Generic orbit recovery from invariants of very low degree

Abstract

Motivated by the multi-reference alignment (MRA) problem and questions in equivariant neural networks we study the problem of recovering the generic orbit in a representation of a finite group from invariant tensors of degree at most three. We explore the similarities and differences between the descriptive power of low degree polynomial and unitary invariant tensors and provide evidence that in many cases of interest they have similar descriptive power. In particular we prove that for the regular representation of a finite group, polynomial invariants of degree at most three separate generic orbits answering a question posed in \cite{bandeira2017estimation}. This complements a previously known result for unitary invariants~\cite{smach2008generalized}. We also investigate these questions for subregular representations of finite groups and prove that for the defining representation of the dihedral group, polynomial invariants of degree at most three separate generic orbits. This answers a question posed in~\cite{bendory2022dihedral} and it implies that the sample complexity of the corresponding MRA problem is $\sim \sigma^6$. On the other hand we also show that for the groups $D_n$ and $A_4$ generic orbits in the {\em complete multiplicity-free} representation cannot be separated by invariants of degree at most three.