Bounding Threshold Dimension: Realizing Graphic Boolean Functions as the AND of Majority Gates

Abstract

AbstractA graph G on n vertices is a threshold graph if there exist real numbers \(a_1,a_2, \ldots , a_n\) and b such that the zero-one solutions of the linear inequality \(\sum \limits _{i=1}^n a_i x_i \le b\) are the characteristic vectors of the cliques of G. Introduced in [Aggregation of inequalities in integer programming. Chvátal and Hammer, Annals of Discrete Mathematics, 1977], the threshold dimension of a graph G, denoted by \(\textrm{dim}_{\textrm{TH}}(G)\), is the minimum number of threshold graphs whose intersection yields G. Given a graph G on n vertices, in line with Chvátal and Hammer, \(f_G:\{0,1\}^n \rightarrow \{0,1\}\) is the Boolean function that has the property that \(f_G(x) = 1\) if and only if x is the characteristic vector of a clique in G. A Boolean function f for which there exists a graph G such that \(f=f_G\) is called a graphic Boolean function. It follows that for a graph G, \(\textrm{dim}_{\textrm{TH}}(G)\) is precisely the minimum number of majority gates whose AND (or conjunction) realizes the graphic Boolean function \(f_G\). The fact that there exist Boolean functions which can be realized as the AND of only exponentially many majority gates motivates us to study threshold dimension of graphs. We give tight or nearly tight upper bounds for the threshold dimension of a graph in terms of its treewidth, maximum degree, degeneracy, number of vertices, size of a minimum vertex cover, etc. We also study threshold dimension of random graphs and graphs with high girth.KeywordsIntersection dimensionThreshold dimensionBoxicityThreshold graphsGraphic Boolean functionMajority gatesDepth-2 circuitsTreewidth