##### Department of Mathematics,

University of California San Diego

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### Algebraic Geometry Seminar

## Anton Geraschenko

## When is a variety a quotient of a smooth variety by a finite group?

##### Abstract:

\indent If a variety $X$ is a quotient of a smooth variety by a finite group, it has quotient singularities---that is, it is \emph{locally} a quotient by a finite group. In this talk, we will see that the converse is true if $X$ is quasi-projective and is known to be a quotient by a torus. In particular, all quasi-projective simplicial toric varieties are global quotients by finite groups! Though the proof is stack-theoretic, the construction of a smooth variety $U$ and finite group $G$ so that $X=U/G$ can usually be made explicit purely scheme-theoretically. \indent To illustrate the construction, I'll produce a smooth variety $U$ with an action of $G=\mathbb{Z}/2$ so that $U/G$ is the blow-up of $\mathbb{P}(1,1,2)$ at a smooth point. This example is interesting because even though $U/G$ is toric, $U$ cannot be taken to be toric. This is joint work with Matthew Satriano.

### October 26, 2011

### 4:00 PM

### AP&M 7218

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