Functions determining few direction: the final missing case

Source abstract

By results of Blokhuis, Ball, Brouwer, Storme and Szőnyi, and of Ball, if a function $f: {\mathbb F}_q\to{\mathbb F}_q$ determines fewer than $(q+3)/2$ directions, then $f-f(0)$ is linear over a subfield of ${\mathbb F}_q$, except possibly when $q$ is even and $f$ determines exactly $q/2+1$ directions. We settle this case: $f-f(0)$ is then additive. Hence every small minimal blocking set of Rédei type in PG(2,$q$) is linear.