Quasi-median hulls in Hamming space are Steiner hulls

  • Authors:
  • Hans-Jürgen Bandelt;Arne Röhl

  • Affiliations:
  • Department of Mathematics, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany;Department of Mathematics, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany

  • Venue:
  • Discrete Applied Mathematics
  • Year:
  • 2009

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Abstract

A Hamming space @L^n consists of all sequences of length n over an alphabet @L and is endowed with the Hamming distance. In particular, any set of aligned DNA sequences of fixed length constitutes a subspace of a Hamming space with respect to mismatch distance. The quasi-median operation returns for any three sequences u,v,w the sequence which in each coordinate attains either the majority coordinate from u,v,w or else (in the case of a tie) the coordinate of the first entry, u; for a subset of @L^n the iterative application of this operation stabilizes in its quasi-median hull. We show that for every finite tree interconnecting a given subset X of @L^n there exists a shortest realization within @L^n for which all interior nodes belong to the quasi-median hull of X. Hence the quasi-median hull serves as a Steiner hull for the Steiner problem in Hamming space.