Path Contraction Faster Than 2^n

Authors Akanksha Agrawal, Fedor V. Fomin, Daniel Lokshtanov, Saket Saurabh, Prafullkumar Tale

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Akanksha Agrawal
  • Ben-Gurion University of the Negev, Beersheba, Israel
Fedor V. Fomin
  • University of Bergen, Bergen, Norway
Daniel Lokshtanov
  • University of California Santa Barbara, Santa Barbara, California
Saket Saurabh
  • Institute of Mathematical Sciences, HBNI and UMI ReLaX Chennai, India
  • University of Bergen, Bergen, Norway
Prafullkumar Tale
  • Institute of Mathematical Sciences, HBNI, Chennai, India

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Akanksha Agrawal, Fedor V. Fomin, Daniel Lokshtanov, Saket Saurabh, and Prafullkumar Tale. Path Contraction Faster Than 2^n. In 46th International Colloquium on Automata, Languages, and Programming (ICALP 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 132, pp. 11:1-11:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)


A graph G is contractible to a graph H if there is a set X subseteq E(G), such that G/X is isomorphic to H. Here, G/X is the graph obtained from G by contracting all the edges in X. For a family of graphs F, the F-Contraction problem takes as input a graph G on n vertices, and the objective is to output the largest integer t, such that G is contractible to a graph H in F, where |V(H)|=t. When F is the family of paths, then the corresponding F-Contraction problem is called Path Contraction. The problem Path Contraction admits a simple algorithm running in time 2^n * n^{O(1)}. In spite of the deceptive simplicity of the problem, beating the 2^n * n^{O(1)} bound for Path Contraction seems quite challenging. In this paper, we design an exact exponential time algorithm for Path Contraction that runs in time 1.99987^n * n^{O(1)}. We also define a problem called 3-Disjoint Connected Subgraphs, and design an algorithm for it that runs in time 1.88^n * n^{O(1)}. The above algorithm is used as a sub-routine in our algorithm for Path Contraction.

Subject Classification

ACM Subject Classification
  • Mathematics of computing → Graph algorithms
  • Theory of computation → Graph algorithms analysis
  • Theory of computation → Parameterized complexity and exact algorithms
  • path contraction
  • exact exponential time algorithms
  • graph algorithms
  • enumerating connected sets
  • 3-disjoint connected subgraphs


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