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A362931
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a(n) = number of pairs (i,j) in [1..n] X [1..n] with integral harmonic mean 2*i*j/(i+j).
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9
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1, 2, 3, 4, 5, 10, 11, 12, 13, 14, 15, 20, 21, 22, 27, 28, 29, 34, 35, 40, 41, 42, 43, 48, 49, 50, 51, 56, 57, 66, 67, 68, 69, 70, 75, 80, 81, 82, 83, 88, 89, 98, 99, 100, 109, 110, 111, 116, 117, 118, 119, 120, 121, 126, 127, 132, 133, 134, 135, 148, 149, 150, 155, 156, 157, 166, 167, 168, 169, 174, 175, 184, 185, 186, 191, 192, 197, 202, 203, 208
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OFFSET
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1,2
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LINKS
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FORMULA
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a(n) = n + Sum_{1<=i<j<=n, (i+j)|2*i*j} 2. - Chai Wah Wu, Aug 28 2023
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MAPLE
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a:= proc(n) option remember; `if`(n=0, 0, a(n-1)-1+
2*add(`if`(irem(2*i*n, i+n)=0, 1, 0), i=1..n))
end:
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PROG
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(Python)
def A362931(n): return n+(sum(1 for x in range(1, n+1) for y in range(1, x) if not (x*y<<1)%(x+y))<<1) # Chai Wah Wu, Aug 28 2023
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CROSSREFS
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Cf. A000982 (arithmetic mean analog), A132188 (geometric mean analog).
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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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