Non-symmetric class $2$ association schemes obtained by doubling of skew-Hadamard matrices are non-schurian

Abstract

We can obtain a non-symmetric class $2$ association scheme by a skew-Hadamard matrix. We begin with a skew-Hadamard matrix of order $n$, construct a skew-Hadamard matrix of order $2n$ by doubling construction, and a non-symmetric class $2$ association scheme of order $2n-1$. We will show that the association scheme obtained in this way never be schurian if $n$ is greater than or equal to $8$.

Ilia Ponomarenko pointed out that the main result is not new. 6 pages