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A016725
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Number of integer solutions to x^2+y^2+z^2 = n^2, allowing zeros and distinguishing signs and order.
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8
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1, 6, 6, 30, 6, 30, 30, 54, 6, 102, 30, 78, 30, 78, 54, 150, 6, 102, 102, 126, 30, 270, 78, 150, 30, 150, 78, 318, 54, 174, 150, 198, 6, 390, 102, 270, 102, 222, 126, 390, 30, 246, 270, 270, 78, 510, 150, 294, 30, 390, 150, 510, 78, 318, 318, 390, 54, 630, 174, 366
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OFFSET
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0,2
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COMMENTS
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Hurwitz found a formula for a(n). See the paper by Olds.
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LINKS
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FORMULA
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a(n) = 6 * b(n) if n>0 where b(n) is multiplicative with b(2^e) = 1, b(p^e) = p^e if p == 1 (mod 4), b(p^e) = p^e + 2 * (p^e - 1) / (p - 1) if p == 3 (mod 4). - Michael Somos, Nov 18 2011
a(n) = [x^(n^2)] theta_3(x)^3, where theta_3() is the Jacobi theta function. - Ilya Gutkovskiy, Apr 20 2018
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EXAMPLE
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1 + 6*x + 6*x^2 + 30*x^3 + 6*x^4 + 30*x^5 + 30*x^6 + 54*x^7 + 6*x^8 + ...
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MAPLE
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for n from 0 to 60 do s:=0: for x from -n to n do for y from -n to n do for z from -n to n do if (x^2+y^2+z^2) = n^2 then s:=s+1 fi od od od: printf("%d, ", s) od: # C. Ronaldo (aga_new_ac(AT)hotmail.com), Dec 13 2004
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MATHEMATICA
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SquaresR[3, Range[0, 100]^2]
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PROG
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(PARI) {a(n) = if( n<1, n==0, polcoeff( sum( k=1, n, 2 * x^k^2, 1 + x * O(x^n^2))^3, n^2))} /* Michael Somos, Nov 18 2011 */
(PARI) {a(n) = local(A, p, e); if( n<1, n==0, A = factor(n); 6 * prod( k=1, matsize(A)[1], if( p = A[k, 1], e = A[k, 2]; if( p==2, 1, p^e + if( p%4 == 1, 0, 2 * (p^e - 1) / (p - 1))))))} /* Michael Somos, Nov 18 2011 */
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CROSSREFS
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KEYWORD
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AUTHOR
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csvcjld(AT)nomvst.lsumc.edu
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EXTENSIONS
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Revised description from C. Ronaldo (aga_new_ac(AT)hotmail.com), Dec 13 2004
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STATUS
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approved
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