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 A081690 #10 Nov 23 2014 03:42:27
%S A081690 0,1,3,4,5,7,8,9,10,12,13,14,15,16,17,18,19,21,22,23,24,25,26,27,28,
%T A081690 29,30,31,32,33,34,35,36,37,39,40,41,42,43,44,45,46,47,48,49,50,51,52,
%U A081690 53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,72,73,74,75,76,77
%N A081690 From P-positions in a certain game.
%H A081690 A. S. Fraenkel, <a href="http://www.emis.de/journals/INTEGERS/papers/eg6/eg6.Abstract.html">New games related to old and new sequences</a>, INTEGERS, Electronic J. of Combinatorial Number Theory, Vol. 4, Paper G6, 2004.
%F A081690 Let a(n) = this sequence, b(n) = A081691. Then a(n) = mex{ a(i), b(i) : 0 <= i < n}, b(0) = 0, b(n) = 2(b(n-1) - a(n-1)) + a(n) + 1 = a(n) + 2^n - 1.
%Y A081690 Apart from initial zero, complement of A081691.
%K A081690 nonn
%O A081690 0,3
%A A081690 _N. J. A. Sloane_, Apr 02 2003
%E A081690 Corrected and extended by _Vladeta Jovovic_, Apr 04 2003