Neighborly boxes and strings with jokers; constructions and asymptotics

Abstract

We study families of axis-aligned boxes in a $d$-dimensional Euclidean space ${\mathbb{R}}^d$ whose placement is restricted by bounds on the dimension of their pairwise intersections. More specifically, two such boxes in ${\mathbb{R}}^d$ are said to be \emph{$k$-neighborly} if their intersection has dimension at least $d-k$ and at most $d-1$. The maximum number of pairwise $k$-neighborly boxes in ${\mathbb{R}}^d$ is denoted by $n(k,d)$. It is known that $n(k,d)=\hat{I}˜(d^k)$, for fixed $1⩽k⩽d$, however, exact formulas are known only in three cases: $k=1$, $k=d-1$, and $k=d$. In particular, the equality $n(1,d)=d+1$ is equivalent to the famous theorem of Graham and Pollak concerning partitions of complete graphs into complete bipartite graphs. In our main result we give a new construction of families of $k$-neighborly boxes which improves the lower bound for $n(k,d)$ when $k$ is close to $d$. Together with some recent upper bounds on $n(k,d)$, it gives the asymptotic equality $n(d-s,d)\sim \frac{2^s+1}{2^{s+1}}\cdot 2^d$, for every fixed $s⩽d/2$. In our constructions we use a familiar interpretation of the problem in the language of Hamming cubes represented by binary strings with a special blank symbol, called \emph{joker}.