'Straight' Parentheses around Matrices

Question

I'm using the txfonts package in order to access the \lambdaup symbol. In order to maintain consistency with other documents, I'm then loading the lmodern package to reset the font families. However, I would like to keep the style of parentheses for large (> 2 line) matrices (via the \pmatrix environment from amsmath) that \txfonts gives me. Is there a way to do this?

Compare the following example with/without the \usepackage{lmodern} line commented out:

\documentclass{article}
\usepackage{amsmath}
\usepackage{txfonts} % \lambdaup
\usepackage{lmodern}
\begin{document}
    $\begin{pmatrix}
    1 & 0 & 0\\
    0 & 1 & 0\\
    0 & 0 & 1
    \end{pmatrix}$
\end{document}

First image shows the 'default' style of parentheses, from CM fonts:

Second image shows the style of parentheses that I am hoping to achieve, from TX fonts:

Third image shows egreg's proposed answer:

Answer 1

I think you shouldn't mix the parentheses of TX with those of CM, but here's how you can do.

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}

%\usepackage{txfonts} % \lambdaup
\makeatletter
\let\iftx@libertine\iffalse
\let\iftx@minion\iffalse
\let\iftx@coch\iffalse
\makeatother
\DeclareFontEncoding{LMX}{}{}
\DeclareFontSubstitution{LMX}{ntxexx}{m}{n}
\DeclareSymbolFont{TXlettersA}{U}{ntxmia}{m}{it}
\SetSymbolFont{TXlettersA}{bold}{U}{ntxmia}{b}{it}
\DeclareSymbolFont{TXlargesymbols}{LMX}{ntxexx}{m}{n}
\SetSymbolFont{TXlargesymbols}{bold}{LMX}{ntxexx}{b}{n}

\DeclareMathSymbol{\lambdaup}{\mathord}{TXlettersA}{21}
\DeclareMathDelimiter{(}{\mathopen}{operators}{"28}{TXlargesymbols}{0}
\DeclareMathDelimiter{)}{\mathclose}{operators}{"29}{TXlargesymbols}{1}


\begin{document}

$\begin{pmatrix}
1 & 0 & \lambdaup \\
0 & 1 & 0\\
0 & 0 & 1
\end{pmatrix}$

$(a+b)c$

$\bigl(a+b\bigr)c$

\end{document}

If you prefer the parentheses in the style of txfonts, you can do like this:

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}

%\usepackage{txfonts} % \lambdaup
\DeclareSymbolFont{TXlettersA}{U}{txmia}{m}{it}
\SetSymbolFont{TXlettersA}{bold}{U}{txmia}{bx}{it}
\DeclareSymbolFont{TXlargesymbols}{OMX}{txex}{m}{n}
\SetSymbolFont{TXlargesymbols}{bold}{OMX}{txex}{bx}{n}

\DeclareMathSymbol{\lambdaup}{\mathord}{TXlettersA}{21}
\DeclareMathDelimiter{(}{\mathopen}{operators}{40}{TXlargesymbols}{0}
\DeclareMathDelimiter{)}{\mathclose}{operators}{41}{TXlargesymbols}{1}


\begin{document}

$\begin{pmatrix}
1 & 0 & \lambdaup \\
0 & 1 & 0\\
0 & 0 & 1
\end{pmatrix}$

$(a+b)c$

$\bigl(a+b\bigr)c$

\end{document}

Answer 2

Instead of loading the deprecated txfonts pacakge, with provides both text and math Times Roman glyphs, you could load the newer newtxmath package, which doesn't modify the text-mode glyphs. Hence, you can keep the Computer Modern text mode font parts.

\documentclass{article}
\usepackage{amsmath}             % for 'pmatrix' env.
\usepackage[cmbraces]{newtxmath} % provides '\lambdaup'
\begin{document}
    $\begin{pmatrix}
    1 & 0 & \lambdaup\\
    0 & 1 & 0\\
    0 & 0 & 1
    \end{pmatrix}$
\end{document}