How to use only selected math symbols of the STIX fonts?
You can just look in stix.sty and extract the definition for any character:
\documentclass{article}
\DeclareFontEncoding{LS1}{}{}
\DeclareFontSubstitution{LS1}{stix}{m}{n}
\DeclareSymbolFont{arrows1} {LS1}{stixsf} {m} {n}
\DeclareMathDelimiter{\DDownarrow} {\mathrel}{arrows1}{"FF}{arrows1}{"FF}
\begin{document}\showoutput
\[\alpha + \sum x_i \DDownarrow \phi \]
\end{document}
Here's how to add the requested symbols. You don't have correctly identified the required fonts and symbols.
Look for \lbrbrak
in stix.sty
, to find
\stix@MathDelimiter{\lbrbrak} {\mathopen} {largesymbols}{"EE}{largesymbols}{"14}
\stix@MathDelimiter{\rbrbrak} {\mathclose}{largesymbols}{"EF}{largesymbols}{"15}
Thus we need to see what font largesymbols
refers to:
\DeclareSymbolFont{largesymbols} {LS2}{stixex} {m} {n}
OK, now we need LS2
:
\DeclareFontEncoding{LS2}{}{\noaccents@}
\DeclareFontSubstitution{LS2}{stix}{m}{n}
Next we have to choose a different symbolic name for the math font and put the pieces together (in reverse order).
\documentclass{amsart}
\makeatletter
\DeclareFontEncoding{LS2}{}{\@noaccents}
\makeatother
\DeclareFontSubstitution{LS2}{stix}{m}{n}
\DeclareSymbolFont{largesymbolsstix}{LS2}{stixex}{m}{n}
\DeclareMathDelimiter{\lbrbrak}{\mathopen}{largesymbolsstix}{"EE}{largesymbolsstix}{"14}
\DeclareMathDelimiter{\rbrbrak}{\mathclose}{largesymbolsstix}{"EF}{largesymbolsstix}{"15}
\newcommand{\newbrak}[2]{\genfrac{\lbrbrak}{\rbrbrak}{0pt}{}{#1}{#2}}
\begin{document}
\begin{equation}
\lbrbrak x\rbrbrak,\quad
\bigl\lbrbrak x\bigr\rbrbrak,\quad
\newbrak{k}{m} = (m n)! \, \frac{\Omega_{k}(z)}{\Omega_{m}(z)}
\end{equation}
\end{document}