Past exam of the algebra, analysis and differential equations 1 course of the University of Oxford / 2008 / 3 / Solution

Reduction modulo the prime $p$ preserves integer addition, multiplication, and one:Every congruence class has an integer representative, so the ring homomorphism is onto. Its kernel is exactly $p\mathbb{Z}=(p)$. The quotient is a field because every class not divisible by $p$ has a multiplicative inverse by Bézout's identity.

For the coefficientwise extension to the polynomial ring, addition is preserved term by term. If $f={\sum }_i{a}_i{x}^i$ and $g={\sum }_j{b}_j{x}^j$, the coefficient of $x^k$ in their product is ${\sum }_{i+j=k}{a}_i{b}_j$. Reducing it modulo $p$ gives ${\sum }_{i+j=k}{\bar{a}}_i{\bar{b}}_j$, the coefficient in $\hat{\varphi }(f)\hat{\varphi }(g)$. Thus multiplication and one are preserved as well. Lift each coefficient of a polynomial over ${\mathbb{Z}}_p$ to an integer to prove surjectivity. The kernel isIndeed, its elements are precisely the polynomials whose coefficients are all divisible by $p$.

In the binomial theorem expansion of $(x-1{)}^p$, every intermediate coefficient $\left(\genfrac{}{}{0ex}{}{p}{j}\right)$ is divisible by $p$: the numerator contains the prime factor $p$, while $j!(p-j)!$ does not for $1\le j<p$. HenceFor $p=2$, the last equality uses $1=-1$ in characteristic two. This is an instance of the Frobenius endomorphism.