1,2

a(a(n))=n, a self-inverse permutation of natural numbers.

Table of n, a(n) for n=1..65.

Index entries for sequences that are permutations of the natural numbers

Multiplicative with a(p) = (if p mod 4 = 3 and p+2 is prime then p+2 else (if p mod 4 = 1 and p-2 is prime then p-2 else p)), p prime.

a(42) = a(2*3*7) = a(2)*a(3)*a(7) = a(2)*a(4*0+3)*a(7) = 2*(4*1+1)*7 = 2*5*7 = 70.

Cf. A072028, A071698, A071699, A072026, A072027, A061898, A064505.

Sequence in context: A266403 A266415 A132664 * A309734 A309668 A238758

Adjacent sequences: A072026 A072027 A072028 * A072030 A072031 A072032

nonn,mult

Reinhard Zumkeller, Jun 07 2002

approved