cp's OEIS Frontend

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.

A162690 Strictly positive numbers n such that 20*n/(20+n) are integers.

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%I A162690 #4 Mar 31 2012 12:38:25
%S A162690 5,20,30,60,80,180,380
%N A162690 Strictly positive numbers n such that 20*n/(20+n) are integers.
%C A162690 The 20th row of A127730.
%C A162690 The ansatz 20*n/(20+n)=j (any integer j) yields n=20*j/(20-j) which demonstrates that the sequence is finite if n>=0. [R. J. Mathar, Jul 13 2009]
%t A162690 f[a_,b_]:=(a*b)/(a+b); a=20;lst={};Do[If[f[a,n]==IntegerPart[f[a,n]], AppendTo[lst,n]],{n,9!}];lst
%Y A162690 Cf. A162688, A162689
%Y A162690 Cf. A127730. [From _Franklin T. Adams-Watters_, Aug 07 2009]
%K A162690 nonn,fini,full
%O A162690 1,1
%A A162690 _Vladimir Joseph Stephan Orlovsky_, Jul 10 2009
%E A162690 Edited by _R. J. Mathar_, Jul 13 2009