07-401/Class Notes for March 7

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In Preparation

The information below is preliminary and cannot be trusted! (v)

Class Plan

Some discussion of the term test and HW6.

Extension Fields

Definition. An extension field [math]\displaystyle{ E }[/math] of [math]\displaystyle{ F }[/math].

Theorem. For every non-constant polynomial [math]\displaystyle{ f }[/math] in [math]\displaystyle{ F[x] }[/math] there is an extension [math]\displaystyle{ E }[/math] of [math]\displaystyle{ F }[/math] in which [math]\displaystyle{ f }[/math] has a zero.

Example [math]\displaystyle{ x^2+1 }[/math] over [math]\displaystyle{ {\mathbb R} }[/math].

Example [math]\displaystyle{ x^5+2x^2+2x+2=(x^2+1)(x^3+2x+2) }[/math] over [math]\displaystyle{ {\mathbb Z}/3 }[/math].

Definition. [math]\displaystyle{ F(a_1,\ldots,a_n) }[/math].

Theorem. If [math]\displaystyle{ a }[/math] is a root of an irreducible polynomial [math]\displaystyle{ p\in F[x] }[/math], within some extension field [math]\displaystyle{ E }[/math] of [math]\displaystyle{ F }[/math], then [math]\displaystyle{ F(a)\cong F[a]/\langle p\rangle }[/math], and [math]\displaystyle{ \{1,a,a^2,\ldots,a^{n-1}\} }[/math] (here [math]\displaystyle{ n=\deg p }[/math]) is a basis for [math]\displaystyle{ F(a) }[/math] over [math]\displaystyle{ F }[/math].

Corollary. In this case, [math]\displaystyle{ F(a) }[/math] depends only on [math]\displaystyle{ p }[/math].

Corollary. If [math]\displaystyle{ p\in F[x] }[/math] irreducible over [math]\displaystyle{ F }[/math], [math]\displaystyle{ \phi:F\to F' }[/math] an isomorphism, [math]\displaystyle{ a }[/math] a root of [math]\displaystyle{ p }[/math] (in some [math]\displaystyle{ E/F }[/math]), [math]\displaystyle{ a' }[/math] a root of [math]\displaystyle{ \phi(p) }[/math] in some [math]\displaystyle{ E'/F' }[/math], then [math]\displaystyle{ F[a]\cong F'[a'] }[/math].

Splitting Fields

Definition. [math]\displaystyle{ f\in F[x] }[/math] splits in [math]\displaystyle{ E/F }[/math], a splitting field for [math]\displaystyle{ f }[/math] over [math]\displaystyle{ F }[/math].

Theorem. A splitting field always exists.

Example. [math]\displaystyle{ x^4-x^2-2=(x^2-2)(x^2+1) }[/math] over [math]\displaystyle{ {\mathbb Q} }[/math].

Example. Factor [math]\displaystyle{ x^2+x+2\in{\mathbb Z}_3[x] }[/math] within its splitting field [math]\displaystyle{ {\mathbb Z}_3[x]/\langle x^2+x+2\rangle }[/math].