Diamond-free Families (1010.5311v3)

Abstract: Given a finite poset P, we consider the largest size La(n,P) of a family of subsets of $[n]:={1,...,n}$ that contains no subposet P. This problem has been studied intensively in recent years, and it is conjectured that $\pi(P):= \lim_{n\rightarrow\infty} La(n,P)/{n choose n/2}$ exists for general posets P, and, moreover, it is an integer. For $k\ge2$ let $\D_k$ denote the $k$-diamond poset ${A< B_1,...,B_k < C}$. We study the average number of times a random full chain meets a $P$-free family, called the Lubell function, and use it for $P=\D_k$ to determine $\pi(\D_k)$ for infinitely many values $k$. A stubborn open problem is to show that $\pi(\D_2)=2$; here we make progress by proving $\pi(\D_2)\le 2 3/11$ (if it exists).