Determination of hyperovals by lines through a few points

Abstract

If S is a set of q+2 points in P^2(F_q) such that some point of S is not on any line containing two other points of S, then in suitable coordinates S has the form S_f:={(c:f(c):1) : c in F_q} U {(1:0:0),(0:1:0)} for some f(X) in F_q[X]. Let T be a subset of S_f which contains the two infinite points and at least 3+log_3(q)/4 finite points. We show that if there is no line passing through a point of T and two other points of S_f, and deg(f)<=q^{1/4}, then no three points of S_f are collinear, so that S_f is a hyperoval. We also determine all f(X) with deg(f)<=q^{1/4} for which S_f is a hyperoval, which strengthens a result that was proved by Caullery and Schmidt using entirely different methods.