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 A226431 #17 Oct 30 2018 04:26:33
%S A226431 1,2,6,21,73,244,786,2458,7510,22527,66579,194408,561988,1610900,
%T A226431 4584426,12966225,36476173,102132412,284785878,791182318,2190833086,
%U A226431 6048706947,16655647911,45752451536,125405039368,343040546984,936651104466,2553146783253,6948573570145
%N A226431 The number of permutations of length n in a particular geometric grid class.
%C A226431 This geometric grid class is given by the array [[0,0,1,0],[0,0,0,1],[0,1,-1,0],[1,0,0,-1]]. A picture is given in the LINKS section.
%C A226431 The simple permutations in this class are A226432.
%H A226431 Jay Pantone, <a href="http://arxiv.org/abs/1309.0832">The Enumeration of Permutations Avoiding 3124 and 4312</a>, arXiv:1309.0832 [math.CO], 2013-2015.
%H A226431 Jay Pantone, <a href="/A226431/a226431.png">Picture of the geometric grid class</a>
%H A226431 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (9,-31,51,-41,15,-2).
%F A226431 G.f.: x*(1-7*x+19*x^2-22*x^3+9*x^4-x^5)/((1-x)*(1-2*x)*(1-3*x+x^2)^2).
%F A226431 a(n) = 3*A001871(n-1)-A001871(n) +2*A001906(n) +2^(n-1)+1. - _R. J. Mathar_, Aug 31 2013
%t A226431 LinearRecurrence[{9, -31, 51, -41, 15, -2}, {1, 2, 6, 21, 73, 244}, 29] (* _Jean-François Alcover_, Oct 30 2018 *)
%o A226431 (PARI) x=x='x+O('x^66); Vec((x-7*x^2+19*x^3-22*x^4+9*x^5-x^6)/((1-x)*(1-2*x)*(1-3*x+x^2)^2) ) \\ _Joerg Arndt_, Jun 19 2013
%K A226431 nonn
%O A226431 1,2
%A A226431 _Jay Pantone_, Jun 06 2013