On strongly walk regular graphs, triple sum sets and their codes

TL;DR

This paper studies strongly walk regular graphs, their connection to projective three-weight codes, and classifies feasible parameters for binary and ternary cases, revealing structural properties and conjecturing limitations for certain eigenvalue configurations.

Contribution

It classifies feasible parameters of strongly walk regular graphs derived from projective three-weight codes and analyzes eigenvalue constraints using algebraic geometry methods.

Findings

Classified feasible parameters for binary and ternary cases.

Proved divisibility properties of code weights in the binary case.

Showed that for s=5 and s=7, only obvious solutions exist for eigenvalue equations.

Abstract

Strongly walk regular graphs (SWRGs or $s$-SWRGs) form a natural generalization of strongly regular graphs (SRGs) where paths of length~2 are replaced by paths of length~$s$. They can be constructed as coset graphs of the duals of projective three-weight codes whose weights satisfy a certain equation. We provide classifications of the feasible parameters of these codes in the binary and ternary case for medium size code lengths. For the binary case, the divisibility of the weights of these codes is investigated and several general results are shown. It is known that an $s$-SWRG has at most 4 distinct eigenvalues $k > \theta_1 > \theta_2 > \theta_3$, and that the triple $(\theta_1, \theta_2, \theta_3)$ satisfies a certain homogeneous polynomial equation of degree $s - 2$ (Van Dam, Omidi, 2013). This equation defines a plane algebraic curve; we use methods from algorithmic arithmetic geometry to show that for $s = 5$ and $s = 7$, there are only the obvious solutions, and we conjecture this to remain true for all (odd) $s \ge 9$.