On the Parameterized Complexity of Contraction to Generalization of Trees *

Akanksha Agrawal, Saket Saurabh, Prafullkumar Tale
For a family of graphs F, the F-Contraction problem takes as an input a graph G and an integer k, and the goal is to decide if there exists S ⊆ E(G) of size at most k such that G/S belongs to F. Here, G/S is the graph obtained from G by contracting all the edges in S. Heggernes et al. [Algorithmica (2014)] were the first to study edge contraction problems in the realm of Parameterized Complexity. They studied F-Contraction when F is a simple family of graphs such as trees and paths. In this
more » ... paths. In this paper, we study the F-Contraction problem, where F generalizes the family of trees. In particular, we define this generalization in a "parameterized way". Let T be the family of graphs such that each graph in T can be made into a tree by deleting at most edges. Thus, the problem we study is T-Contraction. We design an FPT algorithm for T-Contraction running in time O((2 √ + 2) O(k+) · n O(1)). Furthermore, we show that the problem does not admit a polynomial kernel when parameterized by k. Inspired by the negative result for the kernelization, we design a lossy kernel for T-Contraction of size O([k(k + 2)] (α α−1 +1)).