Maximum scattered linear sets and complete caps in Galois spaces

Abstract

Explicit constructions of infinite families of scattered ${\mathbb{F}}_q$--linear sets in $PG(r-1,q^t)$ of maximal rank $\frac{rt}{2}$, for $t$ even, are provided. When $q=2$ and $r$ is odd, these linear sets correspond to complete caps in $AG(r,2^t)$ fixed by a translation group of size $2^{\frac{rt}{2}}$. The doubling construction applied to such caps gives complete caps in $AG(r+1,2^t)$ of size $2^{\frac{rt}{2}+1}$. For Galois spaces of even dimension greater than $2$ and even square order, this solves the long-standing problem of establishing whether the theoretical lower bound for the size of a complete cap is substantially sharp.