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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 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

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