Abstract
A set of integers is primitive if it does not contain an element dividing another. Let f(n) denote the number of maximum-size primitive subsets of {1,…,2n}. We prove that the limit α = lim
n→∞
f(n)1/n exists. Furthermore, we present an algorithm approximating α with (1 + ε) multiplicative error in N(ε) steps, showing in particular that α ≈ 1.318. Our algorithm can be adapted to estimate the number of all primitive sets in {1,…,n} as well.
We address another related problem of Cameron and Erdős. They showed that the number of sets containing pairwise coprime integers in {1,…n} is between
${2^{\pi (n)}} \cdot {e^{(1/2 + o(1))\sqrt n }}$
and
${2^{\pi (n)}} \cdot {e^{(2 + o(1))\sqrt n }}$
. We show that neither of these bounds is tight: there are in fact
${2^{\pi (n)}} \cdot {e^{(1 + o(1))\sqrt n }}$
such sets.