Lower Bounds for Protrusion Replacement by Counting Equivalence Classes

Garnero et al. [SIAM J. Discrete Math. 2015, 29(4):1864-1894] recently introduced a framework based on dynamic programming to make applications of the protrusion replacement technique constructive and to obtain explicit upper bounds on the involved constants. They show that for several graph problems, for every boundary size t one can find an explicit set R_t of representatives. Any subgraph H with a boundary of size t can be replaced with a representative H\u27 in R_t such that the effect of this replacement on the optimum can be deduced from H and H\u27 alone. Their upper bounds on the size of the graphs in R_t grow triple-exponentially with t. In this paper we complement their results by lower bounds on the sizes of representatives, in terms of the boundary size t. For example, we show that each set of planar representatives R_t for the Independent Set problem contains a graph with Omega(2^t / sqrt{4t}) vertices. This lower bound even holds for sets that only represent the planar subgraphs of bounded pathwidth. To obtain our results we provide a lower bound on the number of equivalence classes of the canonical equivalence relation for Independent Set on t-boundaried graphs. We also find an elegant characterization of the number of equivalence classes in general graphs, in terms of the number of monotone functions of a certain kind. Our results show that the number of equivalence classes is at most 2^{2^t}, improving on earlier bounds of the form (t+1)^{2^t}

Equivalence of the filament and overlap graphs of subtrees of limited trees

The overlap graphs of subtrees of a tree are equivalent to subtree filament graphs, the overlap graphs of subtrees of a star are cocomparability graphs, and the overlap graphs of subtrees of a caterpillar are interval filament graphs. In this paper, we show the equivalence of many more classes of subtree overlap and subtree filament graphs, and equate them to classes of complements of cochordal-mixed graphs. Our results generalize the previously known results mentioned above