locally checkable proofs
This work studies decision problems from the perspective of nondeterministic distributed algorithms. For a yes instance there must exist a proof that can be verified with a distributed algorithm: all nodes must accept a valid proof, and at least one node must reject an invalid proof. We focus on locally checkable proofs that can be verified with a constant-time distributed algorithm. For example, it is easy to prove that a graph is bipartite: the locally checkable proof gives a 2-colouring of the graph, which only takes 1 bit per node. However, it is more difficult to prove that a graph is not bipartite - it turns out that any locally checkable proof requires (log n ) bits per node. In this work we classify graph problems according to their local proof complexity, i.e., how many bits per node are needed in a locally checkable proof. We establish tight or near-tight results for classical graph properties such as the chromatic number. We show that the proof complexities form a natural hierarchy of complexity classes: for many classical graph problems, the proof complexity is either 0, (1), (log n ), or poly( n ) bits per node. Among the most difficult graph properties are symmetric graphs, which require ( n ) bits per node, and non-3-colourable graphs, which require ( n /log n ) bits per node - any pure graph property admits a trivial proof of size O ( n ).
decision problems proof complexity graph properties local algorithms nondeterministic algorithms