An Algorithm to compute the Kronecker cone and other moment cones

Abstract: We describe an algorithm that computes the minimal list of inequalities for the moment cone of any representation of a complex reductive group. We give more details for the Kronecker cone (that is the asymptotic support of the Kronecker coefficients) and the fermionic cone. These two cases correspond to the action of $GL_{d_1}(\mathbb C)\times\cdots\times GL_{d_s}(\mathbb C)$ on $\mathbb C^{d_1}\otimes\cdots\otimes \mathbb C^{d_s}$ and $GL_d(\mathbb C)$ on $\bigwedge^r\mathbb C^d$, respectively. These two cases are implemented in Python-Sage. Available at https://ea-icj.github.io/, our algorithm improves in several directions, the one given by Vergne-Walter in Inequalities for Moment Cones of Finite-Dimensional Representations. For example, it can compute the minimal list of 463 inequalities for the Kronecker cone $(5,5,5)$ in 20 minutes. An important ingredient is an algorithm that decides whether an application is birational or not. It could be of independent interest in effective algebraic geometry.