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How can be calculate this following summation efficiently (is there some shorter formula for it) ?

$$\sum_{x=0}^{\frac{n}{k}}{n\choose k\cdot x}$$

Darío A. Gutiérrez
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caretaker
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  • I don't know what you mean by efficiently. But here is anothere closed formula: http://math.stackexchange.com/questions/156635/putting-n-choose-0-n-choose-5-n-choose-10-cdots-n-choose-5k?rq=1 – b00n heT Aug 25 '16 at 09:08

2 Answers2

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$$\frac1k \sum_{j=1}^k (1+\omega^j)^n.$$

Here is the answer $\sum_{i=0}^{\lfloor\frac{n}{k}\rfloor}{n \choose i * k}$ in a closed form

Community
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Darío A. Gutiérrez
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  • thank you for your answer. But how can efficiently compute this sum, we need to deal with complex numbers, which may lead to precision issues. Also, complexity will be n*k, which is very slow i guess. – caretaker Aug 25 '16 at 10:51
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$$\sum_{k=0}^{\frac{n}{x}}\binom{n}{x\cdot k} = \binom{n}{x\cdot 1}+\binom{n}{x\cdot 2}+\binom{n}{x\cdot 3}+...+\underbrace{\binom{n}{x\cdot \frac{n}{x}}}_{\binom{n}{n}}$$

$\forall y \in \mathbb{N} : y = \frac{n}{x}$ $$\sum_{k=0}^{n}\binom{n}{x\cdot k} = \binom{n}{x\cdot 1}+\binom{n}{x\cdot 2}+\binom{n}{x\cdot 3}+\underbrace{\binom{n}{x\cdot y}}_{\binom{n}{n}}+\underbrace{\binom{n}{x\cdot (y+1)}+...+\binom{n}{x\cdot n}}_{0}$$

$$\sum_{k=0}^{\frac{n}{x}}\binom{n}{x\cdot k} =\sum_{k=0}^{n}\binom{n}{x\cdot k} $$

Darío A. Gutiérrez
  • 3,863