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 A137199 #12 Jan 02 2024 08:58:05
%S A137199 1,1,1,5,9,25,57,141,337,817,1969,4757,11481,27721,66921,161565,
%T A137199 390049,941665,2273377,5488421,13250217,31988857,77227929,186444717,
%U A137199 450117361,1086679441,2623476241,6333631925,15290740089,36915112105,89120964297
%N A137199 a(n)=a(n-1)+3a(n-2)+a(n-3).
%H A137199 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,3,1)
%F A137199 O.g.f.: (-1+3*x^2)/{(1+x)(x^2+2*x-1)} . a(n) = (-1)^(n+1)+2*A000129(n-1) if n>=1. - _R. J. Mathar_, Mar 17 2008
%t A137199 LinearRecurrence[{1,3,1},{1,1,1},40] (* _Harvey P. Dale_, Feb 27 2020 *)
%Y A137199 Cf. A097076.
%K A137199 nonn,easy
%O A137199 0,4
%A A137199 _Paul Curtz_, Mar 04 2008