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A139791 Numbers n for which 2n is a multiple of A002326(n). 1
1, 2, 3, 5, 6, 8, 9, 11, 14, 15, 18, 20, 21, 23, 26, 29, 30, 33, 35, 36, 39, 41, 44, 48, 50, 51, 53, 54, 56, 63, 65, 68, 69, 74, 75, 78, 81, 83, 86, 89, 90, 95, 96, 98, 99, 105, 111, 113, 114, 116, 119, 120, 125, 128, 131, 134, 135, 138, 140, 141, 146, 153, 155, 156, 158 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The sequence properly contains A005097. 170 is the first number which is not in A005097. One can prove that A002326(2^(2t-1))=4t. Thus if n=2^(2t-1), where, for any m>0, t=2^(m-1) then 2n is a multiple of A002326(n) while 2n+1 is a Fermat number which, as well known, is not always a prime.

The sequence is the union of A005097 and (A001567 - 1)/2. [Conjectured by Vladimir Shevelev, proved by Ray Chandler, May 26 2008]

LINKS

Table of n, a(n) for n=1..65.

PROG

(PARI) isok(n) = !(2*n % znorder(Mod(2, 2*n+1))); \\ Michel Marcus, Nov 02 2017

CROSSREFS

Cf. A002326, A005097, A001262, A001567, A137576.

Sequence in context: A130290 A005097 A102781 * A027563 A219729 A000534

Adjacent sequences:  A139788 A139789 A139790 * A139792 A139793 A139794

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, May 21 2008, May 24 2008

STATUS

approved

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Last modified March 21 22:19 EDT 2019. Contains 321382 sequences. (Running on oeis4.)