A New Blocking Semioval

Abstract

Let I1 = ( P, L) be a projective plane of order n . A blocking set in IT is a set B of points such that for every line 1 of fl there is at least one point of 1 in B , but 1 is not entirely contained in B . Blocking sets have been extensively studied, see for example, Berardi and Eugeni [2]. A semioval in II is a set S of points such that for every point P E S t here is a unique tangent to S containing P . Here, as usual, a tangent to S is a line of IT meeting S in exactly one point.