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%I #28 Sep 08 2022 08:46:11
%S 9,729,531441,2503155504993241601315571986085849,
%T 4638397686588101979328150167890591454318967698009
%N Numbers n of the form 3^k such that sigma(n) is a prime p.
%C Powers of 3 from A023194 (numbers n such that sigma(n) is a prime).
%H Jaroslav Krizek, <a href="/A255510/b255510.txt">Table of n, a(n) for n = 1..10</a>
%F a(n) = 3^(A028491(n) - 1).
%F sigma(a(n)) = A076481(n).
%t Select[3^Range[0,110],PrimeQ[DivisorSigma[1,#]]&] (* _Harvey P. Dale_, Mar 29 2015 *)
%o (Magma) [(3^n): n in [1..1000] | IsPrime((SumOfDivisors(3^n)))]
%Y Cf. A000203 (sigma), A023194 (sigma(n) is prime).
%Y Cf. A003462 (sigma(3^n)), A028491 (sigma(3^n) is prime) , A076481.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Mar 25 2015