Simeon Ball András Gács Péter Sziklai On the number of directions determined by a pair of functions over a prime field. 505-516 2008 115 J. Comb. Theory, Ser. A 3 http://dx.doi.org/10.1016/j.jcta.2007.08.001 db/journals/jct/jcta115.html#BallGS08