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A000925
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Number of ordered ways of writing n as a sum of 2 squares of nonnegative integers.
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19
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1, 2, 1, 0, 2, 2, 0, 0, 1, 2, 2, 0, 0, 2, 0, 0, 2, 2, 1, 0, 2, 0, 0, 0, 0, 4, 2, 0, 0, 2, 0, 0, 1, 0, 2, 0, 2, 2, 0, 0, 2, 2, 0, 0, 0, 2, 0, 0, 0, 2, 3, 0, 2, 2, 0, 0, 0, 0, 2, 0, 0, 2, 0, 0, 2, 4, 0, 0, 2, 0, 0, 0, 1, 2, 2, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 4, 0, 0, 0, 2, 2, 0, 0, 0, 0, 0, 0, 2, 1, 0, 4
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OFFSET
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0,2
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REFERENCES
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A. Das and A. C. Melissinos, Quantum Mechanics: A Modern Introduction, Gordon and Breach, 1986, p. 47.
E. Grosswald, Representations of Integers as Sums of Squares. Springer-Verlag, NY, 1985.
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LINKS
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FORMULA
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Coefficient of q^k in (1/4)*(1 + theta_3(0, q))^2.
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MATHEMATICA
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a[n_] := (pr = PowersRepresentations[n, 2, 2]; Count[Union[Join[pr, Reverse /@ pr]], {j_ /; j >= 0, k_ /; k >= 0}]); a /@ Range[0, 100] (* Jean-François Alcover, Apr 05 2011 *)
nn = 100; t = CoefficientList[Series[Sum[x^k^2, {k, 0, Sqrt[nn]}]^2, {x, 0, nn}], x] (* T. D. Noe, Apr 05 2011 *)
SquareQ[n_] := IntegerQ[Sqrt[n]]; Table[Count[FrobeniusSolve[{1, 1}, n], {__?SquareQ}], {n, 0, 100}] (* Robert G. Wilson v, Apr 15 2017 *)
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PROG
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(PARI) a(n)=sum(i=0, n, sum(j=0, n, if(i^2+j^2-n, 0, 1)))
(Haskell)
a000925 n = sum $ map (a010052 . (n -)) $ takeWhile (<= n) a000290_list
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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Jacques Haubrich (jhaubrich(AT)freeler.nl)
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STATUS
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approved
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