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 A226432 #20 Jul 21 2018 10:05:07
%S A226432 1,2,0,2,3,7,13,25,46,84,151,269,475,833,1452,2518,4347,7475,12809,
%T A226432 21881,37274,63336,107375,181657,306743,517057,870168,1462250,2453811,
%U A226432 4112479,6884101,11510809,19226950,32084028,53489287,89097893,148290067,246615425,409835844,680609086
%N A226432 The number of simple permutations of length n in a particular geometric grid class.
%C A226432 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 A226432 The sequence of all permutations in this class is given by A226431.
%H A226432 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).
%H A226432 Jay Pantone, <a href="/A226432/a226432.png">Picture of the geometric grid class</a>
%H A226432 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-2,-1).
%F A226432 G.f.: x+2*x^2+ x^4*(1-x)*(2+x)/(1-x-x^2)^2 (corrected, _Joerg Arndt_, Jun 26 2013)
%F A226432 a(n) = A191830(n+2)-A000045(n+2), n>=4. - _R. J. Mathar_, Aug 31 2013
%t A226432 Join[{1, 2}, LinearRecurrence[{2, 1, -2, -1}, {0, 2, 3, 7}, 40]] (* _Jean-François Alcover_, Jul 21 2018 *)
%o A226432 (PARI) x='x+O('x^66); Vec(x+2*x^2+(x^4*(1-x)*(2+x))/((1-x-x^2)^2) ) \\ _Joerg Arndt_, Jun 19 2013
%K A226432 nonn,easy
%O A226432 1,2
%A A226432 _Jay Pantone_, Jun 06 2013