%I #13 Jan 03 2016 17:47:52
%S 3,8,30,140,315,319,460,840,864,936,1040,1287,1836,2480,2600,6076,
%T 6200,7788,11398,12035,12415,12584,13260,13734,15886,18480,26078,
%U 33820,40640,42665,46200,55860,68352,70266,70645,75330,78939,81740,98230
%N Numbers n such that (n, sigma(n)) lies on a circle with integral radius centered at the origin, i.e., n^2 + sigma(n)^2 is a square.
%C Also numbers such that A017665(n)^2 + A017666(n)^2 is a square. - _Michel Marcus_, May 23 2015
%H Harry J. Smith, <a href="/A066764/b066764.txt">Table of n, a(n) for n = 1..731</a>
%e 3^2 + sigma(3)^2 = 9 + 16 = 5^2; so 3 is a term of the sequence.
%t Select[ Range[ 1, 10^5 ], IntegerQ[ Sqrt[ #^2 + DivisorSigma[ 1, # ]^2 ] ] & ]
%o (PARI) { n=0; for (m=1, 10^10, if (issquare(m^2 + sigma(m)^2), write("b066764.txt", n++, " ", m); if (n==1000, return)) ) } \\ _Harry J. Smith_, Mar 24 2010
%K nonn
%O 1,1
%A _Joseph L. Pe_, Jan 17 2002