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%I #24 Nov 26 2019 18:14:12
%S 15,21,27,33,35,39,45,51,55,57,63,65,69,75,77,85,87,91,93,95,99,105,
%T 111,115,117,119,123,125,129,133,135,141,143,145,147,153,155,159,161,
%U 165,171,175,177,183,185,187,189,195,201,203,205,207,209,213,215,217,219
%N Odd nonprime integers n which have an odd number of proper divisors.
%C Odd nonprime numbers with even abundance and even sigma. Is their abundance always negative; i.e. are they always deficient? - _Labos Elemer_, Oct 28 2003
%C Odd nonprime nonsquares. - _Vladeta Jovovic_, Jan 25 2004
%H Harvey P. Dale, <a href="/A082686/b082686.txt">Table of n, a(n) for n = 1..1000</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/UnsolvedProblems.html">Unsolved Problems</a> (see #17). [Today this web page has only 16 sections, so this link is no longer useful. - _N. J. A. Sloane_, Nov 26 2019]
%e a(1) = 15 since 15 is the first odd nonprime number with an odd number of proper divisors.
%e n, #proper divisors, #divisors:
%e 9, 2, {1, 3}
%e 15, 3, {1, 3, 5}
%t Select[Range[1,221,2],!PrimeQ[#]&&EvenQ[DivisorSigma[0,#]]&] (* _Harvey P. Dale_, Dec 16 2015 *)
%o (PARI) isok(n) = (n % 2) && (! isprime(n)) && ((numdiv(n) % 2) == 0) \\ _Michel Marcus_, Jun 10 2013
%Y Cf. A000396, A001065.
%K easy,nonn
%O 1,1
%A Anne Donovan (anned3005(AT)aol.com), May 19 2003
%E Definition corrected by _Zak Seidov_, Jul 21 2009
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