Extension of Period Maps by Polyhedral Fans

TL;DR

This paper explores extending period maps using polyhedral fans, constructing a specific fan for a Calabi-Yau family, and disproving the existence of complete fans in some cases, advancing understanding of compactifications in Hodge theory.

Contribution

It constructs a specific fan for a Calabi-Yau family and demonstrates the non-existence of complete fans in certain cases, challenging previous conjectures.

Findings

Constructed a fan compactifying the period map for a specific Calabi-Yau family.

Disproved the existence of complete fans in some general cases, including the studied example.

Provided insights into the limitations of fan-based compactifications in Hodge theory.

Abstract

Kato and Usui developed a theory of partial compactifications for quotients of period domains D by arithmetic groups {\Gamma}, in an attempt to generalize the toroidal compactifications of Ash-Mumford-Rapoport-Tai to non-classical cases. Their partial compactifications, which aim to fully compactify the images of period maps, rely on a choice of fan which is strongly compatible with {\Gamma}. In particular, they conjectured the existence of a complete fan, which would serve to simultaneously compactify all period maps of a given type. In this paper, we briefly review the theory, and construct a fan which compactifies theimage of a period map arising from a particular two-parameter family of Calabi-Yau threefolds studied by Hosono and Takagi, with Hodge numbers (1,2,2,1). On the other hand, we disprove the existence of complete fans in some general cases, including the (1,2,2,1) case.