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!

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, Dec 06 '20 at 13:13
See also https://tex.stackexchange.com/questions/116506/how-are-big-operators-defined — Steven B. Segletes, Dec 06 '20 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, Dec 06 '20 at 15:01
It may seem strange, but we typically don't thank contributors in the comments. Instead we show thanks by upvoting any useful answers and accepting the one we find most helpful. Welcome to TeX.SX! — Sandy G, Dec 06 '21 at 00:55

score 40 · Answer 1 · answered Dec 06 '20 at 14:56

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}

score 17 · Answer 2 · answered Dec 06 '20 at 13:32

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

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}
\makeatletter
\DeclareRobustCommand\bigop[2][1]{%
  \mathop{\vphantom{\sum}\mathpalette\bigop@{{#2}{#1}}}\slimits@
}
\newcommand{\bigop@}[2]{\bigop@@#1#2}
\newcommand{\bigop@@}[3]{%
  \vcenter{%
    \sbox\z@{$#1\sum$}%
    \hbox{\resizebox{#3\dimexpr\ifx#1\displaystyle.9\fi\dimexpr\ht\z@+\dp\z@}{!}{$\m@th#2$}}%
  }%
}
\makeatother
\newcommand{\bigconcat}{\DOTSB\bigop[0.5]{[,}}
\begin{document}
[
\sum_{i=1}^n \bigconcat_{i=1}^n x_i
\qquad
\textstyle
\sum\bigconcat_{i=1}^n x_i
\qquad
\scriptstyle
\sum\bigconcat_{i=1}^n x_i
\qquad
\scriptscriptstyle
\sum\bigconcat_{i=1}^n x_i
]
\end{document}

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

score 10 · Answer 3 · answered Dec 06 '20 at 17:00

10

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

\def\concat{\mathop{\left[\vphantom{\sum}\right.}}
Test:
$\concat_{n=1}^\infty 
 \displaystyle \concat_{n=1}^\infty
 \scriptstyle \concat_{n=1}^\infty
$
\bye

answered Dec 06 '20 at 17:00

wipet

74,238

How to create a new math operator?

3 Answers3

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