diff --git a/content/Math4302/Math4302_L29.md b/content/Math4302/Math4302_L29.md new file mode 100644 index 0000000..6f4d29a --- /dev/null +++ b/content/Math4302/Math4302_L29.md @@ -0,0 +1,60 @@ +# Math4302 Modern Algebra (Lecture 29) + +## Rings + +### Polynomial Rings + +$$ +R[x]=\{a_0+a_1x+\cdots+a_nx^n:a_0,a_1,\cdots,a_n\in R,n>1\} +$$ + +Then $(R[x],+,\cdot )$ is a ring. + +If $R$ has a unity $1$, then $R[x]$ has a unity $1$. + +If $R$ is commutative, then $(R[x],+,\cdot )$ is commutative. + +#### Definition of evaluation map + +Let $F$ be a field, and $F[x]$. Fix $\alpha\in F$. $\phi_\alpha:F[x]\to F$ defined by $f(x)\mapsto f(\alpha)$ (the evaluation map). + +Then $\phi_\alpha$ is a ring homomorphism. $\forall f,g\in F[x]$, + +- $(f+g)(\alpha)=f(\alpha)+g(\alpha)$ +- $(fg)(\alpha)=f(\alpha)g(\alpha)$ (use commutativity of $\cdot$ of $F$, $f(\alpha)g(\alpha)=\sum_{k=0}^{n+m}c_k x^k$, where $c_k=\sum_{i=0}^k a_ib_{k-i}$) + +#### Definition of roots + +Let $\alpha\in F$ is zero (or root) of $f\in F[x]$, if $f(\alpha)=0$. + +
+Example + +$f(x)=x^3-x, F=\mathbb{Z}_3$ + +$f(0)=f(1)=0$, $f(2)=8-2=2-2=0$ + +but note that $f(x)$ is not zero polynomial $f(x)=0$, but all the evaluations are zero. + +
+ +#### Factorization of polynomials + +Division algorithm. Let $F$ be a field, $f(x),g(x)\in F[x]$ with $g(x)$ non-zero. Then there are unique polynomials $q(x),r(x)\in F[x]$ such that + +$f(x)=q(x)g(x)+r(x)$ + +$r(x)$ is the zero polynomial or $\deg r(x)<\deg g(x)$. + +
+Proof + +Uniqueness: exercise + +--- + +Existence: + +Let $S=\{f(x)-h(x)g(x):h(x)\in F[x]\}$. + +
\ No newline at end of file diff --git a/content/Math4302/_meta.js b/content/Math4302/_meta.js index 6a3d978..5e39aa6 100644 --- a/content/Math4302/_meta.js +++ b/content/Math4302/_meta.js @@ -31,4 +31,5 @@ export default { Math4302_L26: "Modern Algebra (Lecture 26)", Math4302_L27: "Modern Algebra (Lecture 27)", Math4302_L28: "Modern Algebra (Lecture 28)", + Math4302_L29: "Modern Algebra (Lecture 29)", }