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Keywords:

  • k-to-1;
  • continuity;
  • finite discontinuity;
  • graphs;
  • complete graphs;
  • wiggles;
  • 2000 Mathematics Subject Classifications;
  • 57M15;
  • 05C99

Abstract

A function between graphs is k-to-1 if each point in the codomain has precisely k preimages in the domain. In this article, we approach the topic of continuous, or finitely discontinuous, k-to-1 functions between graphs from three different points of view. Harrold (Duke Math J 5 (1939), 789–793) showed that there is no 2-to-1 continuous function from a closed interval onto a circle (i.e., from K2 onto C3). In the first part of this article, we describe all 3-to-1 continuous functions from an edge onto a cycle. Such a description is just one step away from a description of all 3-to-1 continuous functions from inline image onto inline image, which is in fact our main initial emphasis. Second, given two graphs, G and H, and an integer inline image, and considering G and H as subsets of inline image, Jo Heath gave a simple criterion for the existence of a finitely discontinuous k-to-1 function from G onto H. Such functions often involve a limiting construction which we call a wiggle. In the second part of this article, we give a simple formula (related to Jo Heath's construction) which counts the number of wiggles. The question of whether there is a continuous k-to-1 function (i.e., a k-to-1 map in the usual topological sense) from G onto H is more complicated. In the third part of this article, we consider complete graphs inline image and inline image. In the cases where n and m have the same parity, and inline image, then we determine exactly when there is a k-to-1 continuous function from inline image onto inline image. Other cases are considered elsewhere (J. K. Dugdale, S. Fiorini, A. J. W. Hilton, J. B. Gauci, Discrete Math 310 (2010), 330–346 and J. B. Gauci, A. J. W. Hilton, J Graph Theory 65 (2010), 35–60).