Though algebraic geometry over ℂ is often used to describe the closure of the tensors of a given size and complex rank, this variety includes tensors of both smaller and larger rank. Here we focus on the n × n × n tensors of rank n over ℂ, which has as a dense subset the orbit of a single tensor under a natural group action. We construct polynomial invariants under this group action whose nonvanishing distinguishes this orbit from points only in its closure. Together with an explicit subset of the defining polynomials of the variety, this gives a semialgebraic description of the tensors of rank n and multilinear rank (n,n,n). The polynomials we construct coincide with Cayley's hyperdeterminant in the case n=2 and thus generalize it. Though our construction is direct and explicit, we also recast our functions in the language of representation theory for additional insights. We give three applications in different directions: First, we develop basic topological understanding of how the real tensors of complex rank n and multilinear rank (n,n,n) form a collection of path-connected subsets, one of which contains tensors of real rank n. Second, we use the invariants to develop a semialgebraic description of the set of probability distributions that can arise from a simple stochastic model with a hidden variable, a model that is important in phylogenetics and other fields. Third, we construct simple examples of tensors of rank 2n-1 which lie in the closure of those of rank $n$.

Item Type:	Refereed Article
Keywords:	tensor rank, hyperdeterminant, border rank, latent class model, phylogenetics
Research Division:	Mathematical Sciences
Research Group:	Applied mathematics
Research Field:	Biological mathematics
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in the mathematical sciences
UTAS Author:	Jarvis, PD (Dr Peter Jarvis)
UTAS Author:	Sumner, JG (Associate Professor Jeremy Sumner)
ID Code:	87077
Year Published:	2013
Funding Support:	Australian Research Council (DE130100423)
Web of Science® Times Cited:	13
Deposited By:	Mathematics and Physics
Deposited On:	2013-11-06
Last Modified:	2017-01-10
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