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 A089399 #3 Mar 30 2012 18:36:39
%S A089399 0,0,3,4,5,6,8,9,10,10,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,
%T A089399 27,28,29,30,31,33,34,35,35,36,37,38,39,40,41,42,43,44,45,46,47,49,50,
%U A089399 51,51,52,53,54,55,57,58,59,59,61,62,63,64,65,66,67,68,69,69,70,71,72,73
%N A089399 a(n) satisfies: 2^a(n)+1 = sum(k=1,n, A089398(k)*2^(k-1)) for n>2, with a(1)=a(2)=0.
%C A089399 A089398(n) = n-th column sum of binary digits of k*2^(k-1), where summation is over all k>=1, without carrying from columns sums that may exceed 2.
%e A089399 a(7)=8 since 2^8+1=257=(1)+(0)2+(2)2^2+(1)2^3+(1)2^4+(1)2^5+(3)2^6,
%e A089399 and A089398 begins: {1,0,2,1,1,1,3,2,2,0,3,2,2,2,4,3,3,1,...}.
%Y A089399 Cf. A089398.
%K A089399 nonn
%O A089399 1,3
%A A089399 _Paul D. Hanna_, Oct 30 2003