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A298939
Number of ordered ways of writing n^3 as a sum of n squares of positive integers.
3
1, 1, 1, 4, 1, 286, 7582, 202028, 6473625, 226029577, 8338249868, 391526193477, 19990594900630, 1159906506684446, 74890158861242740, 5119732406649036418, 380146984328280974281, 30198665638519565614034, 2555354508318427693497565
OFFSET
0,4
FORMULA
a(n) = [x^(n^3)] (Sum_{k>=1} x^(k^2))^n.
EXAMPLE
a(3) = 4 because we have [25, 1, 1], [9, 9, 9], [1, 25, 1] and [1, 1, 25].
MATHEMATICA
Table[SeriesCoefficient[(-1 + EllipticTheta[3, 0, x])^n/2^n, {x, 0, n^3}], {n, 0, 18}]
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Jan 29 2018
STATUS
approved