\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*}
How can I get i \in B below the \sum symbol without breaking the align into multiple aligns?
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.
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 00:14
gather environment that does exactly what you want, i.e., combine multiple equations without aligning them.
– Pieter
Sep 20 '10 at 08:29
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.)
