Lubell mass and induced partially ordered sets

Abstract

We prove that for every partially ordered set $P$, there exists $c(P)$ such that every family $\mathcal{F}$ of subsets of $[n]$ ordered by inclusion and which contains no induced copy of $P$ satisfies $\sum_{F\in \mathcal{F}}1/{n\choose |F|}\leq c(P)$. This confirms a conjecture of Lu and Milans.