# Double subscript problem

The code

\DeclareMathOperator*{\FAggregate}{F}

$$q_k'(e) = \FAggregate\limits_{a \in R}_a (q_k(S(a)))$$

causes a problem with an additional _a. What I want is the a to show in the subscript position as shown in the following picture:

How could I achieve that?

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I suggest to add the subscript as an argument to \FAggregate:

\documentclass{article}
\usepackage{amsmath}
\newcommand{\FAggregate}[1][]{\operatorname*{F_\mathnormal{#1}}}

\begin{document}
$$q_k'(e) = \FAggregate[a]_{a \in R} (q_k(S(a))) \ne\FAggregate_{a \in R} (q_k(S(a)))$$
\end{document}

If the subscript is mandatory, then define

\newcommand{\FAggregate}[1]{\operatorname*{F_\mathnormal{#1}}}

and use it as

\FAggregate{a}_{a \in R}

Note that \limits is not necessary in display style.

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You may want to change F_\mathnormal{#1}} to F_{\!\mathnormal{#1}}} to mitigate the visual "hole" that would otherwise arise because the lower-right "zone" of the letter "F" is empty. – Mico Feb 26 '13 at 23:09
@Mico This might be a valuable addition in this particular case, due to the shape of the letter. – egreg Feb 26 '13 at 23:13