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Is it possible to simplify the following function by removing the double summation?

$$f(x) = \sum_{n=1}^{x-1} \sum_{m=n+1}^x a_{n}b_{m}$$

Or is there no way of removing the sigmas?

Thanks in advance

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There's not much you can do, but if you like you could write

\begin{align} f(x) &= \sum_{n=1}^{x-1}\sum_{m=n+1}^xa_nb_m \\ &= \sum_{n=1}^{x-1}\sum_{m=n}^{x-1}a_nb_{m+1} \\ &= \sum_{{\scriptstyle m,n=1}\atop{\scriptstyle m\ge n}}^{x-1}a_nb_{m+1} \end{align}

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