A256420 a(n) = 2n unless n is prime, in which case a(n) = first term not yet present in the sequence.

2, 1, 3, 8, 4, 12, 5, 16, 18, 20, 6, 24, 7, 28, 30, 32, 9, 36, 10, 40, 42, 44, 11, 48, 50, 52, 54, 56, 13, 60, 14, 64, 66, 68, 70, 72, 15, 76, 78, 80, 17, 84, 19, 88, 90, 92, 21, 96, 98, 100, 102, 104, 22, 108, 110, 112, 114, 116, 23, 120

Offset

1,1

Comments

This is a permutation of the positive integers: twice nonprimes (A139270), interspersed with (odd numbers and twice primes, A256421).

References

John Mason, Email message, Apr 07 2015

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

N:= 100:
S:= {$1..N}:
R:= NULL;
for n from 1 do
  if isprime(n) then if S = {} then break else t:= min(S) fi else t:= 2*n fi;
  R:= R, t;
  S:= S minus {t}
od:
R;

Crossrefs

Cf. A139270, A256421.

Sequence in context: A260142 A194505 A137307 * A205391 A352858 A078045

Adjacent sequences: A256417 A256418 A256419 * A256421 A256422 A256423

Keyword

nonn

Author

N. J. A. Sloane, Apr 07 2015

Status

approved