Forbidden intersections for codes

Abstract

AbstractDetermining the maximum size of a ‐intersecting code in was a longstanding open problem of Frankl and Füredi, solved independently by Ahlswede and Khachatrian and by Frankl and Tokushige. We extend their result to the setting of forbidden intersections, by showing that for any and large compared with (but not necessarily ) that the same bound holds for codes with the weaker property of being ‐avoiding, that is, having no two vectors that agree on exactly coordinates. Our proof proceeds via a junta approximation result of independent interest, which we prove via a development of our recent theory of global hypercontractivity: we show that any ‐avoiding code is approximately contained in a ‐intersecting junta (a code where membership is determined by a constant number of coordinates). In particular, when , this gives an alternative proof of a recent result of Eberhard, Kahn, Narayanan and Spirkl that symmetric intersecting codes in have size .