On Elementary Abelian Cartesian Groups

J. Hayden [2] proved that, if a finite abelian group is a Cartesian group satisfying a certain "homogeneity condition", then it must be an elementary abelian group. His proof required the character theory of finite abelian groups. In this note we present a shorter, elementary proof of his result.

Elementary Abelian Cartesian Groups Cartesian Groups

Throughout the paper, G will denote an additively written, but not always abelian, group of finite order n; and X = (xij) will denote a square matrix of order n with entries from G and whose rows and columns are numbered 0, 1, …, n − 1. We call X a cartesian array (afforded by G) if(1.1) The sequence {−xmi + xki, i = 0,…, n – 1} contains all elements of G whenever k ≠ m.By a theorem of Jungnickel (see Theorem 2.2 in [5]), the transpose of a cartesian array is also a cartesian array. We call G a cartesian group if there is a cartesian array X afforded by G. In this case, we also call (G, X) a cartesian pair.

Collineations of Affine Moulton Planes

In a recent article on Moulton planes (8), I have generalized the non-Desarguesian planes introduced by F. R. Moulton (6) and Pickert (7, pp. 93-94). Let F = (0, 1; a, b, . . . , x, y, . . .} denote a given field, having P as a multiplicative subgroup of index 2. Define x > 0 or x < 0 according as x lies in P or in the other coset of non-zero elements. The function ϕ is order-preserving, and can be assumed to satisfy ϕ(0) = 0, ϕ(1) = 1. For any n < 0, the maps x —> ϕ(x) and x —> ϕ(x) — nx both carry F onto itself. The elements of F form a Cartesian group, {+,0} being defined so that