Forbidden subgraphs in connected graphs

  • Authors:
  • Vlady Ravelomanana;Loÿs Thimonier

  • Affiliations:
  • LIPN UMR 7030, Université de Paris-Nord 99, Avenue J. B. Clément. F 93430 Villetaneuse, France;LaRIA EA 2083, Université de Picardie 5, Rue du Moulin-Neuf. 80000 Amiens, France

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2004

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Abstract

Given a set ξ = {H1,H2,...} of connected non-acyclic graphs, a ξ-free graph is one which does not contain any member of ξ as copy. Define the excess of a graph as the difference between its number of edges and its number of vertices. Let Wk,ξ be the exponential generating function (EGF for brief) of connected ξ-free graphs of excess equal to k (k ≥ 1). For each fixed ξ, a fundamental differential recurrence satisfied by the EGFs Wk,ξ is derived. We give methods on how to solve this nonlinear recurrence for the first few values of k by means of graph surgery. We also show that for any finite collection ξ of non-acyclic graphs, the EGFs Wk,ξ are always rational functions of the generating function, T, of Cayley's rooted (non-planar) labelled trees. From this, we prove that almost all connected graphs with n nodes and n + k edges are ξ-free, whenever k = o(n1/3) and |ξ| n nodes has approximately n/2 edges. In particular, the probability distribution that it consists of trees, unicyclic components, ..., (q + 1)-cyclic components all ξ-free is derived. Similar results are also obtained for multigraphs, which are graphs where self-loops and multiple-edges are allowed.