A note on periods of Calabi--Yau fractional complete intersections

Abstract: We prove that the GKZ $\mathscr{D}$-module $\mathcal{M}{A}^{\beta}$ arising from Calabi--Yau fractional complete intersections in toric varieties is complete, i.e., all the solutions to $\mathcal{M}{A}^{\beta}$ are period integrals. This particularly implies that $\mathcal{M}_{A}^{\beta}$ is equivalent to the Picard--Fuchs system. As an application, we give explicit formulae of the period integrals of Calabi--Yau threefolds coming from double covers of $\mathbf{P}^{3}$ branch over eight hyperplanes in general position.