For a prime $p$, consider the graph on the vertex set ${\mathbb F}\_p$, in
which every vertex $z$ is adjacent to $z\pm 1$ and also to $z^{-1}$ (unless
$z=0$). I was told that this graph is known to be an expander, but the person
who told me this couldn't recall where exactly this graph has been studied.
Does anybody know the reference? Thanks!