Characterizing pyramidal Hadamard designs with the largest number of fixed points

Abstract

A symmetric $(v,k,\lambda )$-design is said to be $f$-pyramidal, with $f<v-1$, under the action of a group $G$ if $G$ acts as an automorphism group fixing $f$ points and acting sharply transitively on the remaining ones. We show that, necessarily, $f\le v-2(k-\lambda )$. In particular, for a Hadamard design with parameters $(v,k,\lambda )=(4m-1,2m-1,m-1)$, for some $m\in \mathbb{N}$, it follows that $f\le 2m-1$. Recently, working on the complement design, the family $P$ of Hadamard $(4m-1,2m-1,m-1)$-designs admitting an abelian $(2m-1)$-pyramidal automorphism group $G$ has been completely determined in the case $m=2^k>1$. In this paper, we generalize that result by showing that $P$ coincides with the family of Hadamard $(4m-1,2m-1,m-1)$-designs admitting a $(2m-1)$-pyramidal automorphism group, without assuming either that the group is abelian or that $m$ is a power of $2$.

8 pages