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I am struggling to find an answer of the following series

$$\sum_{i=1}^n \frac{1}{1+\exp(a_i+b_ix)}$$

Any suggestion?

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    Any information what $a_i$ and $b_i$ are?2012-10-05
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    This is no series as it has only $n$ terms... What exactly do you want to calculate?2012-10-05
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    I'm confused by the question. What makes you think this series converges? What's the motivation here? It will help to know that.2012-10-05
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    I don't think you will find an expression in terms of $\exp(\sum a_i)$ and $\exp(\sum b_i)$ Note that if one $a_i$ is very large, the corresponding entry into the sum is essentially zero and doesn't matter. When you add all the $a_i$ it will dominate.2012-10-05
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    $a_i$ and $b_i$ are any number in R. I need to find an equal expression which includes $e^{\sum _i^n a_i}$ and $ e^{\sum _i^n b_i}$2012-10-05
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    It should be $\frac{1}{1+exp(\sum a_i+\sum b_i x)}*f(\sum a_i,\sum b_i x)$ . problem is finding function $f$2012-10-05
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    You won't be able to. For an example, let all the $b_i$ be zero. Compare $a_1=a_2=5$ to $a_1=0, a_2=10$ These are not equal, but your expression cannot tell them apart.2012-10-05

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