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%I #23 May 21 2017 21:33:13
%S 3,5,8,13,18,36,37,49,50,61,73,81,100,121,144,157,169,193,225,256,277,
%T 313,361,397,400,421,457,529,541,576,578,613,625,661,673,733,757,841,
%U 877,961,997,1024,1093,1153,1156,1201,1213,1237,1321,1381,1453
%N Numbers n such that the sum of the divisors of n is semiprime.
%C The sequence contains a subset of squares {36, 49, 81, 100, 121, 144, 169, 225, 256, ...}.
%C If n is a term of this sequence and n is nonsquare, then n must be a prime or twice a square. Additionally, if n is in this sequence, then A001221(n) <= 2. - _Altug Alkan_, Jul 17 2016
%H G. C. Greubel, <a href="/A175388/b175388.txt">Table of n, a(n) for n = 1..1000</a>
%e a(6)= 36 with 8 divisors: {1, 2, 3, 4, 6, 9, 12, 18, 36}
%e and the sum of the divisors is 91 = 7*13 (semiprime).
%p with(numtheory):for k from 1 to 1600 do:if bigomega(sigma(k))=2 then printf(`%d, `,k): else fi:od:
%t Select[Range@ 1500, PrimeOmega@ DivisorSigma[1, #] == 2 &] (* _Michael De Vlieger_, Jul 17 2016 *)
%Y Cf. A112381, A023194 (numbers n such that sigma(n) is prime).
%K nonn
%O 1,1
%A _Michel Lagneau_, Aug 20 2011
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