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We are given $2n$ positive integers $a_1,a_2\ldots,a_n$ and and $$b_1,b_2,\ldots,b_n$$ as input.

The question is to find a permutation $O$ on $\{1,2,\ldots,n\}$ that minimizes $$\sum_{i=1}^n \left(a_{O(i)} \cdot \sum_{j=1}^i b_{O(j)}\right).$$

For example, take $n = 3$, $a_1 = 10, a_2 = 1, a_3 = 100, b_1 = 10, b_2 = 100$, and $b_3 = 1$. If we take $O$ so that $O(1) = 1, O(2) = 3$, and $O(3) = 2$, we have $$\sum_{i=1}^n \left(a_{O(i)} \cdot \sum_{j=1}^i b_{O(j)}\right) = 1311.$$

The desired output is $O(1) = 3, O(2) = 1$, and $O(3) = 2$, which yields $$\sum_{i=1}^n \left(a_{O(i)} \cdot \sum_{j=1}^i b_{O(j)}\right) = 321.$$

Trying all possibility is exponential. How can I find an optimal permutation $O$?

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    $\begingroup$ Have you tried any other approaches? Have you tried proving that the problem is hard? $\endgroup$ – Yuval Filmus Nov 13 '17 at 13:04
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    $\begingroup$ Given a solution $O$, consider what happens when you switch $O(i)$ and $O(j)$. When is this beneficial? What features does a local optimum satisfy? Can you extract an algorithm out of this? $\endgroup$ – Yuval Filmus Nov 13 '17 at 13:11
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Did you check what's the class of your problem? I'd try a reduction to a scheduling problem.

If you want to have something so you solve it today, use Branch and Bound. Bounds should be easy to find.

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    $\begingroup$ Could you elaborate your answer a bit? $\endgroup$ – Evil Nov 13 '17 at 19:58
  • $\begingroup$ Which part ? :) The reduction or the Branch and Bound ? $\endgroup$ – Ricocotam Nov 13 '17 at 23:42

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