login
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
OFFSET
1,1
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-François Alcover, Apr 30 2014 *)
CROSSREFS
Cf. A020588.
Sequence in context: A192869 A147513 A075323 * A055072 A059334 A130761
KEYWORD
nonn,fini,full
STATUS
approved