Change math font only in some parts of a document?

Question

Within a larger document with lots of formulas, I use a self-defined "Example" environement for illustrational purposes from time to time. In order to better set it apart from the rest of the text, I have decided to use another font family within those example surroundings.

But - and here's my problem - the code I use only changes the text font. How can I use another math font for the formulas within the defined enviromenent?

\newenvironment{Ex}{\fontfamily{ppl}\selectfont\par\vspace{1ex}\begin{quote}\textbf{Example: }}{\end{quote}\par}

Thanks in advance for any help!

Edit: More details regarding my question. Let me illustrate it with an example:

[Here's some default-formatted text]
    \begin{Example}
     [some example text I want in another font family]
     \begin{equation}
      and now I need a formula within the example-environement set in a different style than the standard layout to underline it's still part of the example-surrounding
     \end{equation}
     [maybe some more text within the example]
    \end{Example}
    [And now back to the main body of the text set once again in the default layout]

I hope this clarifies (rather than confuses) my question.

Answer 1

You can do this using the unicode math package. You need to use xelatex or lualatex for this to work (rather than just pdflatex). As a rather simple example, the following switches from italic to upright font within the example environment.

\documentclass{minimal}
\usepackage{unicode-math}

\setmathfont{xits-math.otf}

\newenvironment{example}{\par\textbf{Example:}\setmathfont[math-style=upright]{xits-math.otf}}{\par}

\begin{document}

\[
  E = m c^2
\]

\begin{example}
A famous equation is
%
\[
  E = m c^2
\]
\end{example}

\[
  E = m c^2
\]

\end{document}

(You need to have the STIX fonts for this particular example to work.) This produces:

For more on the syntax, I recommend that you read the unicode-math documentation. I should also warn you that changing the font several times in a document can slow down the compilation time considerably. It might not, I'm not sure exactly what it is that causes the slow-down, but on some documents where I change maths font a lot then I find that I have to disable the font changing when writing the document and only enable it for the final run.

Answer 2

With the instructions below, only in Example environment the main font (of text and math formulas) will be changed into computer modern sans one.

    \DeclareMathVersion{sfmath}
    \DeclareSymbolFont{sfletters}{OT1}{cmss}{m}{sl}         
        \SetSymbolFont{letters}{sfmath}{OT1}{cmss}{m}{sl}   
    \DeclareSymbolFont{sfoperators}{OT1}{cmss}{m}{n}
        \SetSymbolFont{operators}{sfmath}{OT1}{cmss}{m}{n}  
    \SetMathAlphabet\mathit{sfmath}{OT1}{cmss}{m}{sl}
    \SetMathAlphabet\mathrm{sfmath}{OT1}{cmss}{m}{n}
    \SetMathAlphabet\mathbf{sfmath}{OT1}{cmss}{bx}{n}
    %\SetMathAlphabet\mathbb{sfmath}{OT1}{cmss}{bx}{n}

    \DeclareSymbolFont{greek}{OML}{cmr}{m}{n}   
    \DeclareMathSymbol{\alpha}{0}{greek}{"0B}
    \DeclareMathSymbol{\beta}{0}{greek}{"0C}
    \DeclareMathSymbol{\gamma}{0}{greek}{"0D}
    \DeclareMathSymbol{\delta}{0}{greek}{"0E}
    \DeclareMathSymbol{\epsilon}{0}{greek}{"0F}
    \DeclareMathSymbol{\zeta}{0}{greek}{"10}
    \DeclareMathSymbol{\eta}{0}{greek}{"11}
    \DeclareMathSymbol{\theta}{0}{greek}{"12}
    \DeclareMathSymbol{\iota}{0}{greek}{"13}
    \DeclareMathSymbol{\kappa}{0}{greek}{"14}
    \DeclareMathSymbol{\lambda}{0}{greek}{"15}
    \DeclareMathSymbol{\mu}{0}{greek}{"16}
    \DeclareMathSymbol{\nu}{0}{greek}{"17}
    \DeclareMathSymbol{\xi}{0}{greek}{"18}
    \DeclareMathSymbol{\pi}{0}{greek}{"19}
    \DeclareMathSymbol{\rho}{0}{greek}{"1A}
    \DeclareMathSymbol{\sigma}{0}{greek}{"1B}
    \DeclareMathSymbol{\tau}{0}{greek}{"1C}
    \DeclareMathSymbol{\upsilon}{0}{greek}{"1D}
    \DeclareMathSymbol{\phi}{0}{greek}{"1E}
    \DeclareMathSymbol{\chi}{0}{greek}{"1F}
    \DeclareMathSymbol{\psi}{0}{greek}{"20}
    \DeclareMathSymbol{\omega}{0}{greek}{"21}
    \DeclareMathSymbol{\varepsilon}{0}{greek}{"22}
    \DeclareMathSymbol{\vartheta}{0}{greek}{"23}
    \DeclareMathSymbol{\varpi}{0}{greek}{"24}
    \DeclareMathSymbol{\varrho}{0}{greek}{"25}
    \DeclareMathSymbol{\varsigma}{0}{greek}{"26}
    \DeclareMathSymbol{\varphi}{0}{greek}{"27}

