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A094572
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Number of pairs of integers x, y (of either sign) with x^2 - y^2 = n.
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0
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2, 0, 4, 2, 4, 0, 4, 4, 6, 0, 4, 4, 4, 0, 8, 6, 4, 0, 4, 4, 8, 0, 4, 8, 6, 0, 8, 4, 4, 0, 4, 8, 8, 0, 8, 6, 4, 0, 8, 8, 4, 0, 4, 4, 12, 0, 4, 12, 6, 0, 8, 4, 4, 0, 8, 8, 8, 0, 4, 8, 4, 0, 12, 10, 8, 0, 4, 4, 8, 0, 4, 12, 4, 0, 12, 4, 8, 0, 4, 12, 10, 0, 4, 8, 8, 0, 8, 8, 4, 0, 8, 4, 8, 0, 8, 16, 4, 0, 12, 6
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| The old entry with this sequence number was a duplicate of A058071.
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REFERENCES
| M. N. Huxley, Area, Lattice Points and Exponential Sums, Oxford, 1996; p. 236.
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FORMULA
| a(n) = 2d(n) if n is odd, = 2d(n/4) if n == 0 mod 4, otherwise 0, where d() = A000005().
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MAPLE
| with(numtheory); f:=proc(n) if n mod 2 = 1 then RETURN(2*tau(n)); fi; if n mod 4 = 0 then RETURN(2*tau(n/4)); fi; 0; end;
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CROSSREFS
| Cf. A000005, A093061.
Sequence in context: A155984 A028609 A107490 * A079534 A097042 A196606
Adjacent sequences: A094569 A094570 A094571 * A094573 A094574 A094575
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Nov 02 2008
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