%I
%S 1,90,392448,411264,804384,871416
%N Numbers n such that (n, sigma(n)) lies on the hyperbola y^2  x^2 = m^2, for some natural number m, i.e., sigma(n)^2  n^2 = m^2.
%e sigma(90)^2  90^2 = 234^2  90^2 = 216^2, so 90 is a term of the sequence.
%t Select[ Range[ 1, 10^6 ], IntegerQ[ Sqrt[ DivisorSigma[ 1, # ]^2  #^2 ] ] & ]
%K nonn
%O 1,2
%A _Joseph L. Pe_, Jan 18 2002
