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!

  • 2
    Something like $\mathop{\Big[}\limits_{i=1}^N$?
    – user226564
    Commented Dec 6, 2020 at 13:11
  • 1
    Welcome! If you post a very small code with what you tried, it would be great: people here could find what's wrong without having to guess too much. So please post a minimal working example (MWE) even if it doesn't really work
    – Rmano
    Commented Dec 6, 2020 at 13:13
  • 1
    See also tex.stackexchange.com/questions/116506/… Commented Dec 6, 2020 at 13:37
  • @StevenB.Segletes, after posting my answer, I realize this is essentially the same thing you did in your linked solution. Would you like to post it as an answer? I can remove mine.
    – Sandy G
    Commented Dec 6, 2020 at 15:01
  • 1
    @SandyG Your answer is fine (+1) Commented Dec 6, 2020 at 16:47

3 Answers 3


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:

enter image description here

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: enter image description here

Here is a MWE:





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

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


You can use a modified version of https://tex.stackexchange.com/a/23436/4427 (first code set).




\sum_{i=1}^n \bigconcat_{i=1}^n x_i
\sum\bigconcat_{i=1}^n x_i
\sum\bigconcat_{i=1}^n x_i
\sum\bigconcat_{i=1}^n x_i

The scaling is necessary because of how TeX is treating [.

enter image description here


My idea uses \left[\phantom{\sum}\right. trick:


 \displaystyle \concat_{n=1}^\infty
 \scriptstyle \concat_{n=1}^\infty


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