Induced Subgraph Isomorphism Problem Research Articles

Combinatorics and Algorithms for Quasi-Chain Graphs

The class of quasi-chain graphs is an extension of the well-studied class of chain graphs. This latter class enjoys many nice and important properties, such as bounded clique-width, implicit representation, well-quasi-ordering by induced subgraphs, etc. The class of quasi-chain graphs is substantially more complex. In particular, this class is not well-quasi-ordered by induced subgraphs, and the clique-width is not bounded in it. In the present paper, we show that the universe of quasi-chain graphs is at least as complex as the universe of permutations by establishing a bijection between the class of all permutations and a subclass of quasi-chain graphs. This implies, in particular, that the induced subgraph isomorphism problem is NP-complete for quasi-chain graphs. On the other hand, we propose a decomposition theorem for quasi-chain graphs that implies an implicit representation for graphs in this class and efficient solutions for some algorithmic problems that are generally intractable.

Detecting and Counting Small Pattern Graphs

We study the induced subgraph isomorphism problem and the general subgraph isomorphism problem for small pattern graphs. We present a new general method for detecting induced subgraphs of a host graph isomorphic to a fixed pattern graph by reduction to polynomial testing for nonidentity with zero over a field of finite characteristic. It yields new upper time bounds for several pattern graphs on five vertices and provides an alternative combinatorial method for the majority of pattern graphs on four and three vertices. Since our method avoids the large overhead of fast matrix multiplication, it can be of practical interest even for larger pattern graphs. Next, we derive new upper time bounds on counting the number of isomorphisms between a fixed pattern graph with an independent set of size $s$ and a subgraph of the host graph. We also consider a weighted version of the counting problem, when one counts the number of isomorphisms between the pattern graph and lightest subgraphs, providing a slightly slowe...

Maximum $r$-Regular Induced Subgraph Problem: Fast Exponential Algorithms and Combinatorial Bounds

We show that for a fixed $r$, the number of maximal $r$-regular induced subgraphs in any graph with $n$ vertices is upper bounded by $\mathcal{O}(c^n)$, where $c$ is a positive constant strictly less than $2$. This bound generalizes the well-known result of Moon and Moser, who showed an upper bound of $3^{n/3}$ on the number of maximal independent sets of a graph on $n$ vertices. We complement this upper bound result by obtaining an almost tight lower bound on the number of (possible) maximal $r$-regular induced subgraphs possible in a graph on $n$ vertices. Our upper bound results are algorithmic. That is, we can enumerate all the maximal $r$-regular induced subgraphs in time $\mathcal{O}(c^n n^{\mathcal{O}(1)})$. A related question is that of finding a maximum-sized $r$-regular induced subgraph. Given a graph $G=(V,E)$ on $n$ vertices, the Maximum $r$-Regular Induced Subgraph (M-$r$-RIS) problem asks for a maximum-sized subset of vertices, $R \subseteq V$, such that the induced subgraph on $R$ is $r$-regular. As a by-product of the enumeration algorithm, we get a $\mathcal{O}(c^n)$ time algorithm for this problem for any fixed constant $r$, where $c$ is a positive constant strictly less than $2$. Furthermore, we use the techniques and results obtained in the paper to obtain improved exact algorithms for a special case of the Induced Subgraph Isomorphism problem, namely, the Induced $r$-Regular Subgraph Isomorphism problem, where $r$ is a constant, the $\delta$-Separating Maximum Matching problem and the Efficient Edge Dominating Set problem.

Cleaning Interval Graphs

We investigate a special case of the Induced Subgraph Isomorphism problem, where both input graphs are interval graphs. We show the NP-hardness of this problem, and we prove fixed-parameter tractability of the problem with non-standard parameterization, where the parameter is the difference |V(G)|−|V(H)|, with G and H being the larger and the smaller input graph, respectively. Intuitively, we can interpret this problem as “cleaning” the graph G, regarded as a pattern containing extra vertices indicating errors, in order to obtain the graph H representing the original pattern. We also prove W[1]-hardness for the standard parameterization where the parameter is |V(H)|.

Parameterized graph cleaning problems

We investigate the Induced Subgraph Isomorphism problem with non-standard parameterization, where the parameter is the difference | V ( G ) | − | V ( H ) | with H and G being the smaller and the larger input graph, respectively. Intuitively, we can interpret this problem as “cleaning” the graph G , regarded as a pattern containing extra vertices indicating errors, in order to obtain the graph H representing the original pattern. We show fixed-parameter tractability of the cases where both H and G are planar and H is 3-connected, or H is a tree and G is arbitrary. We also prove that the problem when H and G are both 3-connected planar graphs is NP-complete without parameterization.

The subgraph isomorphism problem for outerplanar graphs

This paper deals with the subgraph isomorphism problem for outerplanar graphs (SUBOUTISOM). In general, since trees and forests are outerplanar, SUBOUTISOM is NP-complete. We show that SUBOUTISOM remains NP-complete even when the strongest connectivity requirements are imposed on both graphs. The same results holds for the induced subgraph isomorphism problem for outerplanar graphs except the case when both graphs are 2-connected; for such graphs we give a polynomial algorithm which verifies whether a 2-connected outerplanar graph is an induced subgraph of another 2-connected outerplanar graph.