New math operator (EDIT: newcommand!) with parentheses and subscript

Question

I would like to declare a new MathOperator for the trace of a matrix / an operator.

So far I have: \DeclareMathOperator{\Tr}{Tr}, but I don't know how to implement the parentheses.

What may make it more complicated, is that I would also like to compute the partial Trace, which I would like to write in the tex file as $ \Tr_{i} { ... } $ and have the effect of $ Tr_{i} \left( ... \right) $. I prefer \left( and \right) to ( and ), respectively because some of the objects I will compute the trace of have hats and \left( and \right) seem to adjust the size of the brackets.

I am new to Latex, so if declaring a math operator is not the way to go about things to accomplish what I want, I am open to other suggestions.

Answer 1

I don't think you are going to save yourself much time by creating a math operator that takes a parenthesis-delimited argument.

Moreover, if you use \left and \right to auto-size the delimiters, you will end up with a typographically incorrect amount of space between Tr and the opening parenthesis. To avoid this incorrect spacing, I suggest you load the mleftright package and use \mleft( and \mright) instead of \left( and \right).

\documentclass{article}
\usepackage{mathtools}  % for '\DeclareMathOperator' macro
\DeclareMathOperator{\Tr}{Tr}
\usepackage{mleftright} % for '\mleft' and '\mright' macros

\begin{document}
$-\Tr(A)$ vs. $-\Tr\mleft(A\mright)$ vs. $-\Tr\left(A\right)$
\end{document}

Answer 2

You can define \newcommand{\Tr}[1]{\operatorname{Tr}\left(#1\right)}.