# Does Fermat's Little Theorem work on polynomials?

Let $p$ be a prime number. Then if $f(x) = (1+x)^p$ and $g(x) = (1+x)$, then is $f \equiv g \mod p$?

I’m trying to prove that for integers $a > b > 0$ and a prime integer $p$, ${pa\choose b} \equiv {a \choose b}.$ To do this I use FLT to show that $(1+x)^{pa} \equiv (1+x)^a \mod p$ and compare the coefficients of $x^b$ to complete the proof. Am I applying FLT correctly?

In general, do most theorems regarding integers/reals generalize to polynomials over the integers/reals? Are there some common pitfalls that I could make when trying to generalize such theorems?

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The answer to the first question is NO. When you expand $g(x)$ by the binomial theorem, you will see that all of the coefficients, except for the first and last, are zero, so $g(x)$ is $1+x^p$ modulo $p.$ The difference between $f(x)$ and $g(x)$ has $p$ roots in the algebraic closure of $\mathbb{F}_p,$ which is not the same as the number of roots of the zero polynomial.

I think that gives an indication of the problems for your second question.

Possibly you have encountered a common stumbling point: confusing polynomial functions with formal polynomials. Over $\,\Bbb F_p,\,$ by $\,\mu$Fermat, $\,f-g\, =\, x^p -x \,$ equals the constant function $\,0,\,$ but it is not equal to $\,0\,$ as a formal polynomial since, by definition, formal polyomials are equal iff their corresponding coefficients are equal.