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 A066784 #13 Jul 12 2024 10:17:55 %S A066784 1,90,392448,411264,804384,871416,1284192,1935360,7456512,396168192, %T A066784 24193572480,43171285248,54585498240,63178786944,123570274464, %U A066784 730078562304,823442861592,1420069242240,2354025332736,2506251331584,3606011136000,3697798293504,9951618862080 %N A066784 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 A066784 sigma(90)^2 - 90^2 = 234^2 - 90^2 = 216^2, so 90 is a term of the sequence. %t A066784 Select[ Range[ 1, 10^6 ], IntegerQ[ Sqrt[ DivisorSigma[ 1, # ]^2 - #^2 ] ] & ] %Y A066784 Cf. A066764. %K A066784 nonn %O A066784 1,2 %A A066784 _Joseph L. Pe_, Jan 18 2002 %E A066784 a(7)-a(10) from _Sean A. Irvine_, Nov 06 2023 %E A066784 a(11)-a(23) from _Martin Ehrenstein_, Jul 12 2024