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 A133648 #19 May 16 2020 02:54:51 %S A133648 5,18,59,184,561,1694,5095,15300,45917,137770,413331,1240016,3720073, %T A133648 11160246,33480767,100442332,301327029,903981122,2711943403, %U A133648 8135830248,24407490785,73222472398,219667417239,659002251764,1977006755341 %N A133648 a(n) = 2*3^n + 3^(n-1) - (n+1). %H A133648 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-7,3). %F A133648 O.g.f.: -x*(5-7*x+4*x^2)/((-1+x)^2*(-1+3*x)). - _R. J. Mathar_, Jan 07 2008 %F A133648 a(n) = A108765(n-1) + A108765(n). - _Klaus Purath_, Apr 13 2020 %e A133648 a(3) = 2*3^3 + 3^2 - 4 = 2*27 + 9 - 4. %p A133648 A133648:=n->2*3^n+3^(n-1)-(n+1): seq(A133648(n), n=1..40); # _Wesley Ivan Hurt_, Apr 18 2017 %t A133648 Table[2*3^n + 3^(n - 1) - (n + 1), {n, 1, 50}] (* _Stefan Steinerberger_, Sep 20 2007 *) %Y A133648 Cf. A000244, A108765, A133649 (inverse binomial transform). %K A133648 nonn,easy %O A133648 1,1 %A A133648 _Gary W. Adamson_, Sep 19 2007