On (q2+q+2, q+2)-arcs in the Projective Plane PG(2, q)

A (k; n)-arc in PG(2; q) is usually deened to be a set K of k points in the plane such that some line meets K in n points but such that no line meets K in more than n points. There is an extensive literature on the topic of (k; n)-arcs. Here we keep the same deenition but allow K to be a multiset, that is, permit K to contain multiple points. The case k = q 2 + q + 2 is of interest because it is the rst value of k for which a (k; n)-arc must be a multiset. The problem of classifying (q 2 + q + 2; q + 2)-arcs is of importance in coding theory, since it is equivalent to classifying 3-dimensional q-ary error-correcting codes of length q 2 + q + 2 and minimum distance q 2. Indeed, it was the coding theory problem which provided the initial motivation for our study. It turns out that such arcs are surprisingly rich in geometric structure. Here we construct several families of (q 2 + q + 2; q + 2)-arcs as well as obtain some bounds and non-existence results. A complete classiication of such arcs seems to be a diicult problem.