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%I #17 Jan 22 2019 17:30:07
%S 3,5,17,27,41,49,59,71,89,101,125,131,167,169,173,289,293,383,529,677,
%T 701,729,743,761,773,827,839,841,857,911,1091,1097,1163,1181,1193,
%U 1217,1373,1427,1487,1559,1583,1709,1811,1847,1849,1931,1973,2129,2197,2273,2309
%N Odd numbers N for which sigma(N^2) is prime.
%C The function sigma(n) (=A000203(n)) takes odd values when n is a square or twice a square. Thus, odd numbers n for which sigma(n) is prime (i.e. which are in A023194) must be odd squares. This sequence consists exactly of the square roots of these terms.
%H T. D. Noe, <a href="/A193070/b193070.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A278911(n)^(1/2). - _Robert Israel_, Jan 22 2019
%t Select[Range[1,2401,2],PrimeQ[DivisorSigma[1,#^2]]&] (* _Harvey P. Dale_, Mar 07 2015 *)
%o (PARI) forstep(N=1, 1e7, 2, isprime(sigma(N^2)) && print1(N", "))
%Y Cf. A000203, A023194, A195382, A278911.
%K nonn
%O 1,1
%A _M. F. Hasler_, Jul 15 2011
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