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%I #5 Mar 30 2012 18:39:11
%S 0,0,0,0,0,1,0,1,0,1,0,2,0,0,1,0,0,1,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,
%T 0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
%U 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
%N Number of pairs (x,y) 1<=x<=y<=n such that 1/x+1/y+1/n = 1/2.
%F It seems that for n>=43 a(n) = 0
%o (PARI) a(n)=sum(i=1,n,sum(j=1,i,if(1/i+1/j+1/n-1/2,0,1)))
%K nonn
%O 1,12
%A _Benoit Cloitre_, Dec 08 2002