Characterizations and enumerations of patterns of signed shifts

Abstract

Shift maps can be interpreted as maps from the unit interval to itself sending x to the fractional part of Nx, whose graph consists of N segments of positive slope. Signed shifts generalize shift maps by allowing some of these segments to have negative slope. Permutations realized by the relative order of the elements in the orbits of these maps have been studied recently by Amigó, Archer and Elizalde. In this paper, we give a complete characterization of the permutations (also called patterns) realized by signed shifts. In the case of the negative shift, which is the signed shift having only negative slopes, we use our characterization to give an exact enumeration of these patterns. Finally, we improve the best known bounds for the number of patterns realized by the tent map, and calculate the topological entropy of signed shifts using these combinatorial methods.