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 A121718 #5 Oct 31 2013 12:17:40
%S A121718 0,1,3,13,24,36,56,77,99,199,300,402,512,623,735,855,976,1098,1298,
%T A121718 1499,1701,1911,2122,2334,2554,2775,2997,3997,4998,6000,7010,8021,
%U A121718 9033,10053,11074,12096,13196,14297,15399,16509,17620,18732,19852,20973,22095
%N A121718 Write 0, 1, ..., n in base 3 and add as if they were decimal numbers.
%C A121718 This is to A007089 as A067894 is to A007088.
%e A121718 a(5) = 36 = 0 + 1 + 2 + 10 + 11 + 12.
%t A121718 f[n_] := Apply[Plus, Table[ FromDigits[ IntegerDigits[i, 3]], {i, 0, n}]]; Table[ f@n, {n, 0, 45}] (* _Robert G. Wilson v_ Sep 10 2006 *)
%Y A121718 Cf. A007088, A007089, A067894.
%K A121718 base,easy,nonn
%O A121718 0,3
%A A121718 _Jonathan Vos Post_, Sep 08 2006