Are there more circular Langford pairings than Langford pairings? – mathoverflow.net
Pranay Gorantla
A Langford pairing is an arrangement of $2n$ numbers, $1, 1, 2, 2, \ldots, n, n$, in a row such that there are $k$ numbers between the two $k$s for any $k=1,2,\ldots,n$. These are well studied and are ...