Weighted noncommutative regular projective curves

Abstract

Let $\mathcal{H}$ be a noncommutative regular projective curve over a perfect field $k$. We study global and local properties of the Auslander–Reiten translation $\tau$ and give an explicit description of the complete local rings, with the involvement of $\tau$. We introduce the $\tau$-multiplicity $e_{\tau }(x)$, the order of $\tau$ as a functor restricted to the tube concentrated in $x$. We obtain a local-global principle for the (global) skewness $s(\mathcal{H})$, defined as the square root of the dimension of the function (skew-) field over its centre. In the case of genus zero we show how the ghost group, that is, the group of automorphisms of $\mathcal{H}$ which fix all objects, is determined by the points $x$ with $e_{\tau }(x)>1$. Based on work of Witt we describe the noncommutative regular (smooth) projective curves over the real numbers; those with $s(\mathcal{H})=2$ we call Witt curves. In particular, we study noncommutative elliptic curves, and present an elliptic Witt curve which is a noncommutative Fourier–Mukai partner of the Klein bottle. If $\mathcal{H}$ is weighted, our main result will be formulae for the orbifold Euler characteristic, involving the weights and the $\tau$-multiplicities. As an application we will classify the noncommutative 2-orbifolds of nonnegative Euler characteristic, that is, the real elliptic, domestic and tubular curves. Throughout, many explicit examples are discussed.