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A078593
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Number of pairs (x,y) 1<=x<=y<=n such that 1/x+1/y+1/n < 1/2.
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0
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0, 0, 0, 0, 0, 0, 4, 9, 16, 24, 33, 42, 55, 68, 81, 96, 111, 127, 145, 163, 183, 203, 224, 246, 270, 294, 319, 345, 372, 400, 429, 459, 490, 522, 555, 589, 624, 660, 697, 735, 774, 814, 856, 898, 941, 985, 1030, 1076, 1123, 1171, 1220, 1270, 1321, 1373, 1426
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OFFSET
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1,7
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LINKS
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FORMULA
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for n>=43 a(n) = (1/2)*(n^2 - 3*n - 8 )
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PROG
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(PARI) a(n)=sum(i=1, n, sum(j=1, i, if(sign(1/i+1/j+1/n-1/2)+1, 0, 1)))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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