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A014201
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Number of solutions to x^2 + x*y + y^2 <= n excluding (0,0).
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4
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0, 6, 6, 12, 18, 18, 18, 30, 30, 36, 36, 36, 42, 54, 54, 54, 60, 60, 60, 72, 72, 84, 84, 84, 84, 90, 90, 96, 108, 108, 108, 120, 120, 120, 120, 120, 126, 138, 138, 150, 150, 150, 150, 162, 162, 162, 162, 162, 168
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OFFSET
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0,2
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LINKS
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FORMULA
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Equals A038589(n) - 1. - Neven Juric, May 10 2010
a(n) = 6*Sum_{k=0..n/3} (floor(n/(3k+1)) - floor(n/(3k+2))).
a(n) is asymptotic to 2*(Pi/sqrt(3))*n.
Conjecture: a(n) = 2*(Pi/sqrt(3))*n + O(n^(1/4 + epsilon)), similar to the Gauss circle or Dirichlet divisor problems. (End)
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MATHEMATICA
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a[n_] := Sum[ Length[ {ToRules[ Reduce[ x^2 + x*y + y^2 == k, {x, y}, Integers]]}], {k, 1, n}]; Table[ a[n], {n, 0, 48}] (* Jean-François Alcover, Feb 23 2012 *)
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PROG
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(PARI) a(n)=6*sum(k=0, n\3, (n\(3*k+1))-(n\(3*k+2))) \\ Benoit Cloitre, Oct 27 2012
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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