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A066535
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Number of ways of writing n as a sum of n squares.
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25
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1, 2, 4, 8, 24, 112, 544, 2368, 9328, 34802, 129064, 491768, 1938336, 7801744, 31553344, 127083328, 509145568, 2035437440, 8148505828, 32728127192, 131880275664, 532597541344, 2153312518240, 8710505815360, 35250721087168, 142743029326162, 578472382307304
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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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a(n) equals the coefficient of x^n in the n-th power of Jacobi theta_3(x) where theta_3(x) = 1 + 2*Sum_{n>=1} x^(n^2). - Paul D. Hanna, Oct 25 2009
a(n) ~ c * d^n / sqrt(n), where d = 4.13273137623493996302796465... (= 1/radius of convergence A166952), c = 0.2820942036723951157919967... . - Vaclav Kotesovec, Sep 12 2014
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EXAMPLE
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There are a(3) = 8 solutions (x,y,z) of 3 = x^2 + y^2 + z^2: (1,1,1), (-1,-1,-1), 3 permutations of (1,1,-1) and 3 permutations of (1,-1,-1).
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MAPLE
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b:= proc(n, t) option remember; `if`(n=0, 1, `if`(n<0 or t<1, 0,
b(n, t-1) +2*add(b(n-j^2, t-1), j=1..isqrt(n))))
end:
a:= n-> b(n$2):
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MATHEMATICA
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Join[{1}, Table[SquaresR[n, n], {n, 24}]]
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PROG
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(PARI) {a(n)=local(THETA3=1+2*sum(k=1, sqrtint(n), x^(k^2))+x*O(x^n)); polcoeff(THETA3^n, n)} /* Paul D. Hanna, Oct 25 2009 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Peter Bertok (peter(AT)bertok.com), Jan 07 2002
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EXTENSIONS
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STATUS
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approved
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