Stacky curve

Template:Short description In mathematics, a stacky curve is an object in algebraic geometry that is roughly an algebraic curve with potentially "fractional points" called stacky points. A stacky curve is a type of stack used in studying Gromov–Witten theory, enumerative geometry, and rings of modular forms.

Stacky curves are closely related to 1-dimensional orbifolds and therefore sometimes called orbifold curves or orbicurves.

Definition

A stacky curve $𝔛$ over a field Template:Mvar is a smooth proper geometrically connected Deligne–Mumford stack of dimension 1 over Template:Mvar that contains a dense open subscheme.

Properties

A stacky curve is uniquely determined (up to isomorphism) by its coarse space Template:Mvar (a smooth quasi-projective curve over Template:Mvar), a finite set of points Template:Mvar (its stacky points) and integers Template:Mvar (its ramification orders) greater than 1. The canonical divisor of $𝔛$ is linearly equivalent to the sum of the canonical divisor of Template:Mvar and a ramification divisor Template:Mvar:

$K_{𝔛}\sim K_X+R.$

Letting Template:Mvar be the genus of the coarse space Template:Mvar, the degree of the canonical divisor of $𝔛$ is therefore:

$d=\mathrm{deg}{K}_{𝔛}=2-2g-\sum _{i=1}^r\frac{n_i-1}{n_i}.$

A stacky curve is called spherical if Template:Mvar is positive, Euclidean if Template:Mvar is zero, and hyperbolic if Template:Mvar is negative.

Although the corresponding statement of Riemann–Roch theorem does not hold for stacky curves, there is a generalization of Riemann's existence theorem that gives an equivalence of categories between the category of stacky curves over the complex numbers and the category of complex orbifold curves.

Applications

The generalization of GAGA for stacky curves is used in the derivation of algebraic structure theory of rings of modular forms.

The study of stacky curves is used extensively in equivariant Gromov–Witten theory and enumerative geometry.