# Borrowing the integral and summation signs from mathabx package

I am trying to use only the integral and summation symbols from mathabx package.

In this question, someone provided codes for adding the integral signes, so I tried to tweak it to add the summation sign (\sum), but failed...

In general, if I have a particular sybol that I would like to borrow from come package, and I want to use only that, how can I accomplish it?

Thank you!

The \sum symbol as well as the other math symbols of mathabx are defined in mathabx.dcl file. In particular, the font family of\sum is mathx, the same family of the symbols defined in the post you mentioned. So you just need to add these two lines to the solution of https://tex.stackexchange.com/a/289379/231952.

\let\sum\relax
\DeclareMathSymbol{\sum}{\mathop}{mathx}{"B0}

\let\sum\relax is required to overwriter the definition given by newtxmath.

% !TeX program = pdflatex
% !TeX encoding = UTF-8
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{libertine}
\usepackage[libertine]{newtxmath}
\usepackage{amsmath}

\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{<->mathx10}{}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}

\DeclareMathSymbol{\intop}  {\mathop}{mathx}{"B3}
\DeclareMathSymbol{\iintop} {\mathop}{mathx}{"B4}
\DeclareMathSymbol{\iiintop}{\mathop}{mathx}{"B5}
\DeclareMathSymbol{\ointop} {\mathop}{mathx}{"B6}
\DeclareMathSymbol{\oiintop}{\mathop}{mathx}{"B7}

\let\sum\relax
\DeclareMathSymbol{\sum}{\mathop}{mathx}{"B0}

\begin{document}

$$\sum_{k=m}^{n} f(k) = \dots$$

\end{document}