Symmetric subrank and its border analogue
This work extends asymptotic rank theory to symmetric tensors, providing foundational results for the study of polynomial complexity and invariant theory.
The paper determines the asymptotic behavior of symmetric subrank and symmetric border subrank for degree-d forms as the number of variables grows, and shows that for cubic and quartic forms these ranks coincide when the border subrank is small.
The symmetric subrank of homogeneous polynomial is the largest number of terms in a diagonal form to which it can be specialized by a (typically non-invertible) linear variable substitution. Building on earlier work by Derksen-Makam-Zuiddam and Biaggi-Chang-Draisma-Rupniewski for ordinary tensors, we determine the asymptotic behavior of symmetric subrank and symmetric border subrank of degree-d forms as the number of variables tends to infinity. Furthermore, by using results from geometric invariant theory we show that for cubic (resp. quartic) forms the symmetric subrank and symmetric border subrank coincide if the latter is at most three (resp. two).