—μ„œ λͺ¨λ“  non-constant $f(x)$λŠ” primitive의 μƒμˆ˜κ³±μ΄λΌλŠ” 말이닀.)

proof.

Let $f(x) \in D[x]$ be a non-constant polynomial; $f(x) = a_0 + a_1 x + \cdots a_n x^n$

Let $c$ be a gcd of all $a_i$.

Then for each $i$, we have $a_i = c \cdot q_i$ for some $q_i \in D$.

By the distributive law, we have $f(x) = (c) g(x)$.

By definition of gcd $c$, the left polynomial $g(x)$ is a primitive polynomial. $\blacksquare$



Lemma 45.25 Gauss’s Lemma

If $D$ is a UFD, then a product of two primitive polynomials in $D[x]$ is again primitive.

proof.

Let $f(x) = a_0 + a_1 x + \cdots a_n x^n$ and $g(x) = b_0 + b_1 x + \cdots b_m x^m$ be primitive in $D[x]$,

and let $h(x) = f(x)g(x)$.

Let $p$ be an irreducible in $D$.

이미 $f(x)$, $g(x)$κ°€ primitiveμ΄λ―€λ‘œ $p$κ°€ $a_i$ μ „λΆ€λ₯Ό, 또 $b_j$ μ „λΆ€λ₯Ό λ‚˜λˆ„μ§€λŠ” λͺ» ν•œλ‹€.

Let $a_r$ be the first coefficient of $f(x)$ not divisible by $p$;

that is $p \mid a_i$ for $i < r$, but $p \not\mid a_r$.

Similarly, let $b_s$ be the first coefficient of $g(x)$ not divisible by $p$.

The coefficient of $x^{r+s}$ in $h(x) = f(x)g(x)$ is

\[c_{r+s} = (a_0 b_{r+s} + \cdots + a_{r-1} b_{s+1}) + a_r b_s + (a_{r+1} b_{s-1} \cdots a_{r+s} b_0)\]

$p \mid a_i$ for $i < r$μ΄λ―€λ‘œ $p \mid (a_0 b_{r+s} + \cdots + a_{r-1} b_{s+1})$

λ§ˆμ°¬κ°€μ§€λ‘œ $p \mid b_j$ for $j < s$μ΄λ―€λ‘œ $p \mid (a_{r+1} b_{s-1} \cdots a_{r+s} b_0)$

ν•˜μ§€λ§Œ, $p$κ°€ $a_r$, $b_s$λ₯Ό λ‚˜λˆ„μ§„ λͺ» ν•˜λ―€λ‘œ $p \not\mid a_r b_s$이닀.

μ’…ν•©ν•˜λ©΄, μ–΄λ–€ irreducible $p \in D$일지라도 $f(x)g(x)$의 κ³„μˆ˜λ₯Ό λ‚˜λˆ„μ§€ λͺ» ν•˜λŠ” 지점이 있기 λ•Œλ¬Έμ— $f(x)g(x)$의 κ³„μˆ˜λŠ” μ–΄λ–€ irreducible $p$라도 common divisor둜 κ°€μ§ˆ 수 μ—†λ‹€.

λ”°λΌμ„œ $f(x)g(X)$λŠ” primitiveλ‹€. $\blacksquare$



이제 이 Gauss’s Lemmaλ₯Ό ν™œμš©ν•΄ 본래의 λͺ©μ μΈ

"If $D$ is a UFD, then $D[x]$ is a UFD."

λ₯Ό 증λͺ…ν•΄λ³΄μž!

λ‹€μŒ 포슀트: Poylnomial over UFD