On Betti numbers of flag complexes with forbidden induced subgraphs
  We analyze the asymptotic extremal growth rate of the Betti numbers of clique complexes of graphs on n vertices not containing a fixed forbidden induced subgraph H.   In particular, we prove a theorem of the alternative: for any H the growth rate achieves exactly one of five possible exponentials, that is, independent of the field of coefficients, the nth root of the maximal total Betti number over n-vertex graphs with no induced copy of H has a limit, as n tends to infinity, and, ranging over all H, exactly five different limits are attained.   For the interesting case where H is the 4-cycle, the above limit is 1, and we prove a slightly superpolynomial upper bound. 
Karim Adiprasito, Eran Nevo and Martin Tancer
math.CO