Complexity of finding dense subgraphs

  • Authors:
  • Yuichi Asahiro;Refael Hassin;Kazuo Iwama

  • Affiliations:
  • Department of Computer Science and Communication Engineering, Kyushu University, Fukuoka 812-8581, Japan;Department of Statistics & Operations Research, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel;School of Informatics, Kyoto University, Kyoto 606-8501, Japan

  • Venue:
  • Discrete Applied Mathematics
  • Year:
  • 2002

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Abstract

The k-f(k) dense subgraph problem ((k, f(k))-DSP) asks whether there is a k-vertex subgraph of a given graph G which has at least f(k) edges. When f(k)=k(k - 1)/2, (k,f(k))-DSP is equivalent to the well-known k-clique problem. The main purpose of this paper is to discuss the problem of finding slightly dense subgraphs. Note that f(k) is about k2 for the k-clique problem. It is shown that (k,f(k))-DSP remains NP-complete for f(k)= Θ(k1+ε) where ε may be any constant such that 0 (k, f(k))-DSP is NP-complete for f(k)= ek2/υ2(1 +O(υε-1)), where υ is the number of G's vertices and e is the number of G's edges. This condition is quite tight because the answer to (k, f(k))-DSP is always yes for f(k)= ek2/υ2(1 -(υ- k)/(υk- k)) that is the average number of edges in a subgraph of k vertices. Also, we show that the hardness of (k, f(k))-DSP remains for regular graphs: (k, f(k))-DSP is NP-complete for Θ(υε1)-regular graphs if f(k)= Θ(k1-ε2) for any 0 1, ε2