why is this the case? I don't think it's true.well, it simply follows from the followingsum_i p_i BS(S,K P,Sigma_i) = BS(S,K P,Sigma_Implied(S,K P))ATM we have S=K P, hence, using homogeneitysum_i p_i BS(1,1,Sigma_i) = BS(1,1,Sigma_Implied(1,1))All dependance on the spot drops out...Jiri