Rigid polyboxes and Keller's conjecture

Abstract: A cube tiling of R^d is a family of pairwise disjoint cubes $[0,1)^d+T={[0,1)^d+t:t\in T}$ such that $\bigcup_{t\in T}([0,1)^d+t)=R^d$. Two cubes $[0,1)^d+t$, $[0,1)^d+s$ are called a twin pair if their closures have a complete facet in common, that is if $|t_j-s_j|=1$ for some $j\in [d]={1,..., d}$ and $t_i=s_i$ for every $i\in [d]\setminus {j}$. In 1930, Keller conjectured that in every cube tiling of R^d there is a twin pair. Keller's conjecture is true for dimensions $d\leq 6$ and false for all dimensions $d\geq 8$. For $d=7$ the conjecture is still open. Let $x\in R^d$, $i\in [d]$, and let L(T,x,i) be the set of all $i$th coordinates $t_i$ of vectors $t\in T$ such that $([0,1)^d+t)\cap ([0,1]^d+x)\neq \emptyset$ and $t_i\leq x_i$. Let $r^-(T)=\min_{x\in R^d}\; \max_{1\leq i\leq d}|L(T,x,i)|$ and $r^+(T)=\max_{x\in R^d}\; \max_{1\leq i\leq d}|L(T,x,i)|$. It is known that Keller's conjecture is true in dimension seven for cube tilings $[0,1)^7+T$ for which $r^-(T)\leq 2$. In the present paper we show that it is also true for $d=7$ if $r^+(T)\geq 6$. Thus, if $[0,1)^d+T$ is a counterexample to Keller's conjecture in dimension seven, then $r^-(T),r^+(T)\in {3,4,5}$.