A078593 - OEIS

%I #5 Mar 30 2012 18:39:11

%S 0,0,0,0,0,0,4,9,16,24,33,42,55,68,81,96,111,127,145,163,183,203,224,

%T 246,270,294,319,345,372,400,429,459,490,522,555,589,624,660,697,735,

%U 774,814,856,898,941,985,1030,1076,1123,1171,1220,1270,1321,1373,1426

%N Number of pairs (x,y) 1<=x<=y<=n such that 1/x+1/y+1/n < 1/2.

%F for n>=43 a(n) = (1/2)*(n^2 - 3*n - 8 )

%o (PARI) a(n)=sum(i=1,n,sum(j=1,i,if(sign(1/i+1/j+1/n-1/2)+1,0,1)))

%K nonn

%O 1,7

%A _Benoit Cloitre_, Dec 08 2002