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 A278043 #23 Jul 07 2022 02:30:48
%S A278043 0,1,1,2,1,2,2,1,2,2,3,2,3,1,2,2,3,2,3,3,2,3,3,4,1,2,2,3,2,3,3,2,3,3,
%T A278043 4,3,4,2,3,3,4,3,4,4,1,2,2,3,2,3,3,2,3,3,4,3,4,2,3,3,4,3,4,4,3,4,4,5,
%U A278043 2,3,3,4,3,4,4,3,4,4,5,4,5,1,2,2,3,2,3,3,2,3,3,4,3,4,2,3,3,4,3,4
%N A278043 Number of 1's in tribonacci representation of n (cf. A278038).
%H A278043 N. J. A. Sloane, <a href="/A278043/b278043.txt">Table of n, a(n) for n = 0..20000</a>
%F A278043 a(n) = A000120(A003726(n+1)). - _John Keith_, May 23 2022
%t A278043 t[1] = 1; t[2] = 2; t[3] = 4; t[n_] := t[n] = t[n - 1] + t[n - 2] + t[n - 3]; a[0] = 0; a[n_] := Module[{s = {}, m = n, k}, While[m > 0, k = 1; While[t[k] <= m, k++]; k--; AppendTo[s, k]; m -= t[k]; k = 1]; DigitCount[Total[2^(s - 1)], 2, 1]]; Array[a, 100, 0] (* _Amiram Eldar_, Mar 04 2022 *)
%Y A278043 Cf. A001590, A003726, A007895, A278038, A278042, A278044.
%K A278043 nonn,base
%O A278043 0,4
%A A278043 _N. J. A. Sloane_, Nov 18 2016