Jimm, a Fundamental Involution

Abstract

We study the involution of the real line induced by the outer automorphism of the extended modular group PGL(2,Z). This `modular' involution is discontinuous at rationals but satisfies a surprising collection of functional equations. It preserves the set of real quadratic irrationalities mapping them in a highly non-obvious way to each other. It commutes with the Galois action on real quadratic irrationals. More generally, it preserves set-wise the orbits of the modular group, thereby inducing an involution of the moduli space of real rank-two lattices. It induces a duality of Beatty partitions of the set of positive integers. This involution conjugates (though not topologically) the Gauss' continued fraction map to an intermittent dynamical system on the unit interval with an infinite invariant measure. The transfer operator (resp. the functional equation) naturally associated to this dynamical system is closely related to the Mayer transfer operator (resp. the Lewis' functional equation). We give a description of this involution as the boundary action of a certain automorphism of the infinite trivalent tree. We prove that its derivative exists and vanishes almost everywhere. It is conjectured that algebraic numbers of degree at least three are mapped to transcendental numbers under this involution.