# Refuting FPT algorithms for some parameterized problems under Gap-ETH

Akanksha Agrawal, Ravi Kiran Allumalla, Varun Teja Dhanekula

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

4 Scopus citations

## Abstract

In this article we study a well-known problem, called Bipartite Token Jumping and not-so-well known problem(s), which we call, Half (Induced-) Subgraph, and show that under Gap-ETH, these problems do not admit FPT algorithms. The problem Bipartite Token Jumping takes as input a bipartite graph G and two independent sets S, T in G, where |S| = |T | = k, and the objective is to test if there is a sequence of exactly k-sized independent sets 〈I0, I1, · · ·, Iℓ〉 in G, such that: i) I0 = S and I = T, and ii) for every j ∈ [ℓ], Ij is obtained from Ij−1 by replacing a vertex in Ij−1 by a vertex in V (G) \ Ij−1. We show that, assuming Gap-ETH, Bipartite Token Jumping does not admit an FPT algorithm. We note that this result resolves one of the (two) open problems posed by Bartier et al. (ISAAC 2020), under Gap-ETH. Most of the known reductions related to Token Jumping exploit the property given by triangles (i.e., C3s), to obtain the correctness, and our results refutes FPT algorithm for Bipartite Token Jumping, where the input graph cannot have any triangles. For an integer k ∈ N, the half graph Sk,k is the graph with vertex set V (Sk,k) = Ak ∪ Bk, where Ak = {a1, a2, · · ·, ak} and Bk = {b1, b2, · · ·, bk}, and for i, j ∈ [k], {ai, bj} ∈ E(Tk,k) if and only if j ≥ i. We also study the Half (Induced-)Subgraph problem where we are given a graph G and an integer k, and the goal is to check if G contains Sk,k as an (induced-)subgraph. Again under Gap-ETH, we show that Half (Induced-)Subgraph does not admit an FPT algorithm, even when the input is a bipartite graph. We believe that the above problem (and its negative) result maybe of independent interest and could be useful obtaining new fixed parameter intractability results. There are very few reductions known in the literature which refute FPT algorithms for a parameterized problem based on assumptions like Gap-ETH. Thus our technique (and simple reductions) exhibits the potential of such conjectures in obtaining new (and possibly easier) proofs for refuting FPT algorithms for parameterized problems.

Original language English 16th International Symposium on Parameterized and Exact Computation, IPEC 2021 Petr A. Golovach, Meirav Zehavi Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing 9783959772167 https://doi.org/10.4230/LIPIcs.IPEC.2021.2 Published - 1 Nov 2021 Yes 16th International Symposium on Parameterized and Exact Computation, IPEC 2021 - Virtual, Lisbon, PortugalDuration: 8 Sep 2021 → 10 Sep 2021

### Publication series

Name Leibniz International Proceedings in Informatics, LIPIcs 214 1868-8969

### Conference

Conference 16th International Symposium on Parameterized and Exact Computation, IPEC 2021 Portugal Virtual, Lisbon 8/09/21 → 10/09/21

## Keywords

• Bipartite graphs
• Fixed parameter intractability
• Gap-exponential time hypothesis
• Half graphs
• Token jumping

• Software