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I'm not sure how to evaluate the following :

$$ \sum_{k=i}^n \frac{1}{k!(n-k)!} $$

Where $i,n \in \mathbb{N}, n > i$ are given.

I don't have any working for this, I just looked it at and don't have any idea how to go about evaluating it.

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    $\begingroup$ You can at most write it as $\frac1{n!}\sum_{k=i}^n\binom{n}{k}$ but there is no closed form for it. $\endgroup$ – drhab Aug 30 '18 at 11:00
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    $\begingroup$ You have one in the case $i = 0$. $\endgroup$ – ncmathsadist Aug 30 '18 at 11:02
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    $\begingroup$ @ncmathsadist I think for $i=0$ the sum is $\frac{2^n}{n!}$ $\endgroup$ – Henry Aug 30 '18 at 11:02
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    $\begingroup$ @drhab - there may be a form related to the regularized incomplete beta function though this is may not be helpful $\endgroup$ – Henry Aug 30 '18 at 11:06
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    $\begingroup$ @Henry I see. It somehow shifts from sums to integrals (which can always be labeled with some name). $\endgroup$ – drhab Aug 30 '18 at 11:14
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No Closed Form

As pointed out by @drhab, your sum is equivalent to $\frac{1}{n!}\sum_{k=i}^n\binom{n}{k}$. Given that $\sum_{k=0}^n\binom{n}{k}=2^n$, your sum can be rearranged to $$\frac{2^n}{n!}-\frac1{n!}\sum_{k=0}^{i-1}\binom{n}{k}$$

Hence, the partial sum of binomial coefficients, $\sum_{k=0}^{i-1}\binom{n}{k}$, is the heart of your problem. The sum can be expressed in various other ways (such as with hypergeometric or beta functions) and, as pointed out by @Henry, it has some nice expressions for specific $n,i$. But unfortunately, it has no closed form in terms of the sum of a fixed number of hypergeometric terms (Petrovsek, 1996. Theorem 5.6.3, pp. 88, 94, 102; 2, p. 6). But then again, it could possibly have a different type of closed form.

References and Further Reading

Despite having no closed form, the sum has been studied before and can be approximated and bounded:

  1. M. Petkovˇsek, G. S. Wilf and D. Zeilberger, 'A=B' (1996) (purchase eBook) (PDF)

  2. M. Boardman, 'The Egg-Drop Numbers' (2004) (link)

  3. Bounds and algorithms. (link)

  4. Approximations. (link)

  5. Asymptotics. (link)

  6. Relation to hyperplanes. (link)

  7. Wikipedia article. (link)

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