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 A066782 #15 Dec 19 2024 21:16:05
%S A066782 1,5,13,25,34,41,61,68,113,125,136,169,181,219,222,272,313,390,421,
%T A066782 444,482,544,578,613,625,657,666,761,780,888,964,979,1013,1088,1156,
%U A066782 1170,1201,1301,1332,1560,1681,1741,1776,1861
%N A066782 Numbers k such that (k, phi(k)) lies on the hyperbola x^2 - y^2 = m^2, for some natural number m, i.e., k^2 - phi(k)^2 is a square.
%H A066782 Harry J. Smith, <a href="/A066782/b066782.txt">Table of n, a(n) for n = 1..1000</a>
%e A066782 5^2 - phi(5)^2 = 25 - 16 = 3^2, so 5 is a term of the sequence.
%t A066782 Select[ Range[ 1, 10^4 ], IntegerQ[ Sqrt[ #^2 - EulerPhi[ # ]^2 ] ] & ]
%o A066782 (PARI) isok(k) = { issquare(k^2 - eulerphi(k)^2) } \\ _Harry J. Smith_, Mar 25 2010
%Y A066782 Cf. A066763.
%K A066782 nonn
%O A066782 1,2
%A A066782 _Joseph L. Pe_, Jan 18 2002