A simple proof of the Borsuk-Ulam theorem for Z_p-actions

In this note, we give a simple proof of the Borsuk-Ulam theorem for $Z_p$-actions. We prove that, if $S^n$ and $S^m$ are equipped with free $Z_p$-actions (p prime) and $f:S^n\to S^m$ is a $Z_p$-equivariant map, then $n\le m$.