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%I #19 Jul 02 2019 18:06:14
%S 1,3,15,21,25,33,39,45,51,57,69,75,81,87,93,111,123,129,141,159,177,
%T 183,189,201,213,219,225,237,249,267,291,303,309,315,321,327,339,343,
%U 381,393,405,411,417,447,453,471,489,495,501,519,525,537,543,567,573
%N Square root of odd refactorable numbers.
%C Odd refactorable numbers are always squares.
%H Amiram Eldar, <a href="/A036897/b036897.txt">Table of n, a(n) for n = 1..1001</a>
%H S. Colton, <a href="http://www.cs.uwaterloo.ca/journals/JIS/colton/joisol.html">Refactorable Numbers - A Machine Invention</a>, J. Integer Sequences, Vol. 2, 1999, #2.
%H S. Colton, <a href="http://web.archive.org/web/20070831060523/http://www.dai.ed.ac.uk/homes/simonco/research/hr/">HR - Automatic Theory Formation in Pure Mathematics</a>
%F a(n) = sqrt(A036896(n)). - _Amiram Eldar_, Jul 02 2019
%e 15^2 is refactorable because 225 has 9 divisors and 9 divides 225.
%t Select[Range[1, 1000, 2], Divisible[#^2, DivisorSigma[0,#^2]] &] (* _Amiram Eldar_, Jul 02 2019 *)
%o (PARI) isrefac(n) = ! (n % numdiv(n));
%o lista(nn) = {forstep (n = 1, nn, 2, if (isrefac(n), print1(sqrtint(n), ", ")););} \\ _Michel Marcus_, Aug 31 2013
%Y Cf. A033950, A036896.
%K nonn
%O 1,2
%A Simon Colton (simonco(AT)cs.york.ac.uk)
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