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In Luca Trevisan's paper "Psuedorandomness and Combinatorial Constructions", he says that in a graph drawn from $G_{n,1/2}$, for every two disjoint sets A and B there are $(1/2 + o_n(1)) |A||B|$ edges between A and B.

I'm confused as to the exact meaning of this statement. Is it saying that given two disjoint subsets of n vertices selected at random, that's the expected value of the number of edges between them? If so, then doesn't linearity of expectation give |A||B|/2? What is this extra factor of o(1)?

Thanks!

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Perhaps what's implicit after "there are" is "asymptotically almost surely", which means that the property holds with probability tending to 1 as n tends to infinity; this is stronger than "in expectation". You might want to check out a standard textbook on random graphs (e.g. by Alon-Spencer, Bollobas, or Janson-Luczak-Rucinski).

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