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 A096019 #17 Feb 10 2024 13:58:36
%S A096019 3,7,23,67,203,607,1823,5467,16403,49207,147623,442867,1328603,
%T A096019 3985807,11957423,35872267,107616803,322850407,968551223,2905653667,
%U A096019 8716961003,26150883007,78452649023,235357947067,706073841203,2118221523607
%N A096019 a(0)=3, a(n) = 3*a(n-1) + 2*(-1)^n.
%C A096019 The number of Pythagorean quadruples mod 3^n is given by a(n) 3^(2n-1). See A096018.
%H A096019 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,3).
%F A096019 a(n) = 3^(n+1)-(3^n-(-1)^n)/2.
%F A096019 a(n) = 2*a(n-1)+3*a(n-2). G.f.: (3+x)/((1+x)*(1-3*x)). [_Colin Barker_, Mar 26 2012]
%t A096019 LinearRecurrence[{2,3},{3,7},30] (* _Harvey P. Dale_, Feb 10 2024 *)
%Y A096019 Cf. A096018.
%K A096019 nonn,easy
%O A096019 0,1
%A A096019 _T. D. Noe_, Jun 15 2004