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%I #4 Mar 31 2012 12:38:25
%S 5,20,30,60,80,180,380
%N Strictly positive numbers n such that 20*n/(20+n) are integers.
%C The 20th row of A127730.
%C 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 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 Cf. A162688, A162689
%Y Cf. A127730. [From _Franklin T. Adams-Watters_, Aug 07 2009]
%K nonn,fini,full
%O 1,1
%A _Vladimir Joseph Stephan Orlovsky_, Jul 10 2009
%E Edited by _R. J. Mathar_, Jul 13 2009