From Haar to Lebesgue via Domain Theory

Abstract

If C ≃ 2ℕ is the Cantor set realized as the infinite product of two-point groups, then a folklore result says the Cantor map from C into [0,1] sends Haar measure to Lebesgue measure on the interval. In fact, C admits many distinct topological group structures. In this note, we show that the Haar measures induced by these distinct group structures are all the same. We prove this by showing that Haar measure for any group structure is the same as Haar measure induced by a related abelian group structure. Moreover, each abelian group structure on C supports a natural total order that determines a map onto the unit interval that is monotone, and hence sends intervals in C to subintervals of the unit interval. Using techniques from domain theory, we show this implies this map sends Haar measure on C to Lebesgue measure on the interval, and we then use this to prove any two group structures on C have the same Haar measure.