Lattices with many congruences are planar

Abstract

Let $L$ be an $n$-element finite lattice. We prove that if $L$ has strictly more than $2^{n-5}$ congruences, then $L$ is planar. This result is sharp, since for each natural number $n\ge 8$, there exists a non-planar lattice with exactly $2^{n-5}$ congruences.

10 pages, 2 figures