Short proof of two cases of Chvátal's conjecture

Abstract

In 1974 Chvátal conjectured that no intersecting family $\mathcal{F}$ in a downset can be larger than the largest star. In the same year Kleitman and Magnanti proved the conjecture when $\mathcal{F}$ is contained in the union of two stars, and Sterboul when $\mathrm{rank}(\mathcal{F})\le 3$. We give short self-contained proofs of these two statements.

3 pages, updated with additional references