Lasserre Hierarchy for Graph Isomorphism and Homomorphism Indistinguishability

We show that feasibility of the tth level of the Lasserre semidefinite programming hierarchy for graph isomorphism can be expressed as a homomorphism indistinguishability relation. In other words, we define a class Lt of graphs such that graphs G and H are not distinguished by the tth level of the Lasserre hierarchy if and only if they admit the same number of homomorphisms from any graph in Lt. By analysing the treewidth of graphs in Lt, we prove that the 3tth level of Sherali--Adams linear programming hierarchy is as strong as the tth level of Lasserre. Moreover, we show that this is best possible in the sense that 3t cannot be lowered to 3t-1 for any t. The same result holds for the Lasserre hierarchy with non-negativity constraints, which we similarly characterise in terms of homomorphism indistinguishability over a family Lt^+ of graphs. Additionally, we give characterisations of level-t Lasserre with non-negativity constraints in terms of logical equivalence and via a graph colouring algorithm akin to the Weisfeiler--Leman algorithm. This provides a polynomial time algorithm for determining if two given graphs are distinguished by the tth level of the Lasserre hierarchy with non-negativity constraints.