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Let $ \chi_{s,k}$ be the indicator function of integers $ n$ which are expressed as sum of $ s$ positive $ k$ -th powers i.e $ \chi_{s,k}(n)=1$ if and only if $ n=a_1^k+…+a_s^k.$ Examples of this type of functions is well-studied specially when $ k$ and $ s$ are of small values: $ \chi_{2,2}(n)=1 $Read more