Action-angle and complex coordinates on toric manifolds

Abstract

In this article, we provide an exposition about symplectic toric manifolds, which are symplectic manifolds $(M^{2n}, \omega)$ equipped with an effective Hamiltonian $\mathbb{T}^n\cong (S^1)^n$-action. We summarize the construction of $M$ as a symplectic quotient of $\mathbb{C}^d$, the $\mathbb{T}^n$-actions on $M$ and their moment maps, and Guillemin's K\"ahler potential on $M$. While the theories presented in this paper are for compact toric manifolds, they do carry over for some noncompact examples as well, such as the canonical line bundle $K_M$, which is one of our main running examples, along with the complex projective space $\mathbb{P}^n$ and its canonical bundle $K_{\mathbb{P}^n}$. One main topic explored in this article is how to write the moment map in terms of the complex homogeneous coordinates $z\in \mathbb{C}^d$, or equivalently, the relationship between the action-angle coordinates and the complex toric coordinates. We end with a brief review of homological mirror symmetry for toric geometries, where the main connection with the rest of the paper is that $K_M$ provides a prototypical class of examples of a Calabi-Yau toric manifold $Y$ which serves as the total space of a symplectic fibration $W: Y \to \mathbb{C}$ with a singular fiber above $0$, known as a Landau-Ginzburg model in mirror symmetry. Here we write $W$ in terms of the action-angle coordinates, which will prove to be useful in understanding the geometry of the fibration in our forthcoming work [ACLL].