Abstract
The quintic threefold X is the most studied Calabi-Yau 3-fold in the mathematics literature. In this article, using Čech-to-derived spectral sequences, we investigate the mod 2 and integral cohomology groups of a real Lagrangian , obtained as the fixed locus of an anti-symplectic involution in the mirror to X. We show that is the disjoint union of a 3-sphere and a rational homology sphere. Analyzing the mod 2 cohomology further, we deduce a correspondence between the mod 2 Betti numbers of and certain counts of integral points on the base of a singular torus fibration on X. By work of Batyrev, this identifies the mod 2 Betti numbers of with certain Hodge numbers of X. Furthermore, we show that the integral cohomology groups of are 2-primary for , we conjecture that this holds in much greater generality.
Acknowledgments
We thank to Mark Gross and Tom Coates for many useful and inspiring conversations.