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A251561 A permutation of the natural numbers: interchange p and 2p for every prime p. 3
1, 4, 6, 2, 10, 3, 14, 8, 9, 5, 22, 12, 26, 7, 15, 16, 34, 18, 38, 20, 21, 11, 46, 24, 25, 13, 27, 28, 58, 30, 62, 32, 33, 17, 35, 36, 74, 19, 39, 40, 82, 42, 86, 44, 45, 23, 94, 48, 49, 50, 51, 52, 106, 54, 55, 56, 57, 29, 118, 60, 122, 31, 63, 64, 65, 66 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(A001751(n)) != A001751(n). - Reinhard Zumkeller, Dec 27 2014

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000

A. B. Frizell, Certain non-enumerable sets of infinite permutations. Bull. Amer. Math. Soc. 21 (1915), no. 10, 495-499.

Index entries for sequences that are permutations of the natural numbers

MATHEMATICA

a251561[n_] := Block[{f}, f[x_] := Which[PrimeQ[x], 2 x, PrimeQ[x/2], x/2, True, x]; Array[f, n]]; a251561[66] (* Michael De Vlieger, Dec 26 2014 *)

PROG

(Python)

from sympy import isprime

def A251561(n):

....if n == 2:

........return 4

....q, r = divmod(n, 2)

....if r :

........if isprime(n):

............return 2*n

........return n

....if isprime(q):

........return q

....return n # Chai Wah Wu, Dec 26 2014

(Haskell)

a251561 1 = 1

a251561 n | q == 1                    = 2 * p

          | p == 2 && a010051' q == 1 = q

          | otherwise                 = n

          where q = div n p; p = a020639 n

-- Reinhard Zumkeller, Dec 27 2014

CROSSREFS

Cf. A064614, A253046.

Cf. A020639, A010051, A001751.

Sequence in context: A095196 A074828 A282028 * A159193 A118967 A256508

Adjacent sequences:  A251558 A251559 A251560 * A251562 A251563 A251564

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Dec 26 2014

STATUS

approved

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Last modified November 19 08:42 EST 2019. Contains 329318 sequences. (Running on oeis4.)