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 A367537 #16 Jul 09 2024 08:48:04 %S A367537 0,2,48,2652,251328,34213080,6362490816 %N A367537 a(n) is the number of ways to make a Secret Santa assignment among n couples such that no one gets their partner's name and if A gets B's name, B does not get A's name. %C A367537 Equivalent to the number of derangements of [2n] such that (i) there are no 2-cycles, and (ii) neither k nor k+n maps to the other (k <= n). %C A367537 A367538(n) <= a(n) <= A038205(2n) <= A000166(2n), and all the inequalities are strict except for n = 1. %C A367537 Inspired by a question from a coworker about enumerating all possible Secret Santa assignments for an actual Secret Santa exchange for Xmas 2023, subject to these constraints. %Y A367537 Cf. A000166, A038205, A367538. %K A367537 nonn,more %O A367537 1,2 %A A367537 _Jeremy F. Alm_, Nov 21 2023