Asymptotic nonexistence of difference sets in dihedral groups

We prove that for any primes p(1),..., p(s), there are only finitely many numbers Pi(i=1)(s) p(i)(alphai) , alpha(i) is an element of Z(+), which can be orders of dihedral difference sets. We show that, with the possible exception of n = 540, 225, there is no difference set of order 11 with 1 < n less than or equal to 10(6) in any dihedral group. (C) 2002 Elsevier Science (USA).