# Factor a polynomial over a finite field

I have a polynomial: $x^6 + x^4 + x^3 + x + 1$, and I need to factor it over $\mathbb{F}_2$, $\mathbb{F}_4$ and $\mathbb{F}_{16}$. I need to do it manually.

I wasn't even able to factor it over $\mathbb{F}_2$. I found a list of all irreducible polynomials of degree $\leqslant 5$ (here) and tried to divide the original polynomial by each of those, but with each one of them I got a remainder not equal to 0. So I either made a mistake with division or with the way I tried to solved this problem.

I have tried to read about various factorization algorithms, such as Berlekamp's algorithm, but they seem overcomplicated for this particular problem. I think there should be an easier approach to this problem.

In general, how to I try to come up with the factors? How do I know which polynomials are irreducible and which irreducible polynomials should I "look for" in my given polynomial?

• You've already factored it over GF(2); it's irreducible. – Parcly Taxel May 25 '17 at 13:26

One very useful fact when dealing with polynomials over finite fields, is that all irreducible polynomials over $F_p$ of degree $r$ divide the polynomial $x^{p^r} - x$; and all other irreducible factors of that polynomial have a degree dividing $r$. While this takes some time to apply when $r$ is large, it gives you a way to know how many irreducible polynomials of degree $r$ there are, which is useful since most of the times you can find some irreducile polynomials by hand. This is particularly easy when $r$ is prime: in this case, all factors of $x^{p^r} - x$ of degree different than $r$ are those of degree one, and there are exactly $p$ of them: one for each root (every element of $F_p$ is a root). So the polynomial has $\frac{p^r - p}{r}$ irreducible factors of degree $r$, and these are actually all the polynomials of degree $r$ over $F_p$. I hope this helps!