    \DeclareSymbolFont{Greek}{OT1}{cmr}{m}{n}
    \DeclareMathSymbol{\Gamma}{0}{Greek}{"00}
    \DeclareMathSymbol{\Delta}{0}{Greek}{"01}
    \DeclareMathSymbol{\Theta}{0}{Greek}{"02}
    \DeclareMathSymbol{\Lambda}{0}{Greek}{"03}
    \DeclareMathSymbol{\Xi}{0}{Greek}{"04}
    \DeclareMathSymbol{\Pi}{0}{Greek}{"05}
    \DeclareMathSymbol{\Sigma}{0}{Greek}{"06}
    \DeclareMathSymbol{\Upsilon}{0}{Greek}{"07}
    \DeclareMathSymbol{\Phi}{0}{Greek}{"08}
    \DeclareMathSymbol{\Psi}{0}{Greek}{"09}
    \DeclareMathSymbol{\Omega}{0}{Greek}{"0A}

    \DeclareSymbolFont{mainsymbols}{OT1}{cmr}{m}{n}     
    \DeclareMathSymbol{+}{2}{mainsymbols}{"2B}
    \DeclareMathSymbol{=}{3}{mainsymbols}{"3D}

    \DeclareSymbolFont{othersymbols}{OML}{cmr}{m}{n}        
    \DeclareMathSymbol{<}{3}{othersymbols}{"3C}
    \DeclareMathSymbol{/}{0}{othersymbols}{"3D}
    \DeclareMathSymbol{>}{3}{othersymbols}{"3E}
    \DeclareMathSymbol{\triangleright}{3}{othersymbols}{"2E}
    \DeclareMathSymbol{\partial}{0}{othersymbols}{"40}
    \DeclareMathSymbol{\ell}{0}{othersymbols}{"60}

    \DeclareSymbolFont{parentesi}{OT1}{cmr}{m}{n}       
    \DeclareMathSymbol{(}{4}{parentesi}{"28}
    \DeclareMathSymbol{)}{5}{parentesi}{"29}
    \DeclareMathSymbol{[}{4}{parentesi}{"5B}
    \DeclareMathSymbol{]}{5}{parentesi}{"5D}

    \DeclareMathDelimiter{(}{\mathopen}{parentesi}{"28}{largesymbols}{"00}
    \DeclareMathDelimiter{)}{\mathclose}{parentesi}{"29}{largesymbols}{"01}
    \DeclareMathDelimiter{[}{\mathopen}{parentesi}{"5B}{largesymbols}{"02}
    \DeclareMathDelimiter{]}{\mathclose}{parentesi}{"5D}{largesymbols}{"03}

    \newenvironment{Example}{%
    \mathversion{sfmath}%
    \bigbreak\noindent\textbf{Example.}}
    {\par}

However, dots and commas are not correct: their output is colon and semicolon because of the position of these digits into the computer modern sans font table. You may correct this error with

    \DeclareMathSymbol{,}{6}{mainsymbols}{"2C}
    \DeclareMathSymbol{.}{6}{mainsymbols}{"2E}

although, in my opinion, it still sounds like an "error".