[Tex/LaTex] How to create a new math operator

Question

How can I create my own scalable math operator with limits just like the sum symbol? Specifically I want to invent a "concatenation" symbol that looks alike to: "[". For example I want to put

[ _{i=1}^N

etc. where the subscript/superscripts are positioned below and above the symbol as in "eqnarray" environment.

I have tried with \newcommand and creating a drawing similar to "[" with \begin{picture} \end{picture}, but it didn't work.

Thanks for the help!

Answer 1

A simple solution would be to use the \DeclareMathOperator* command from the amsmath package. The unstarred version places sub- and superscript limits to the right of the operator; the starred version places limits above and below the operator when it is in displaystyle.

\DeclareMathOperator*{\concat}{[} will produce the following results:

You could leave it like that, but for me, the displayed version produces a [ symbol that is too small. Using the scalerel package, you could make the symbol the same size as a \sum symbol:

\DeclareMathOperator*{\concat}{\scalerel*{[}{\sum}}, which will produce the following:

Here is a MWE:

\documentclass{article}

\usepackage{amsmath,scalerel}

%\DeclareMathOperator*{\concat}{[}
\DeclareMathOperator*{\concat}{\scalerel*{[}{\sum}}

\begin{document}

\[
\concat_{n=1}^{\infty} a_n
\]

Inline: $\concat_{n=1}^{\infty} a_n$.

\end{document}