Generalized ovals, 2.5-dimensional additive codes, and multispreads

Abstract: We present constructions and bounds for additive codes over a finite field in terms of their geometric counterpart, i.e.\ projective systems. It is known that the maximum number of $(l-1)$-spaces in $\operatorname{PG}(2,q)$, such that no hyperplane contains three, is given by $q^l+1$ if $q$ is odd. Those geometric objects are called generalized ovals. We show that cardinality $q^l+2$ is possible if we decrease the dimension a bit. We completely determine the minimum possible lengths of additive codes over $\mathbb{F}_9$ of dimension $2.5$ and give improved constructions for other small parameters. As an application, we consider multispreads in $\operatorname{PG}(4,q)$, in particular, completing the characterization of parameters of $\mathbb{F}_4$-linear $64$-ary one-weight codes.