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%I #18 Sep 28 2020 01:03:38
%S 1,2,3,4,5,6,7,8,9,10,12,13,14,15,16,17,18,19,20,21,24,25,26,27,28,30,
%T 31,32,34,35,36,38,39,40,42,45,48,49,50,51,52,54,56,57,60,61,62,63,64,
%U 65,68,70,72,75,76,78,80,81,84,85,89,90,91,93,95,96,98,100,102,104,105
%N Numbers whose prime divisors consist of primes p such that 2^p-1 is prime.
%C Multiplicative closure of A000043. - _Charles R Greathouse IV_, Feb 21 2013
%H Ivan Neretin, <a href="/A048737/b048737.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A122254(n) = A122260(n) for n < 22. - _Reinhard Zumkeller_, Aug 29 2006
%F Sum_{n>=1} 1/a(n) = Product_{p in A000043} p/(p-1) = 5.7838... - _Amiram Eldar_, Sep 27 2020
%e 10 = 2 * 5 is a term since both 2 and 5 are Mersenne exponents (A000043).
%t p = Join[{1}, MersennePrimeExponent @ Range[12]]; Select[Range[p[[-1]]], AllTrue[FactorInteger[#][[;; , 1]], MemberQ[p, #] &] &] (* _Amiram Eldar_, Sep 27 2020 *)
%Y Cf. A000043, A122254, A122260.
%K nonn
%O 1,2
%A _J. Lowell_
%E More terms from _James A. Sellers_, Apr 22 2000
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