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A102568
a(n+1) = if a(n-1)=a(n)-1 then 2*a(n) else smallest number not occurring earlier; a(1)=1, a(2)=2.
4
1, 2, 4, 3, 5, 6, 12, 7, 8, 16, 9, 10, 20, 11, 13, 14, 28, 15, 17, 18, 36, 19, 21, 22, 44, 23, 24, 48, 25, 26, 52, 27, 29, 30, 60, 31, 32, 64, 33, 34, 68, 35, 37, 38, 76, 39, 40, 80, 41, 42, 84, 43, 45, 46, 92, 47, 49, 50, 100, 51, 53, 54, 108, 55, 56, 112, 57, 58, 116, 59, 61
OFFSET
1,2
COMMENTS
Permutation of the natural numbers with inverse A102569;
a(n) <= 2*n; a(n) <> n for n>6;
A102570(n) = a(a(n)).
MATHEMATICA
Nest[Append[#, If[#[[-1]] == #[[-2]] + 1, 2 #[[-1]], Min[Complement[Range[Max[#] + 1], #]]]] &, {1, 2}, 69] (* Ivan Neretin, Sep 08 2015 *)
CROSSREFS
Sequence in context: A338252 A086434 A086433 * A338834 A338835 A210536
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Jan 15 2005
STATUS
approved