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I want to do something similar to this but do not know how:

Multi valued function

I do know how to write multi-valued functions by using cases or align or whatever but do not know how to achieve a text that is to the right and in the MIDDLE between two cases.

How can this be done?

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If your write \[ \begin{cases} … \end{cases} \; k > \frac{B}{2}; \] you get what you want. But may be it's not the optimal way to do it. – Manuel Jan 19 '13 at 13:11
Thanks alot guys – Muaz Aljarhi Jan 19 '13 at 13:43

Although amsmath provides a cases environment, this isn’t a case for cases. The construct contains two (sub-)formulas. The aligned environment can be used here. The left brace \{ scales correctly with the height of the aligned environment when used with \left which needs a final \right..

I also took the liberty to use mathtools\mathclap to not create horizontal gaps around the \sum signs.


   x_{k+l} &= x_k - y_k \sum_{i=k}^{\mathclap{k+l-1}} r_i 2^{-i}, \\
   y_{k+l} &= y_k - x_k \sum_{i=k}^{\mathclap{k+l-1}} r_i 2^{-i}, \\
 k > \frac{B}{2};


enter image description here

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