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How do I prove that $x^p-x+a$ is irreducible in a field of $p$ elements when $a\neq 0$?

How to prove that $x^p - x - 1$ is irreducible over $F_{p}[x]$. I thought proving that this polynomial has no roots over $F_p$ but that is not enough to irreducibility.

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    I hate to spoil your fun, but this is an [exact duplicate](http://math.stackexchange.com/q/81583/11619).2012-09-02

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In a splitting field $K$, if $\alpha\in K$ is a root, then $(\alpha+1)^p= \alpha^p+1$ implies that $\alpha+1$ is a root as well. We conclude that the cyclic group of order $p$ operates on the roots (by addition of multiples of $1$), and necessarily does so transitively (as there are $p$ roots). A non-trivial factor of the polynomial would correspond to a non-trivial subgroup, which does not exist.

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    +1 Simple and beautiful. Yet, perhaps the following *more expanded* argument (which already exists in Hagen's answer) could prevent doubt: it's been proved $\,\alpha\,$ is a root $\,\Longrightarrow \alpha+1\,$ is a root. Now, the same argument shows that $\alpha +1\,\,\text{is a root}\Longrightarrow \,\alpha +1+1=\alpha +2$ is a root , and etc. One gets that $\,\alpha, \alpha+1,...,\alpha+p-1$ are all different roots of the polynomial...2012-09-03