A maximization problem

By Randall J. Elzinga

Suppose that \sum_{i=1}^m s_i=s and \sum_{i=1}^n t_i=t for some m\leq n. What is

\max_{\pi\in \Pi} \sum_{i=1}^m s_it_{\pi(i)},

where \Pi is the set of functions from [m] to [n] ([m]=\{1,\ldots,m\})?

That is, if two sets of numbers (possibly multisets) sum to numbers s and t, what is the maximum sum of products of pairs of numbers, where each pair contains exactly one element from each set?

Is the question different if we restrict to the case where s_i and t_j are integral for all i and j?

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