 
Summary: This lemma, in turn, can be used (within an induction) to prove that for
every k, 0 Ÿ k Ÿ log N \Gamma 1, there exists an execution of 2 k+1 processes with k
rounds and ending with a bivalent configuration. Taking k = log N \Gamma 1 we get
the desired lower bound:
Theorem13. In the totally anonymous model, the round complexity of a pro
tocol solving binary consensus among N processes
is\Omega (log N ).
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