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 A137223 #16 Sep 08 2022 08:45:32
%S A137223 3,18,234,1110,15018,70998,961194,4543830,61516458,290805078,
%T A137223 3937053354,18611524950,251971414698,1191137596758,16126170540714,
%U A137223 76232806192470,1032074914605738,4878899596318038,66052794534767274,312249574164354390
%N A137223 a(n) = A135574(3*n) + A135574(3*n+1) + A135574(3*n+2).
%H A137223 G. C. Greubel, <a href="/A137223/b137223.txt">Table of n, a(n) for n = 0..1000</a>
%H A137223 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,64,64).
%F A137223 From _R. J. Mathar_, Feb 07 2009: (Start)
%F A137223 a(n) = -a(n-1) + 64*a(n-2) + 64*a(n-3).
%F A137223 a(n) = (-8*(-1)^n + 35*8^n + 9*(-8)^n)/12.
%F A137223 G.f.: 3*(1 +7*x +20*x^2)/((1-8*x)*(1+8*x)*(1+x)). (End)
%t A137223 LinearRecurrence[{-1,64,64}, {3,18,234}, 41] (* _G. C. Greubel_, Jan 05 2022 *)
%o A137223 (Magma) I:=[3,18,234]; [n le 3 select I[n] else -Self(n-1) +64*Self(n-2) +64*Self(n-3): n in [1..30]]; // _G. C. Greubel_, Jan 05 2022
%o A137223 (Sage) [((35 + 9*(-1)^n)*8^n - 8*(-1)^n)/12 for n in (0..40)] # _G. C. Greubel_, Jan 05 2022
%Y A137223 Cf. A135574.
%K A137223 nonn
%O A137223 0,1
%A A137223 _Paul Curtz_, Mar 07 2008
%E A137223 More terms from _R. J. Mathar_, Feb 07 2009