Standard AMS-Sum Operator using MnSymbols?

Question

Because a professor who reads one of the lectures I attend this term writes so slowly, I typeset a lecture script for that lecture in TeX. For the use of some symbols and for better underbrace rendering I use MnSymbol. As those of you who are familiar with MnSymbol may know, it offers a different sum operator than the amsmath-package. It also offers a different integral operator, but I already fixed that by loading "esint" as last math font package, as I like those amsmath math operators better. Is there a way of easily "fixing" the sum operators to be the standard ones without having to stop using MnSymbol? Any help would be appreciated.

Answer 1

See FOLLOW UP below for importing CM symbols into MnSymbol. But first...

Import MnSymbols:

If you only use a few MnSymbols, it is easiest to import just those, rather than loading MnSymbol package. Here is how it is done (I used, for MnSymbol specific reference, Dashed left arrow over symbol, though other questions on this site about importing symbols are also useful).

It helps to look through the MnSymbol package document (or better still, mnsymbol.sty) to find from which font family the symbol derives, and then it helps to use \fonttable (package fonttable) to display the font family in a table, from which you can determine the slot number corresponding to the desired glyph.

First, to determine font families, I look through mnsymbol.sty for the glyph name, and use it to determine the font family:

From the above pictures, I see that the \bigcupdot glyphs (\displaystyle and\textstyle) are \mathops and come from the symbols font. The name symbols is associated with MnSymbolF font family. These are needed in my MWE. The \cupdot glyph comes from a different font family (MnSymbolC).

In the MWE below, I show the importation of 3 glyphs from two different font families: \cupdot, \tbigcupdot (textstyle \bigcupdot) and \dbigcupdot (displaystyle \bigcupdot). I then use \mathchoice, to tell LaTeX to use the proper bigcupdot style in the appropriate math style.

If you uncomment my commented lines concerning fonttable, you can see the font tables from which I determined the glyph's slot numbers.

\documentclass{article}
% =============================================
%Import symbols from font MnSymbol without importing the whole package
% =============================================
\DeclareFontFamily{U} {MnSymbolC}{}

\DeclareFontShape{U}{MnSymbolC}{m}{n}{
  <-6> MnSymbolC5
  <6-7> MnSymbolC6
  <7-8> MnSymbolC7
  <8-9> MnSymbolC8
  <9-10> MnSymbolC9
  <10-12> MnSymbolC10
  <12-> MnSymbolC12}{}
\DeclareFontShape{U}{MnSymbolC}{b}{n}{
  <-6> MnSymbolC-Bold5
  <6-7> MnSymbolC-Bold6
  <7-8> MnSymbolC-Bold7
  <8-9> MnSymbolC-Bold8
  <9-10> MnSymbolC-Bold9
  <10-12> MnSymbolC-Bold10
  <12-> MnSymbolC-Bold12}{}

\DeclareSymbolFont{MnSyC} {U} {MnSymbolC}{m}{n}

\DeclareMathSymbol{\cupdot}{\mathbin}{MnSyC}{60}
% =============================================
\DeclareFontFamily{U} {MnSymbolF}{}

\DeclareFontShape{U}{MnSymbolF}{m}{n}{
  <-6> MnSymbolF5
  <6-7> MnSymbolF6
  <7-8> MnSymbolF7
  <8-9> MnSymbolF8
  <9-10> MnSymbolF9
  <10-12> MnSymbolF10
  <12-> MnSymbolF12}{}
\DeclareFontShape{U}{MnSymbolF}{b}{n}{
  <-6> MnSymbolF-Bold5
  <6-7> MnSymbolF-Bold6
  <7-8> MnSymbolF-Bold7
  <8-9> MnSymbolF-Bold8
  <9-10> MnSymbolF-Bold9
  <10-12> MnSymbolF-Bold10
  <12-> MnSymbolF-Bold12}{}

\DeclareSymbolFont{SymbolF} {U} {MnSymbolF}{m}{n}

\DeclareMathSymbol{\dbigcupdot}{\mathop}{SymbolF}{35}
\DeclareMathSymbol{\tbigcupdot}{\mathop}{SymbolF}{34}
\def\bigcupdot{\mathchoice{\dbigcupdot}{\tbigcupdot}{\tbigcupdot}{\tbigcupdot}}
% =============================================
%\usepackage{fonttable}
\begin{document}
\centering
$ x \cupdot y$\par
$ x \bigcupdot y \quad \scriptstyle x \bigcupdot y
 \quad \scriptscriptstyle x \bigcupdot y$
\[ x \bigcupdot y\]
%\clearpage\fonttable{MnSymbolF8}
%\clearpage\fonttable{MnSymbolC10}
\end{document}

---

FOLLOW UP:

The OP asked if MnSymbol can be the default, with (for example), \sum being imported from default LaTeX. As barbara points out in the comment, the default \sum comes from the cmex font set. Here, I import it as \Xsum (I am not sure if my \DeclareFontShape invocation is appropriate for cm fonts, but I mimicked what had been done for MnSymbol):

\documentclass{article}
\usepackage{mnsymbol}
% =============================================
%Import symbols from font cmex without importing the whole package
% =============================================
\DeclareFontFamily{U} {cmex}{}

\DeclareFontShape{U}{cmex}{m}{n}{
  <-6> cmex5
  <6-7> cmex6
  <7-8> cmex7
  <8-9> cmex8
  <9-10> cmex9
  <10-12> cmex10
  <12-> cmex12}{}

\DeclareSymbolFont{Xcmex} {U} {cmex}{m}{n}

\DeclareMathSymbol{\Xdsum}{\mathop}{Xcmex}{88}
\DeclareMathSymbol{\Xtsum}{\mathop}{Xcmex}{80}
\DeclareMathOperator*{\Xsum}{\mathchoice{\Xdsum}{\Xtsum}{\Xtsum}{\Xtsum}}
% =============================================
\usepackage{fonttable}
\begin{document}
\centering
sum under MnSymbol:\par
$\sum_{i=1}^2 x_i $
\[\sum_{i=1}^2 x_i \]
Defined Xsum from cmex:\par
$\Xsum_{i=1}^2 x_i $
\[\Xsum_{i=1}^2 x_i \]
\tiny\fonttable{cmex8}
\end{document}