Polynomials

Definition (Polynomials)

Let F\mathbb{F} be a field and consider the ring F[x]\mathbb{F}[x] of polynomials over F\mathbb{F}. Explicitly this is the set F[x]={a0+a1x++anxn:aiF,n0}\mathbb{F}[x]=\{ a_{0}+a_{1}x+\dots+a_{n}x^{n}:a_{i}\in\mathbb{F},n\ge0 \}

Definition (Monic)

We say a polynomial f(x)=anxn++a1x+a0f(x)=a_{n}x^{n}+\dots+a_{1}x+a_{0} of degree nn is monic if an=1a_{n}=1

Theorem (Degree is closed under addition)

Let f,gF[x]f,g\in\mathbb{F}[x] be . Then degfg=degf+degg\deg fg=\deg f+\deg g

Theorem (Division of Polynomials)

Let f,gF[x]f,g\in\mathbb{F}[x] be . We say that gg divides ff i.e. gfg|f if hF[x] s.t. f=gh\exists h\in\mathbb{F}[x]\text{ s.t. }f=gh

Definition (Nonzero constant polynomials)

The Nonzero Constant are a class of functions fF[x]f\in\mathbb{F}[x] s.t. degf=0 and fg  gF[x]\deg f=0\text{ and }f|g \ \ \forall g\in\mathbb{F}[x] These are the units of F[x]\mathbb{F}[x].

Definition (Associate)

Let f,gF[x]f,g\in\mathbb{F}[x] be polynomials. We say ff and gg are associates if fg\frac{f}{g} is a unit, i.e. degfg=0\deg \frac{f}{g}=0 fg\frac{f}{g} is a nonzero constant polynomial.

Definition (Irreducible)

A polynomial fF[x]f\in\mathbb{F}[x] is said to be irreducible if ff is NOT divisible by any gF[x]g\in\mathbb{F}[x] where 1degg<degf1\le\deg g<\deg f

Theorem (Irreducible factorization)

Every non-constant polynomial fF[x]f\in\mathbb{F}[x] is the product of irreducible polynomials.

Theorem (Division Algorithm for Polynomials)

Let F\mathbb{F} be a field and suppose we have a,bF[x]a,b\in\mathbb{F}[x] with b0b\not=0. Then, q,rF[x]\exists q,r\in\mathbb{F}[x] s.t. a=bq+ra=bq+rwith either r=0r=0 or r0r\not=0 and degr<degb\deg r<\deg b.

Remark

Analogue to Euclidean Algorithm.

Theorem

Given two non-zero elements a,bF[x]a,b\in\mathbb{F}[x], the gcd, dd of aa and bb exists and is unique up to units. Moreover, u,vF[x]\exists u,v\in\mathbb{F}[x] s.t. au+bv=dau+bv=d

Remark

Analogue to Theorem 1.1.

Proposition (If an Irreducible Polynomial divides ab then it divides a or b)

If lF[x]\mathscr{l}\in\mathbb{F}[x] is an irreducible polynomial and lab\mathscr{l}|ab then either la\mathscr{l}|a or lb\mathscr{l}|b.

Remark

Analogue to If a Prime divides ab it divides one of a or b.

Definition (Relatively prime)

Given two polynomials aa and bb, they are said to be relatively prime if their gcd is a unit.

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