Published online by Cambridge University Press: 20 November 2018
Let be a projective plane and
a subplane of
. If l is a line of
, we let
denote the group of all elations in
that have
as axis and leave Q invariant. In [12, p. 921], Ostrom asked for a description of all finite planes
that have a Baer subplane
with the property that
for all lines l of
. Here
denotes the order of G. Both the desarguesian planes of square order and the generalized Hughes planes have this property (Hughes [10], Ostrom [14], Dembowski [6]). One of the aims of this paper is to show that these are the only planes having such a Baer subplane.