Generalised Lüroth expansions and a family of Minkowski's question-mark functions

Abstract

Minkowski's question-mark function is a singular homeomorphism of the unit interval that maps the set of quadratic surds into the rationals. This function has deserved the attention of several authors since the beginning of the twentieth century. Using different representations of real numbers by infinite sequences of integers, called α-Lüroth expansions, we obtain different instances of the standard shift map on infinite symbols, all of them topologically conjugated to the Gauss Map. In this note we prove that each of these conjugations share properties with Minkowski's question-mark function: all of them are singular homeomorphisms of the interval, and in the “rational” cases, they map the set of quadratic surds into the set of rational numbers. In this sense, this family is a natural generalisation of Minkowski's question-mark function.