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How many irreducible polynomials over $\mathbb Z_p$ of the form $x^2+ax+b$ are there?

No idea.

Germain
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  • An irreducible polynomial of degree 2 must have no roots, and vice versa. – ronash Apr 13 '13 at 12:36
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    Hint: First count the reducible ones. Or use the search engine, this was asked (for arbitrary degree) at least 10 times on math.SE. – Martin Brandenburg Apr 13 '13 at 12:37
  • @MartinBrandenburg: if you were aware of that, why didn't you vote to close this question as duplicate? The arbitrary degree answer might be a little too complicated (what with Mobius function and all...), but the one I marked is a simple enough generalization. – tomasz Apr 13 '13 at 12:46

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The formula is $$ \frac{p^2 - p}{2}. $$ The reason is that the polynomial $$ x^{p^{2}} - x $$ factors as the product of all the distinct irreducible, monic polynomials of degree dividing $2$, thus of degree $1$ or $2$, And there are $p$ monic polynomials of degree $1$.

More generally, $$ x^{p^{d}} - x $$ factors as the product of all the distinct irreducible, monic polynomials of degree dividing $d$. In general, to recover the number of the irreducible, monic polynomials of degree $d$, you have to use Moebius inversion.

Andreas Caranti
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