This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A066764 #16 Dec 19 2024 21:16:09
%S A066764 3,8,30,140,315,319,460,840,864,936,1040,1287,1836,2480,2600,6076,
%T A066764 6200,7788,11398,12035,12415,12584,13260,13734,15886,18480,26078,
%U A066764 33820,40640,42665,46200,55860,68352,70266,70645,75330,78939,81740,98230
%N A066764 Numbers k such that (k, sigma(k)) lies on a circle with integral radius centered at the origin, i.e., k^2 + sigma(k)^2 is a square.
%C A066764 Also numbers such that A017665(k)^2 + A017666(k)^2 is a square. - _Michel Marcus_, May 23 2015
%H A066764 Harry J. Smith, <a href="/A066764/b066764.txt">Table of n, a(n) for n = 1..731</a>
%e A066764 3^2 + sigma(3)^2 = 9 + 16 = 5^2; so 3 is a term of the sequence.
%t A066764 Select[ Range[ 1, 10^5 ], IntegerQ[ Sqrt[ #^2 + DivisorSigma[ 1, # ]^2 ] ] & ]
%o A066764 (PARI) isok(k) = { issquare(k^2 + sigma(k)^2) } \\ _Harry J. Smith_, Mar 24 2010
%Y A066764 Cf. A017665, A017666, A066763.
%K A066764 nonn
%O A066764 1,1
%A A066764 _Joseph L. Pe_, Jan 17 2002