In my previous post, I said I would write some account of what Kolmogorov and Barzdin did in their paper, where unnamed expanders first appear. I had not really started writing this second post, which turns out to be a good thing, since the arXiv carried this morning a new paper of M. Gromov and L.Guth where the Kolmogorov-Barzdin paper is discussed and expanded, all certainly done with much greater insight than I would have brought…