TY - GEN

T1 - Split contraction

T2 - 34th Symposium on Theoretical Aspects of Computer Science, STACS 2017

AU - Agrawal, Akanksha

AU - Lokshtanov, Daniel

AU - Saurabh, Saket

AU - Zehavi, Meirav

N1 - Funding Information:
The research leading to these results has received funding from the European Research Council (ERC) via grant PARAPPROX, reference 306992.
Publisher Copyright:
© Akanksha Agrawal, Daniel Lokshtanov, Saket Saurabh, and Meirav Zehavi.

PY - 2017/3/1

Y1 - 2017/3/1

N2 - The edit operation that contracts edges, which is a fundamental operation in the theory of graph minors, has recently gained substantial scientific attention from the viewpoint of Parameterized Complexity. In this paper, we examine an important family of graphs, namely the family of split graphs, which in the context of edge contractions, is proven to be significantly less obedient than one might expect. Formally, given a graph G and an integer k, SPLIT CONTRACTION asks whether there exists X ⊆ E(G) such that G/X is a split graph and |X| ≤ k. Here, G/X is the graph obtained from G by contracting edges in X. It was previously claimed that SPLIT CONTRACTION is fixed-parameter tractable. However, we show that SPLIT CONTRACTION, despite its deceptive simplicity, is W[1]-hard. Our main result establishes the following conditional lower bound: under the Exponential Time Hypothesis, SPLIT CONTRACTION cannot be solved in time 2o(ℓ2) · nO(1) where ℓ is the vertex cover number of the input graph. We also verify that this lower bound is essentially tight. To the best of our knowledge, this is the first tight lower bound of the form 2o(ℓ2) · nO(1) for problems parameterized by the vertex cover number of the input graph. In particular, our approach to obtain this lower bound borrows the notion of harmonious coloring from Graph Theory, and might be of independent interest.

AB - The edit operation that contracts edges, which is a fundamental operation in the theory of graph minors, has recently gained substantial scientific attention from the viewpoint of Parameterized Complexity. In this paper, we examine an important family of graphs, namely the family of split graphs, which in the context of edge contractions, is proven to be significantly less obedient than one might expect. Formally, given a graph G and an integer k, SPLIT CONTRACTION asks whether there exists X ⊆ E(G) such that G/X is a split graph and |X| ≤ k. Here, G/X is the graph obtained from G by contracting edges in X. It was previously claimed that SPLIT CONTRACTION is fixed-parameter tractable. However, we show that SPLIT CONTRACTION, despite its deceptive simplicity, is W[1]-hard. Our main result establishes the following conditional lower bound: under the Exponential Time Hypothesis, SPLIT CONTRACTION cannot be solved in time 2o(ℓ2) · nO(1) where ℓ is the vertex cover number of the input graph. We also verify that this lower bound is essentially tight. To the best of our knowledge, this is the first tight lower bound of the form 2o(ℓ2) · nO(1) for problems parameterized by the vertex cover number of the input graph. In particular, our approach to obtain this lower bound borrows the notion of harmonious coloring from Graph Theory, and might be of independent interest.

KW - Edge contraction

KW - Parameterized complexity

KW - Split graph

UR - http://www.scopus.com/inward/record.url?scp=85016179632&partnerID=8YFLogxK

U2 - 10.4230/LIPIcs.STACS.2017.5

DO - 10.4230/LIPIcs.STACS.2017.5

M3 - Conference contribution

AN - SCOPUS:85016179632

T3 - Leibniz International Proceedings in Informatics, LIPIcs

BT - 34th Symposium on Theoretical Aspects of Computer Science, STACS 2017

A2 - Vallee, Brigitte

A2 - Vollmer, Heribert

PB - Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing

Y2 - 8 March 2017 through 11 March 2017

ER -