Tutorial on Rational Rotation $C^*$--Algebras

Abstract

The rotation algebra $\mathcal A_θ$ is the universal $C^*$--algebra generated by unitary operators $U, V$ satisfying the commutation relation $UV = ωV U$ where $ω= e^{2πi θ}.$ They are rational if $θ= p/q$ with $1 \leq p \leq q-1,$ othewise irrational. Operators in these algebras relate to the quantum Hall effect \cite{boca,rammal,simon}, kicked quantum systems \cite{lawton1, wang}, and the spectacular solution of the Ten Martini problem \cite{avila}. Brabanter \cite{brabanter} and Yin \cite{yin} classified rational rotation $C^*$--algebras up to $*$-isomorphism. Stacey \cite{stacey} constructed their automorphism groups. They used methods known to experts: cocycles, crossed products, Dixmier-Douady classes, ergodic actions, K--theory, and Morita equivalence. This expository paper defines $\mathcal A_{p/q}$ as a $C^*$--algebra generated by two operators on a Hilbert space and uses linear algebra, Fourier series and the Gelfand-Naimark-Segal construction \cite{gelfand} to prove its universality. It then represents it as the algebra of sections of a matrix algebra bundle over a torus to compute its isomorphism class. The remarks section relates these concepts to general operator algebra theory. We write for mathematicians who are not $C^*$--algebra experts.