On symmetric intersecting families of vectors

Abstract

A family of vectors $A\subset [k{]}^n$ is said to be intersecting if any two elements of $A$ agree on at least one coordinate. We prove, for fixed $k\ge 3$, that the size of a symmetric intersecting subfamily of $[k{]}^n$ is $o(k^n)$, which is in stark contrast to the case of the Boolean hypercube (where $k=2$). Our main contribution addresses limitations of existing technology: while there is now some spectral machinery, developed by Ellis and the third author, to tackle extremal problems in set theory involving symmetry, this machinery relies crucially on the interplay between up-sets and biased product measures on the Boolean hypercube, features that are notably absent in the problem at hand; here, we describe a method for circumventing these barriers.

6 pages; It has been brought to our attention that our main result (with slightly worse estimates) may be deduced from earlier work of Dinur, Friedgut and Regev, and this revision acknowledges this fact