You might take a look at Chapter VII of Bollobas. In particular,
Theorem VII.1.7 -- which is simple enough that he doesn't bother providing a proof -- states that the expected number of k-cliques in G(n,M) is, setting N=nchoose2 and K=kchoose2,
nchoosekn−KchooseM−KNchooseM−1.
Also, Theorem VII.3.7 states that if M=o(n−2/(k−1)) then with probability tending to one, Gn,M contains no k-clique, whereas if M/n−2/(k−1)toinfty then with probability tending to one Gn,M does contain a k-clique. I know this doesn't fully answer your question but it may help.
Incidentally, (you probably already realize that) it is a priori possible (though I don't think it is the case) that, for example, tk(M+1)−tk(M)geqfrac1mathrmpoly(n) for all kchoose2leqMleqlceilfrac(k−1)Nkrceil, since all we really know by Turán is that
sumlceil(k−1)N/krceilM=K(tk(M+1)−tk(M))=1.
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