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 A193775 #6 Mar 31 2012 10:30:53
%S A193775 1,2,3,4,10,12,52,58,394,418
%N A193775 Number of signed permutations invariant under the reverse complement and avoiding (-2, -1), (-1, 2), and (1, -2).
%F A193775 a(n) = (\lfloor n/2\rfloor)! 2^{\lfloor n/2\rfloor} + \sum_{k=0}^{\lfloor n/2\lceil} k!
%e A193775 For n = 3, the permutations are (1, 2, 3), (3, 2, 1), (-1, -2, -3), and (-3, 2, -1).
%K A193775 nonn
%O A193775 0,2
%A A193775 _Andy Hardt_, Aug 04 2011