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 A026121 #24 Jul 08 2025 17:50:28
%S A026121 0,3,36,351,3240,29403,265356,2390391,21520080,193700403,1743362676,
%T A026121 15690441231,141214502520,1270932117003,11438393835996,
%U A026121 102945558872871,926510072902560,8338590785263203,75047317454789316,675425858255365311
%N A026121 a(n) = 3^n*(3^n-1)/2.
%C A026121 Sum of all base-3 numbers with length at most n. - _Alois P. Heinz_, Feb 23 2020
%H A026121 Vincenzo Librandi, <a href="/A026121/b026121.txt">Table of n, a(n) for n = 0..200</a>
%H A026121 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (12,-27).
%F A026121 a(n) = C(3^n,2), n>=0. - _Zerinvary Lajos_, Jan 07 2008
%F A026121 a(0) = 0, a(1)=3, a(n)=12*a(n-1)-27*a(n-2). - _Harvey P. Dale_, Oct 07 2012
%F A026121 G.f.: 3*x / ( (9*x-1)*(3*x-1) ). a(n) = 3*A016142(n-1). - _R. J. Mathar_, Mar 24 2013
%p A026121 seq(binomial(3^n,2),n=0..19); # _Zerinvary Lajos_, Jan 07 2008
%t A026121 Binomial[3^Range[0,20],2] (* or *) LinearRecurrence[{12,-27},{0,3},30] (* _Harvey P. Dale_, Oct 07 2012 *)
%o A026121 (Magma) [3^n*(3^n-1)/2: n in [0..30]]; // _Vincenzo Librandi_, May 01 2011
%o A026121 (PARI) a(n)=binomial(3^n,2) \\ _Charles R Greathouse IV_, Jun 11 2015
%Y A026121 Cf. A000217, A000244.
%K A026121 nonn,easy
%O A026121 0,2
%A A026121 _N. J. A. Sloane_