A Common Characterization of Finite Projective Spaces and Affine Planes

Abstract

Let S be a finite linear space for which there is a non-negative integer s such that for any two disjoint lines L , L ' of S and any point p outside L and L ' there are exactly s lines through p intersecting the two lines L and L '. We prove that one of the following possibilities occurs: (i) S is a generalized projective space, and if the dimension of S is at least 4, then any line of S has exactly two points. (ii) S is an affine plane, an affine plane with one improper point, or a punctured projective plane. (iii) S is the Fano-quasi -plane.