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A020575 Smallest nonempty set S containing prime divisors of 2k + 1 for each k in S. 3
3, 5, 7, 11, 13, 19, 23, 47 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

2 * 3 + 1 = 7, which is in the set.

2 * 5 + 1 = 11, which is also in the set.

2 * 7 + 1 = 15 = 3 * 5, both of which are in the set.

2 * 11 + 1 = 23, which is in the set.

2 * 13 + 1 = 27 = 3^3, and 3 is in the set.

2 * 19 + 1 = 39 = 3 * 13, both of which are in the set.

2 * 23 + 1 = 47, which is in the set.

2 * 47 + 1 = 95 = 5 * 19, both of which are in the set.

MATHEMATICA

setSeek[s_List] := FactorInteger[2 * # + 1][[All, 1]]& /@ s // Flatten // Union; FixedPoint[Union[#, setSeek[#]]&, setSeek[{1}]] (* Jean-Fran├žois Alcover, Apr 30 2014 *)

CROSSREFS

Cf. A020588.

Sequence in context: A192869 A147513 A075323 * A055072 A059334 A130761

Adjacent sequences:  A020572 A020573 A020574 * A020576 A020577 A020578

KEYWORD

nonn,fini,full

AUTHOR

David W. Wilson

STATUS

approved

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Last modified August 10 20:05 EDT 2022. Contains 356039 sequences. (Running on oeis4.)