Connections between Combinations Without Specified Separations and Strongly Restricted Permutations, Compositions, and Bit Strings

Abstract: Let $S_n$ and $S_{n,k}$ be, respectively, the number of subsets and $k$-subsets of $\mathbb{N}n={1,\ldots,n}$ such that no two subset elements differ by an element of the set $\mathcal{Q}$, the largest element of which is $q$. We prove a bijection between such $k$-subsets when $\mathcal{Q}={m,2m,\ldots,jm}$ with $j,m>0$ and permutations $\pi$ of $\mathbb{N}{n+jm}$ with $k$ excedances satisfying $\pi(i)-i\in{-m,0,jm}$ for all $i\in\mathbb{N}{n+jm}$. We also identify a bijection between another class of restricted permutation and the cases $\mathcal{Q}={1,q}$. This bijection allows us to prove a conjectured recursion relation for the number of such permutations which corresponds to the case $\mathcal{Q}={1,4}$. We also obtain the generating function for $S_n$ in the case $\mathcal{Q}={1,5}$ by first obtaining generating functions for the numbers of closed walks of a given length on a particular class of directed pseudograph. We give some classes of $\mathcal{Q}$ for which $S_n$ is also the number of compositions of $n+q$ into a given set of allowed parts. A bijection between the $k$-subsets for any $\mathcal{Q}$ and bit strings is also noted. Aided by this, an efficient algorithm for finding $S_n$ and $S{n,k}$ is given. We also prove a bijection between $k$-subsets for a class of $\mathcal{Q}$ and the set representations of size $k$ of equivalence classes for the occurrence of a given length-($q+1$) subword within bit strings. We then formulate a straightforward procedure for obtaining the generating function for the number of such equivalence classes.