  This paper studies the maximal size of product-free sets in Z/nZ. These are
sets of residues for which there is no solution to ab == c (mod n) with a,b,c
in the set. In a previous paper we constructed an infinite sequence of integers
(n_i)_{i > 0} and product-free sets S_i in Z/n_iZ such that the density
|S_i|/n_i tends to 1 as i tends to infinity, where |S_i|$ denotes the
cardinality of S_i. Here we obtain matching, up to constants, upper and lower
bounds on the maximal attainable density as n tends to infinity.
