Multipole magnet

In mathematics, and more specifically in the theory of C*-algebras, the noncommutative tori A_θ (also known as irrational rotation algebras when θ is irrational) are a family of noncommutative C*-algebras which generalize the algebra of continuous functions on the 2-torus. Many topological and geometric properties of the classical 2-torus have algebraic analogues for the noncommutative tori, and as such they are fundamental examples of a noncommutative space in the sense of Alain Connes.

Definition

For any irrational number θ, the noncommutative torus A_θ is the C*-subalgebra of $B(L^2(\mathbb{𝕋}))$, the algebra of bounded linear operators of square-integrable functions on the unit circle of $\mathbb{ℂ}$ generated by unitary elements $U$ and $V$, where $U(f)(z)=zf(z)$ and $V(f)(z)=f(e^{-2\pi i\theta }z)$. A quick calculation shows that $VU=e^{-2\pi i\theta }UV$.

Alternative characterizations

• Universal property: A_θ can be defined (up to isomorphism) as the universal C*-algebra generated by two unitary elements U and V satisfying the relation $VU=e^{2\pi i\theta }UV.$ This definition extends to the case when θ is rational. In particular when θ=0, A_θ is isomorphic to continuous functions on the 2-torus by the Gelfand transform.
• Irrational rotation algebra: Let the infinite cyclic group Z act on the circle S¹ by the rotation action by angle 2πiθ. This induces an action of Z by automorphisms on the algebra of continuous functions C(S¹). The resulting C*-crossed product C(S¹) ⋊ Z is isomorphic to A_θ. The generating unitaries are the generator of the group Z and the identity function on the circle z : S¹ → C.
• Twisted group algebra: The function σ : Z² × Z² → C; σ((m,n), (p,q)) = e^2πinpθ is a group 2-cocycle on Z², and the corresponding twisted group algebra C*(Z²; σ) is isomorphic to A_θ.

Classification and K-theory

The K-theory of A_θ is Z² in even dimension and trivial in odd dimension, and so does not distinguish the irrational rotation algebras. But as ordered groups, K₀ ≃ Z + θZ. Therefore two noncommutative tori $A_{\theta }$ and $A_{\eta }$ are isomorphic if and only if either θ+η or θ-η is an integer.

Two irrational rotation algebras A_θ and A_η are strongly Morita equivalent if and only if θ and η are in the same orbit of the action of SL(2, Z) on R by fractional linear transformations. In particular, the noncommutative tori with θ rational are Morita equivalent to the classical torus. On the other hand, the noncommutative tori with θ irrational are simple C*-algebras.