Let $\mathcal{ K}$ be a discrete valued field with finite residue field. In analogy with orthogonality in the Euclidean space $\mathbb{ R}^n$, there is a well-studied notion of "ultrametric orthogonality" in $\mathcal{ K}^n$. In this paper, motivated by a question of Erd{ \H{ o}}s in the real case, given integers $k \geq \ell \geq 2$, we investigate the maximum size of a subset $S \subseteq \mathcal{ K}^n \setminus\{ { \bf 0}\}$ satisfying the following property: for any $E \subseteq S$ of size $k$, there exists $F \subseteq E$ of size $\ell$ such that any two distinct vectors in $F$ are orthogonal. Other variants of this property are also studied.
Categories: math.NT math.AC math.CO