On the automorphism groups of rank-4 primitive coherent configurations

TL;DR

This paper proves Babai's conjecture for the automorphism groups of rank-4 primitive coherent configurations, showing they have large minimal degree proportional to the number of vertices, using structural and spectral methods.

Contribution

It extends Babai's lower bound on minimal degree from rank-3 to rank-4 primitive coherent configurations, confirming the conjecture in this case.

Findings

Automorphism groups of rank-4 primitive coherent configurations have minimal degree at least a constant times the number of vertices.

The proof combines structural and spectral techniques.

Confirms Babai's conjecture for rank-4 case.

Abstract

The minimal degree of a permutation group $G$ is the minimum number of points not fixed by non-identity elements of $G$. Lower bounds on the minimal degree have strong structural consequences on $G$. Babai conjectured that if a primitive coherent configuration with $n$ vertices is not a Cameron scheme, then its automorphism group has minimal degree $\geq cn$ for some constant $c>0$. In 2014, Babai proved the desired lower bound on the minimal degree of the automorphism groups of strongly regular graphs, thus confirming the conjecture for primitive coherent configurations of rank 3. In this paper, we extend Babai's result to primitive coherent configurations of rank 4, confirming the conjecture in this special case. The proofs combine structural and spectral methods.