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%I #23 Mar 05 2015 10:56:22
%S 1,4,6,2,8,3,9,5,7,11,10,13,12,17,19,23,14,29,15,31,37,41,16,43,47,53,
%T 59,61,18,67,20,71,73,79,83,89,21,97,101,103,22,107,24,109,113,127,25,
%U 131,137,139,149,151,26,157,163,167,173,179,27,181,28,191,193,197,199
%N Beginning with the natural numbers, swap the k-th prime and k-th composite.
%C Involution (self-inverse permutation) of [positive] natural numbers.
%H Alois P. Heinz, <a href="/A026239/b026239.txt">Table of n, a(n) for n = 1..10000</a>
%H <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%F a(1) = 1 and a(n) = if n is prime then A002808(A049084(n)) else A000040(A066246(n)) for n>1. - _Reinhard Zumkeller_, Dec 13 2001
%t Composite[n_Integer] := FixedPoint[n + PrimePi@# + 1 &, n + PrimePi@n + 1]; f[1] = 1; f[n_] := If[ PrimeQ@n, Composite@ PrimePi@n, Prime[n - 1 - PrimePi@n]]; Array[f, 65] (* _Robert G. Wilson v_, Jun 08 2010 *)
%o (Haskell)
%o a026239 1 = 1
%o a026239 n | a010051 n == 1 = a002808 $ a049084 n
%o | otherwise = a000040 $ a066246 n
%o -- _Reinhard Zumkeller_, Mar 30 2014
%Y Cf. A236854.
%K nonn
%O 1,2
%A _Clark Kimberling_
%E More terms from _Robert G. Wilson v_, Jun 08 2010
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