# Birational geometry of sextic double solids with a compound $A_n$ singularity

Paemurru, Erik. (2021) Birational geometry of sextic double solids with a compound $A_n$ singularity. Preprints Fachbereich Mathematik, 2021 (02).

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Official URL: https://edoc.unibas.ch/81503/

Sextic double solids, double covers of $\mathbb P^3$ branched along a sextic surface, are the lowest degree Gorenstein Fano 3-folds, hence are expected to behave very rigidly in terms of birational geometry. Smooth sextic double solids, and those which are $\mathbb Q$-factorial with ordinary double points, are known to be birationally rigid. In this article, we study sextic double solids with an isolated compound $A_n$ singularity. We prove a sharp bound $n \leq 8$, describe models for each $n$ explicitly and prove that sextic double solids with $n > 3$ are birationally non-rigid.