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 A111586 #13 May 18 2020 11:32:11 %S A111586 0,1,2,5,12,28,62,136,299,661,1456,3213,7085,15628,34467,76021,167668, %T A111586 369805,815629,1798928,3967659,8750949,19300824,42569309,93889565, %U A111586 207079956,456729219,1007348005,2221775964,4900281149,10807910301,23837596568,52575474283,115958858869 %N A111586 G.f.: -x*(1+x+x^2+2*x^3+3*x^4+3*x^5+x^6-x^7-x^9)/((1+x)*(-1+2*x+x^3)). %D A111586 V. Vatter, The Enumeration and Structure of Permutation Classes, Ph. D. Dissertation, Rutgers Univ., Math. Dept., 2006. %H A111586 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,1,1). %F A111586 a(n) = 2*a(n-1) + a(n-3) - 2*(-1)^n for n >= 10. - _Greg Dresden_, May 18 2020 %K A111586 nonn %O A111586 0,3 %A A111586 _N. J. A. Sloane_, Nov 17 2005