Difference sets with n = 5 p r

Let D be a (v, k, λ)-difference set in an abelian group G, and (v, 31) = 1. If n = 5p r with p a prime not dividing v and r a positive integer, then p is a multiplier of D. In the case 31|v, we get restrictions on the parameters of such difference sets D for which p may not be a multiplier.