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.
%I A380394 #17 Jan 28 2025 21:54:43
%S A380394 1,1,2,6,22,94,426,1938,8724,38724,169438,731390,3119052,13162228
%N A380394 a(n) = number of possible pairs of descent sets of a permutation of 1,2,...,n and its inverse.
%H A380394 Richard Stanley, <a href="https://mathoverflow.net/q/486548">Descent sets and inverse descent sets of a permutation</a>, MathOverflow, 2025.
%e A380394 For n=3 there are the six pairs (empty,empty), (1,1), (1,2), (2,1), (2,2), (12,12).
%o A380394 (Sage) def a380394(n): return len({ (tuple(p.descents()), tuple(p.idescents())) for p in Permutations(n) }) # _Max Alekseyev_, Jan 24 2025
%K A380394 nonn,more
%O A380394 0,3
%A A380394 _Richard Stanley_, Jan 23 2025
%E A380394 a(0)=1 prepended by _Alois P. Heinz_, Jan 25 2025
%E A380394 a(11)-a(13) from _Max Alekseyev_, Jan 28 2025