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 A236391 #12 Feb 06 2014 03:24:52 %S A236391 5,10,20,30,70 %N A236391 Positive numbers n such that 100*n^2/(100+n^2) are integers. %C A236391 Rewrite 100n^2/(100+n^2) as an I : n=sqrt(100I/(100-I)), where a finite substitution for I results. %C A236391 Instead of 100, we could use 72 and get n=8,24,48,64; we could use 162 and get n=54,108,144,150. Many other values are possible. %e A236391 for n=20, 100*20^2/(100+20^2)=I or 80. %t A236391 Select[Range[1000], IntegerQ[100*#^2/(100 + #^2)] &] (* _T. D. Noe_, Jan 29 2014 *) %Y A236391 Cf. A162688. %K A236391 nonn,fini,full %O A236391 1,1 %A A236391 _Larry J Zimmermann_, Jan 24 2014