Cheryl E. Praeger, Jing Xu
November 6, 2008
We prove that, for a positive integer n and subgroup H of automorphisms of a cyclic group Z of order n , there is up to isomorphism a unique connected circulant digraph based on Z admitting an arc-transitive action of Z ⋊ H . We refine the Kovács–Li classification of arc-transitive circulants to determine those digraphs with automorphism group larger than Z ⋊ H . As an application we construct, for each prime power q , a digraph with q – 1 vertices and automorphism group equal to the semilinear group ΓL(1, q ), thus proving that ΓL(1, q ) is 2-closed in the sense of Wielandt.