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 A137213 #14 Sep 08 2022 08:45:32
%S A137213 0,1,4,15,52,173,560,1779,5576,17305,53308,163287,497980,1513541,
%T A137213 4587944,13878075,41910032,126395953,380795380,1146267039,3448170436,
%U A137213 10367130845,31156000928,93599839107,281117798360,844121793481
%N A137213 First differences of A137212.
%H A137213 G. C. Greubel, <a href="/A137213/b137213.txt">Table of n, a(n) for n = 0..1000</a>
%H A137213 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-5,-3).
%F A137213 From _R. J. Mathar_, Mar 17 2008: (Start)
%F A137213 O.g.f.: 1/(1-3*x) - 1/(1-2*x-x^2) = x*(1-x)/( (1-3*x)*(1-2*x-x^2) ).
%F A137213 a(n) = 3^n - A000129(n+1). (End)
%t A137213 LinearRecurrence[{5,-5,-3},{0,1,4},30] (* _Harvey P. Dale_, Apr 18 2019 *)
%o A137213 (Magma) [n le 3 select (n-1)^2 else 5*Self(n-1) -5*Self(n-2) -3*Self(n-3): n in [1..31]]; // _G. C. Greubel_, Jan 05 2022
%o A137213 (Sage) [3^n - lucas_number1(n+1,2,-1) for n in (0..30)] # _G. C. Greubel_, Jan 05 2022
%Y A137213 Cf. A000129, A132894, A137212.
%K A137213 nonn
%O A137213 0,3
%A A137213 _Paul Curtz_, Mar 06 2008
%E A137213 More terms from _R. J. Mathar_, Mar 17 2008