Undesired behaviour within systeme

Question

When I try to write a sum over some index in systeme* within an align environment, the sum is not displayed in the usual way, with the index displayed under the summation symbol, but instead on the lower right as if in inline math mode. Any workarounds?

\begin{align}
    \systeme*{\sum_{i \in I_1} f_i  = C}
\end{align} 

Answer 1

You want to use \systeme for explicit linear systems, when you want to align the variables. In your case there's nothing to align (and just one formula), so you want

\begin{equation}
\biggl\{\,\sum_{i\in I_1} f_i = C
\end{equation}

If you have more than one summation, you can use an inner aligned.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

If you have just one equation to display
\begin{equation}
\biggl\{\,\sum_{i\in I_1} f_i = C
\end{equation}
but if you have more than one, you can do
\begin{equation}
\left\{
  \begin{aligned}
  \sum_{i\in I_1} f_i &= C_1 \\
  \sum_{i\in I_2} f_i &= C_2 \\
  \vdots\quad \\
  \sum_{i\in I_n} f_i &= C_n
  \end{aligned}
\right.
\end{equation}

\end{document}

For the first case, the alternative of \Biggl\{ (or \left\{ with a matching \right. at the end) is not as attractive: