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 A123720 #22 Jun 08 2025 01:12:47
%S A123720 2,4,9,20,43,90,185,376,759,1526,3061,6132,12275,24562,49137,98288,
%T A123720 196591,393198,786413,1572844,3145707,6291434,12582889,25165800,
%U A123720 50331623,100663270,201326565,402653156,805306339,1610612706,3221225441,6442450912,12884901855
%N A123720 a(n) = 2^n + 2^(n-1) - n.
%H A123720 Vincenzo Librandi, <a href="/A123720/b123720.txt">Table of n, a(n) for n = 1..1000</a>
%H A123720 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (4,-5,2).
%F A123720 a(n) = A007283(n-1) - n.
%F A123720 O.g.f.: x(2 - 4x + 3x^2)/((1-x)^2*(1-2x)). - _R. J. Mathar_, Jun 08 2008
%F A123720 E.g.f.: (1/2)*(-3 - 2*x*exp(x) + 3*exp(2*x)). - _G. C. Greubel_, Oct 26 2017
%t A123720 Table[2^n + 2^(n-1) - n, {n,1,50}] (* _G. C. Greubel_, Oct 26 2017 *)
%o A123720 (PARI) for(n=1,31,print1(2^n+2^(n-1)-n,","))
%o A123720 (Magma) [2^n + 2^(n-1) - n: n in [1..40] ]; // _Vincenzo Librandi_, May 18 2011
%Y A123720 Cf. A007283.
%K A123720 nonn,easy
%O A123720 1,1
%A A123720 _Klaus Brockhaus_, Oct 09 2006