Combinatorics: How Many Vectors with Square Sum = K?

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Given a Natural number K, how many combinations \[<br /> x = \left( {x_1 ,...,x_N } \right)<br /> \]<br /> of Natural numbers vectors are there, so that \[<br /> \sum\limits_{i = 1}^N {x_i ^2 } = K<br /> \]<br />?

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