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 A230555 #12 Oct 28 2013 13:30:31 %S A230555 1,1,2,4,10,25,66,173,460,1218,3240,8602,22878,60794,161668,429752, %T A230555 1142758,3038173,8078606,21479469,57113888 %N A230555 Number of involutions avoiding 3421. %H A230555 Miklós Bóna, Cheyne Homberger, Jay Pantone, and Vince Vatter, <a href="http://arxiv.org/abs/1310.7003">Pattern-avoiding involutions: exact and asymptotic enumeration</a>, arxiv:1310.7003, 2013. %e A230555 Of the 26 involutions of length 5, only 45312 contains the pattern 3421, so a(5) = 25. %Y A230555 Cf. A000085, A001006, A121704, A230551-A230556. %K A230555 nonn %O A230555 0,3 %A A230555 _Cheyne Homberger_, Oct 23 2013