# A Geometric Description of Feasible Singular Values in the Tensor Train Format

## TL;DR

This paper characterizes the set of feasible singular values in tensor train formats, linking it to quantum marginal problems and eigenvalue inequalities, and provides algorithms for feasibility checking and tensor construction.

## Contribution

It establishes a geometric description of feasible singular values in tensor trains, connecting it to quantum marginal problems and eigenvalue inequalities, with practical algorithms.

## Key findings

- Feasible TT-singular values form polyhedral cones in three dimensions.
- The tensor feasibility problem is equivalent to the quantum marginal problem.
- Algorithms for checking feasibility and constructing tensors with prescribed singular values are provided.

## Abstract

Tree tensor networks such as the tensor train format are a common tool for high dimensional problems. The associated multivariate rank and accordant tuples of singular values are based on different matricizations of the same tensor. While the behavior of such is as essential as in the matrix case, here the question about the $\textit{feasibility}$ of specific constellations arises: which prescribed tuples can be realized as singular values of a tensor and what is this feasible set? We first show the equivalence of the $\textit{tensor feasibility problem (TFP)}$ to the $\textit{quantum marginal problem (QMP)}$. In higher dimensions, in case of the tensor train (TT-)format, the conditions for feasibility can be decoupled. By present results for three dimensions for the QMP, it then follows that the tuples of squared, feasible TT-singular values form polyhedral cones. We further establish a connection to eigenvalue relations of sums of Hermitian matrices, which in turn are described by sets of interlinked, so called $\textit{honeycombs}$, as they have been introduced by Knutson and Tao. Besides a large class of universal, necessary inequalities as well as the vertex description for a special, simpler instance, we present a linear programming algorithm to check feasibility and a simple, heuristic algorithm to construct representations of tensors with prescribed, feasible TT-singular values in parallel.