Empirical bounds for commuting dilations of free unitaries and the universal commuting dilation constant

Abstract

For a tuple $T$ of Hilbert space operators, the 'commuting dilation constant' is the smallest number $c$ such that the operators of $T$ are a simultaneous compression of commuting normal operators of norm at most $c$. We present numerical experiments giving a strong indication that the commuting dilation constant of a pair of independent random $N\times N$ unitary matrices converges to $\sqrt{2}$ as $N\to \infty$ almost surely. Under the assumption that this is the case, we prove that the commuting dilation constant of an arbitrary pair of contractions is strictly smaller than $2$. Our experiments are based on a simple algorithm that we introduce for the purpose of computing dilation constants between tuples of matrices.

accepted for publication in Journal of Experimental Mathematics; changes in v2: new Remark 2.8, updated references and acknowledgements, minor corrections; 13 pages, 5 figures