A151587 Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 6 adjacent element pairs in decreasing order.
0, 4, 11565, 2580588, 229630758, 12719963448, 531411968223, 18510889687040
Offset: 3
This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.