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%I #9 Jul 13 2013 12:03:39
%S 1,1,2,2,3,4,3,5,6,4,7,8,9,10,5,11,12,6,13,14,15,16,7,17,18,8,19,20,
%T 21,22,9,23,24,25,26,27,28,10,29,30,11,31,32,33,34,35,36,12,37,38,39,
%U 40,13,41,42,14,43,44,45,46,15,47,48,49,50,51,52,16,53,54,55,56,57,58,17,59
%N Start with x=1 and repeat: if x is a prime number then (append i and then x, with x=prime(i)) else (only append x), continue with x:=x+1.
%C All positive integers occur exactly twice: A095117 and A095116 give positions of first and second occurrences.
%H Reinhard Zumkeller, <a href="/A165634/b165634.txt">Table of n, a(n) for n = 1..10000</a>
%e 1,(1,2),(2,3),4,(3,5),6,(4,7),8,9,10,(5,11),12, ... .
%o (Haskell)
%o a165634 n = a165634_list !! (fromInteger n - 1)
%o a165634_list = concatMap (\x ->
%o if a010051 x == 1 then [a049084 x, x] else [x]) [1..]
%o -- _Reinhard Zumkeller_, Apr 17 2012
%Y A049084, A000040.
%K nonn
%O 1,3
%A _Reinhard Zumkeller_, Sep 23 2009