Tensor decompositions on simplicial complexes with invariance

TL;DR

This paper introduces a comprehensive framework for invariant tensor decompositions on simplicial complexes, unifying various existing tensor network methods and establishing new theoretical results on ranks and invariance properties.

Contribution

It develops a general theory of invariant tensor decompositions on simplicial complexes, proving existence, rank inequalities, and extending to nonnegative and positive semidefinite cases.

Findings

Existence of invariant decompositions after enriching the simplicial complex

Inequalities relating different tensor ranks under invariance constraints

Recovery of known tensor decompositions as special cases

Abstract

We develop a framework to analyse invariant decompositions of elements of tensor product spaces. Namely, we define an invariant decomposition with indices arranged on a simplicial complex, and which is explicitly invariant under a group action. We prove that this decomposition exists for all invariant tensors after possibly enriching the simplicial complex. As a special case we recover tensor networks with translational invariance and the symmetric tensor decomposition. We also define an invariant separable decomposition and purification form, and prove similar existence results. Associated to every decomposition there is a rank, and we prove several inequalities between them. For example, we show by how much the rank increases when imposing invariance in the decomposition, and that the tensor rank is the largest of all ranks. Finally, we apply our framework to nonnegative tensors, where we define a nonnegative and a positive semidefinite decomposition on arbitrary simplicial complexes with group action. We show a correspondence to the previous ranks, and as a very special case recover the nonnegative, the positive semidefinite, the completely positive and the completely positive semidefinite transposed decomposition.