Diffeology and Arithmetic of Irrational Tori

Abstract: The irrational torus, $\mathrm{T}\alpha$, originally introduced as a geometric model for quasicrystals, is a foundational object in the theory of diffeology. This paper, after recalling its main algebraic properties, provides a comprehensive analysis of a new geometric invariant for this singular space: the group of flows, $\mathbf{Fl}(\mathrm{T}\alpha, \mathbf{R})$. This invariant, which is trivial for all manifolds, arises as the core of the obstruction to the de Rham theorem in the diffeological setting. We provide a complete computation and geometric interpretation of this group, proving the isomorphism $\mathbf{Fl}(\mathrm{T}\alpha, \mathbf{R}) \simeq \mathbf{R} \times \coker(\Delta\alpha)$.