We consider sets of positive integers containing no sum of two elements in
the set and also no product of two elements. We show that the upper density of
such a set is strictly smaller than 1/2 and that this is best possible.
Further, we also find the maximal order for the density of such sets that are
also periodic modulo some positive integer.
We show that there are sets of integers with asymptotic density arbitrarily
close to 1 in which there is no solution to the equation ab=c, with a,b,c in
the set. We also consider some natural generalizations, as well as a specific
numerical example of a product-free set of integers with asymptotic density
greater than 1/2.