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HALF NIKULIN SURFACES AND MODULI OF PRYM CURVES

Published online by Cambridge University Press:  29 November 2019

Andreas Leopold Knutsen
Affiliation:
Department of Mathematics, University of Bergen, Postboks 7800, 5020Bergen, Norway (andreas.knutsen@math.uib.no)
Margherita Lelli-Chiesa
Affiliation:
Dipartimento di Ingegneria e Scienze dell’Informazione e Matematica, Università degli Studi dell’Aquila, Via Vetoio, località Coppito, 67100L’Aquila, Italy (margherita.lellichiesa@univaq.it)
Alessandro Verra
Affiliation:
Dipartimento di Matematica, Università Roma Tre, Largo San Leonardo Murialdo, 00146Roma, Italy (verra@mat.uniroma3.it)

Abstract

Let ${\mathcal{F}}_{g}^{\mathbf{N}}$ be the moduli space of polarized Nikulin surfaces $(Y,H)$ of genus $g$ and let ${\mathcal{P}}_{g}^{\mathbf{N}}$ be the moduli of triples $(Y,H,C)$, with $C\in |H|$ a smooth curve. We study the natural map $\unicode[STIX]{x1D712}_{g}:{\mathcal{P}}_{g}^{\mathbf{N}}\rightarrow {\mathcal{R}}_{g}$, where ${\mathcal{R}}_{g}$ is the moduli space of Prym curves of genus $g$. We prove that it is generically injective on every irreducible component, with a few exceptions in low genus. This gives a complete picture of the map $\unicode[STIX]{x1D712}_{g}$ and confirms some striking analogies between it and the Mukai map $m_{g}:{\mathcal{P}}_{g}\rightarrow {\mathcal{M}}_{g}$ for moduli of triples $(Y,H,C)$, where $(Y,H)$ is any genus $g$ polarized $K3$ surface. The proof is by degeneration to boundary points of a partial compactification of ${\mathcal{F}}_{g}^{\mathbf{N}}$. These represent the union of two surfaces with four even nodes and effective anticanonical class, which we call half Nikulin surfaces. The use of this degeneration is new with respect to previous techniques.

Type
Research Article
Copyright
© Cambridge University Press 2019

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