On Maximum Symmetric Subgraphs

Let G be an n-node graph. We address the problem of computing a maximum symmetric graph H from G by deleting nodes, deleting edges, and contracting edges. This NP-complete problem arises naturally from the objective of drawing G as symmetrically as possible. We show that its tractability for the special cases of G being a plane graph, an ordered tree, and an unordered tree, depends on the type of operations used to obtain H from G. Moreover, we give an O(log n)-approximation algorithm for the intractable case that H is obtained from a tree G by contracting edges. As a by-product, we give an O(log n)-approximation algorithm for an NP-complete edit-distance problem.