@Article{Paper,
author={Omer Reingold and Salil Vadhan and
Avi Wigderson},
title={Entropy waves, the zig-zag graph
product, and new constant-degree
expanders},
series={Ann. Math},
pages={157-187},
booktitle={Annals of Mathematics, Vol.
155, No. 1},
year={2002},
}