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 A012886 #12 Jul 08 2025 02:47:28
%S A012886 1,2,3,8,30,121,493,2011,8204,33469,136540,557028,2272449,9270673,
%T A012886 37820597,154292742,629451995,2567909604,10476032782,42737977489,
%U A012886 174353665921,711292452431,2901785575960,11838111735997
%N A012886 a(n+3) = 5*a(n+2)-4*a(n+1)+a(n).
%H A012886 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-4,1).
%F A012886 G.f. ( -1+3*x+3*x^2 ) / ( -1+5*x-4*x^2+x^3 ). a(n) = -3*A012814(n)+A012814(n+1) -3*A012814(n-1). - _R. J. Mathar_, Sep 20 2012
%t A012886 LinearRecurrence[{5,-4,1},{1,2,3},30] (* _Harvey P. Dale_, Sep 16 2023 *)
%Y A012886 Cf. A012855.
%K A012886 nonn,easy
%O A012886 0,2
%A A012886 _N. J. A. Sloane_