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 A226508 #29 Sep 04 2020 13:11:53
%S A226508 3,33,315,2889,26163,235953,2125035,19129689,172180323,1549662273,
%T A226508 13947078555,125524061289,1129717614483,10167461718993,91507165036875,
%U A226508 823564514029689,7412080712360643,66708726669526113,600378540800575995,5403406869529706889
%N A226508 a(n) = Sum_{i=3^n..3^(n+1)-1} i.
%C A226508 Partial sums give 3, 36, 351, 3240, 29403,...: A026121.
%C A226508 a(n) is the sum of all integers having n+1 digits in their ternary expansion (without leading zeros). - _Jonathan Vos Post_, Sep 07 2006
%H A226508 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (12,-27).
%F A226508 G.f.: 3*(1-x)/(1-12*x+27*x^2). [_Bruno Berselli_, Jun 11 2013]
%F A226508 a(n) = 3^(n-1)*(4*3^(n+1)-3). [_Bruno Berselli_, Jun 11 2013]
%F A226508 a(0)=3, a(1)=33, a(n)=12*a(n-1)-27*a(n-2). - _Harvey P. Dale_, Jun 19 2013
%e A226508 a(0) = 1+2 = 3,
%e A226508 a(1) = 3+4+5+6+7+8 = 33.
%t A226508 Table[3^(n - 1) (4 3^(n + 1) - 3), {n, 0, 20}] (* _Bruno Berselli_, Jun 11 2013 *)
%t A226508 LinearRecurrence[{12,-27},{3,33},30] (* _Harvey P. Dale_, Jun 19 2013 *)
%o A226508 (PARI) a(n) = sum(i=3^n, 3^(n+1)-1, i) \\ _Michel Marcus_, Jun 11 2013
%Y A226508 Cf. A010035, A010036 (base 2), A026121, A101291 (base 10).
%Y A226508 Cf. A007089 (numbers in base 3).
%K A226508 nonn,easy
%O A226508 0,1
%A A226508 _Michel Marcus_, Jun 10 2013