A Strahler bijection between Dyck paths and planar trees

  • Authors:
  • Xavier Gérard Viennot

  • Affiliations:
  • LaBRI, Université Bordeaux I, 351 cours de la Liberation, 33405 Talence, France

  • Venue:
  • Discrete Mathematics
  • Year:
  • 2002

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Abstract

The Strahler number of binary trees has been introduced by hydrogeologists and rediscovered in computer science in relation with some optimization problems. Explicit expressions have been given for the Strahler distribution, i.e. binary trees enumerated by number of vertices and Strahler number. Two other Strahler distributions have been discovered with the logarithmic height of Dyck paths and the pruning number of forests of planar trees in relation with molecular biology. Each of these three classes are enumerated by the Catalan numbers, but only two bijections preserving the Strahler parameters have been explicited: by Françon between binary trees and Dyck paths, by Zeilberger between binary trees and forests of planar trees. We present here the missing bijection between forests of planar trees and Dyck paths sending the pruning number onto the logarithmic height. A new functional equation for the Strahler generating function is deduced. Some orthogonal polynomials appear, they are one parameter Tchebycheff polynomials.