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%I #6 Jul 29 2020 03:18:21
%S 6,28,8128,137438691328,2305843008139952128
%N Perfect numbers whose number of proper divisors is prime.
%C The next term is too large to include in the data section. If there are no odd perfect numbers then the next term is a(6) = 2^606 * (2^607 - 1) = 1.410... * 10^365. - _Amiram Eldar_, Jul 29 2020
%F a(n) = A006516(A172461(n)), assuming that odd perfect numbers do not exist. - _Amiram Eldar_, Jul 29 2020
%e 28 is member because the number of proper divisors of 28 is 5, a prime number.
%t Table[2^(p-1)*(2^p-1), {p, Select[MersennePrimeExponent[Range[8]], PrimeQ[2# - 1] &]}] (* _Amiram Eldar_, Jul 29 2020 *)
%Y Cf. A000396, A006516, A032741, A133033, A154896, A172461.
%K nonn
%O 1,1
%A _Omar E. Pol_, Jan 25 2009
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