Elusiveness of finding degrees

Abstract

Listen

Translate article

We address the complexity of finding a vertex with specific (or maximum) (out)degree in undirected graphs, directed graphs and tournaments in a model where we count only the queries/edge-probes to the adjacency matrix of the graph. Improving upon some earlier bounds, using adversary arguments, we show that the following problems require n2 probes to the adjacency matrix of a graph with n vertices: •[-] determining whether a given directed graph has a vertex of outdegree k (for a non-negative integer k≤(n+1)∕2);•[-] determining whether an undirected graph has a degree 0, 1 or 2 vertex;•[-] finding the maximum (out)degree in a directed or an undirected graph, and•[-] finding all vertices with the maximum outdegree in a tournament. A property of a simple graph is elusive, if any algorithm to determine the property requires all the relevant probes to the adjacency matrix of the graph in the worst case. So the above results imply that determining whether a directed graph has a vertex of (out)degree k (for a non-negative integer k≤(n+1)∕2) or an undirected graph has a vertex of degree 0,1 or 2 vertex are elusive properties.In contrast, we show that one can find a maximum outdegree in a tournament using at most n2−1 probes. By substantially improving a known lower bound, we show that, for this problem n2−2 probes are necessary if n is odd, and n2−n∕2−2 probes are necessary if n is even. For determining the existence of a vertex with degree k>2 in an undirected graph, we give a lower bound of .42n2 improving on the earlier lower bound of .25n2.