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 A230556 #15 Oct 24 2021 04:33:49 %S A230556 1,1,2,4,9,21,51,128,327,858,2272,6146,16716,46246,128414,361493, %T A230556 1020506,2913060,8335405,24067930,69646035 %N A230556 Number of involutions avoiding the pattern 4231. %H A230556 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 A230556 Of the 26 involutions of length 5, only 53241, 42315, 52431, 52341, and 15342 contain the pattern 4231, so a(5) = 21. %Y A230556 Cf. A000085, A001006, A121704, A230551-A230555. %K A230556 nonn,more %O A230556 0,3 %A A230556 _Cheyne Homberger_, Oct 23 2013