错误:搜索内容不能为空,请输入英文关键词
错误:关键词超出字数限制,请精简
高级检索

Abelian covers and the second fundamental form

  • Paola Frediani

摘要

We give some conditions on a family of abelian covers of \({\mathbb P}^1\) P 1 of genus g curves, that ensure that the family yields a subvariety of \({\mathsf A}_g\) A g which is not totally geodesic, hence it is not Shimura. As a consequence, we show that for any abelian group G, there exists an integer M which only depends on G such that if \(g >M\) g > M , then the family yields a subvariety of \({\mathsf A}_g\) A g which is not totally geodesic. We prove then analogous results for families of abelian covers of \({\tilde{C}}_t \rightarrow {\mathbb P}^1 = {\tilde{C}}_t/{\tilde{G}}\) C ~ t P 1 = C ~ t / G ~ with an abelian Galois group \({\tilde{G}}\) G ~ of even order, proving that under some conditions, if \(\sigma \in {\tilde{G}}\) σ G ~ is an involution, the family of Pryms associated with the covers \({\tilde{C}}_t \rightarrow C_t= {\tilde{C}}_t/\langle \sigma \rangle \) C ~ t C t = C ~ t / σ yields a subvariety of \({\mathsf A}_{p}^{\delta }\) A p δ which is not totally geodesic. As a consequence, we show that if \({\tilde{G}}=(\mathbb Z/N\mathbb Z)^m\) G ~ = ( Z / N Z ) m with N even, and \(\sigma \) σ is an involution in \({\tilde{G}}\) G ~ , there exists an integer M(N) which only depends on N such that, if \({\tilde{g}}= g({\tilde{C}}_t) > M(N)\) g ~ = g ( C ~ t ) > M ( N ) , then the subvariety of the Prym locus in \({{\mathsf A}}^{\delta }_{p}\) A p δ induced by any such family is not totally geodesic (hence it is not Shimura).