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 A063664 #10 Feb 09 2021 02:05:24 %S A063664 2,8,18,20,32,50,72,80,90,98,128,144,162,180,200,242,272,288,320,338, %T A063664 360,392,450,468,500,512,576,578,648,650,720,722,800,810,882,968,980, %U A063664 1058,1088,1152,1250,1280,1296,1332,1352,1440,1458,1568,1620,1682,1800 %N A063664 Numbers whose reciprocal is the sum of two reciprocals of squares. %C A063664 These are numbers which can be written either as b^2*c^2*(b^2+c^2)*d^2 or if (b^2+c^2) is a square then as b^2*c^2*d^2, since 1/(b*(b^2+c^2)*d)^2+1/(c*(b^2+c^2)*d)^2 =1/(b^2*c^2*(b^2+c^2)*d^2) and 1/(b*sqrt(b^2+c^2)*d)^2+1/(c*sqrt(b^2+c^2)*d)^2 = 1/(b^2*c^2*d^2). %e A063664 98 is in the sequence since 1/98=1/10^2+1/70^2 (also 1/98=1/14^2+1/14^2). %o A063664 (Python) %o A063664 from fractions import Fraction %o A063664 def aupto(lim): %o A063664 sqr_recips = [Fraction(1, i*i) for i in range(1, lim+2)] %o A063664 ssr = set(f + g for i, f in enumerate(sqr_recips) for g in sqr_recips[i:]) %o A063664 representable = [f.denominator for f in ssr if f.numerator == 1] %o A063664 return sorted(r for r in representable if r <= lim) %o A063664 print(aupto(1800)) # _Michael S. Branicky_, Feb 08 2021 %Y A063664 Either products of terms in A063663 and A000290, or squares of A008594. %Y A063664 Cf. A001481, A000404, A063665, A063669. %K A063664 nonn %O A063664 1,1 %A A063664 _Henry Bottomley_, Jul 28 2001 %E A063664 Offset changed to 1 by _Derek Orr_, Jun 23 2015