Revisiting Block-Diagonal SDP Relaxations for the Clique Number of the Paley Graphs

This work addresses the block-diagonal semidefinite program (SDP) relaxations for the clique number of the Paley graphs. The size of the maximal clique (clique number) of a graph is a classic NP-complete problem; a Paley graph is a deterministic graph where two vertices are connected if their difference is a quadratic residue modulo certain prime powers. Improving the upper bound for the Paley graph clique number for odd prime powers is an open problem in combinatorics. Moreover, since quadratic residues exhibit pseudorandom properties, Paley graphs are related to the construction of deterministic restricted isometries, an open problem in compressed sensing and sparse recovery. Recent work provides evidence that the current upper bounds can be improved by the sum-of-squares (SOS) relaxations. In particular the bounds given by the SOS relaxations of degree 4 (SOS-4) are asymptotically growing at an order smaller than square root of the prime. However computations of SOS-4 become intractable with respect to large graphs. Gvozdenovic et al. introduced a more computationally efficient block-diagonal hierarchy of SDPs that refines the SOS hierarchy. They computed the values of these SDPs of degrees 2 and 3 (L2 and L3 respectively) for the Paley graph clique numbers associated with primes p less or equal to 809. These values bound from the above the values of the corresponding SOS-4 and SOS-6 relaxations respectively. We revisit these computations and determine the values of the L2 relaxation for larger p's. Our results provide additional numerical evidence that the L2 relaxations, and therefore also the SOS-4 relaxations, are asymptotically growing at an order smaller than the square root of p.