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%I #2 Mar 30 2012 17:31:29
%S 0,1,7,6,5,4,12,13,14,15,8,9,10,11,21,20,23,22,17,16,19,18,29,28,31,
%T 30,25,24,27,26,38,39,36,37,34,35,32,33,46,47,44,45,42,43,40,41,54,55,
%U 52,53,50,51,48,49,62,63,60,61,58,59,56,57,71,70,69,68,67,66,65,64,79,78
%N a(n) = n XOR floor(Log_2(n))+1, where XOR is bitwise XOR.
%C Except for 2 & 3 which are missing, is every integer present just once?
%t f[n_] := BitXor[n, Floor[Log[2, n] + 1]]; t = Array[f, 75, 2];
%K easy,nonn
%O 2,3
%A _Robert G. Wilson v_, Jun 09 2010