Three layer $Q_2$-free families in the Boolean lattice

Abstract: We prove that the largest $Q_2$-free family of subsets of $[n]$ which contains sets of at most three different sizes has at most $(3 + 2\sqrt {3})N/3 + o(N) \approx 2.1547N + o(N)$ members, where $N = {n \choose {\lfloor n/2 \rfloor}}$. This improves an earlier bound of $2.207N + o(N)$ by Axenovich, Manske, and Martin.