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\begin{align*}
\begin{array}{c}
\min \: A = \min \: \sum_{i\in B} C_i 
\\ \\
\min \: D = \min \: \sum_{i\in B} \pi_i \cdot F_i
\end{array}
\end{align*}

See http://i.stack.imgur.com/TFIfY.png for an image of the equation. How can I get {i \in B} below the Sigma symbol without breaking the align into multiple aligns?

Thanks!

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2  
I strongly advise you to consider Hendrik's answer: the fact that \sum is typeset in textstyle is a signal that array is not the right environment to typeset your equations. –  Martijn Sep 20 '10 at 10:28

2 Answers 2

up vote 8 down vote accepted

By putting a \displaystyle in front of the \sum. You basically want the font to behave like it does in a top-level environment, because it's nested in an array it gets assigned a \textstyle font size.

Why you're not aligning the equations on the equals sign beats me though. It might be better to use two equation environments in this case, the code wouldn't look so bloated as it does now.

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Thanks. I do not align them on the equal sign because in reality one is much larger than the other. Aligning them does not look very nice... –  Frank Seifert Sep 19 '10 at 13:37
1  
@Frank: Then you shouldn't be using align. You should take a look at the other math environments provided by amsmath and pick one that's more suited to what you're doing. Using array is wrong. –  TH. Sep 20 '10 at 0:14
    
@TH: I only knew of equation and align. I'll look into it. –  Frank Seifert Sep 20 '10 at 1:08
1  
@Frank: There is a gather environment that does exactly what you want, i.e., combine multiple equations without aligning them. –  Pieter Sep 20 '10 at 8:29

I would typeset this as follows:

\begin{align*}
\min \: A &= \min \: \sum_{i\in B} C_i \\
\min \: D &= \min \: \sum_{i\in B} \pi_i \cdot F_i
\end{align*}

Then it is aligned, and the sums are as you want them. If you do not want it aligned, use gather instead of align. If you want more vertical space between the two lines, use \\[1ex] instead of \\. (Incidentally, I would not use \: after the \min.)

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Thanks for the hint to \[1ex]! –  Frank Seifert Sep 20 '10 at 13:52

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