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A256420 a(n) = 2n unless n is prime, in which case a(n) = first term not yet present in the sequence. 2
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 (list; graph; refs; listen; history; text; internal format)
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
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
Sequence in context: A260142 A194505 A137307 * A205391 A352858 A078045
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 07 2015
STATUS
approved

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Last modified May 24 23:50 EDT 2024. Contains 372782 sequences. (Running on oeis4.)