On the structure of low-rank matrices that approximate the identity matrix

Abstract

Consider a matrix $A$ of rank $n$ that approximates the $N\times N$ identity matrix with elementwise error at most $1/3$. We give a lower bound on the number of elements s.t. $|A_{i,j}|>\gamma$, for a certain threshold. Two corollaries are obtained. 1. If $n\le K\log N$ with some $K$, then at least $c(K)N^2$ elements satisfy $|A_{i,j}|>c(K)n^{-1/2}$. This answers a question of B.S. Kashin. 2. The number of nonzero elements in $A$ is at least $c\log (N)/(n\log (2+n/\log N))$.