The spectrum of random lifts

Details The spectrum of random lifts The spectrum of random lifts

2010-12-18

arxiv.org/abs/1012.4097v3

Mathematics

Combinatorics

35 pages; substantially revised, constants in the main results improved

Scientific paper

For a fixed d-regular graph H, a random n-lift is obtained by replacing each vertex v of H by a "fibre" containing n vertices, then placing a uniformly random matching between fibres corresponding to adjacent vertices of H. We show that with extremely high probability, all eigenvalues of the lift that are not eigenvalues of H, have order O(sqrt(d)). In particular, if H is Ramanujan then its n-lift is with high probability nearly Ramanujan. We also show that any exceptionally large eigenvalues of the n-lift that are not eigenvalues of H, are overwhelmingly likely to have been caused by a dense subgraph of size O(|E(H)|).

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