\everymath, AMS equations, beamer, and tikz not working together

Question

In a beamer document, I have $ expressions embedded in displayed equations (using tikz to make, e.g., colored boxes around some expressions). I put \everymath{\displaystyle} in the preamble so that these expressions aren't show in text style. However, when the $ expression is in an AMS equation environment rather than equation, this \everymath is not honored:

\documentclass{beamer}

\usepackage{tikz}
\usepackage{amsmath}

\everymath{\displaystyle}

\begin{document}
  $\sum_i$ \$ expr\\                 % works
  \tikz \node {$\sum_i$ \$ in tikz}; % works
  \begin{equation}
    \tikz \node {$\sum_i$ tikz in equation}; % works
    \text{$\sum_i$ text in equation}         % works
  \end{equation}
  \begin{gather}
    \tikz \node {$\sum_i$ tikz in gather};   % problem
    \text{$\sum_i$ text in gather}           % works
  \end{gather}
\end{document}

Note that this happens only inside \tikz, not \text; and only with the beamer document class. I am intrigued: why does this happen? And is there a simple workaround?

(Of course there is a simple workaround: put \displaymath in every $ expression ... But maybe something more elegant?)

Answer 1

Something that I'm not sure yet resets the math rendering (probably due to the box font settings or something related to the gather but I'm not really good at that math settings differences about amsmath package). So if you push the command to be executed at every start of the node, it looks like the problem is gone. Also I recommend mathtools instead of amsmath.

\documentclass{beamer}

\usepackage{tikz}
\usepackage{mathtools,lmodern}

\everymath{\displaystyle}
\tikzset{execute at begin node={\everymath{\displaystyle}}} 
\begin{document}
\begin{frame}
  $\sum_i$ \$ expr\\                 % works
  \tikz \node {$\sum_i$ \$ in tikz}; % works
  \begin{equation}
    \tikz[baseline] \node {$\sum_i$ tikz in equation}; % works
    \text{$\sum_i$ text in equation}         % works
  \end{equation}
  \begin{gather}
    \tikz[baseline] \node {$\sum_i$ tikz in gather};   % problem
    \text{$\sum_i$ text in gather}           % works
  \end{gather}
\end{frame}
\end{document}

Answer 2

beamerbasecolor.sty (loaded by beamer.cls) redefines \start@gather to execute \donotcoloroutermaths which in turn does

\everymath{\everymath{\beamer@setmathcolor}}

You can solve the issue by adding \displaystyle:

\documentclass{beamer}

\usepackage{tikz}
\usepackage{amsmath}

\everymath{\displaystyle}
\makeatletter
\renewcommand\donotcoloroutermaths{%
  \everymath{\everymath{\displaystyle\beamer@setmathcolor}}%
}
\makeatother

\begin{document}
\begin{gather}
  \tikz \node {$\sum_i$ tikz in gather};
\end{gather}
\end{document}

The fix should work also with align and the other amsmath environments.

Answer 3

egreg and percusse both provided answers that solved the problem for the test case I provided. However, in my real document, things got a bit more complicated, so I thought I would share what I learned.

Specifically, I have one \tikz inside an \intertext, and that only works with percusse's solution. On the other hand, I have one $ expression inside a tabular, and that only works with egreg's solution:

\documentclass{beamer}

\usepackage{tikz}
\usepackage{amsmath}

\everymath{\displaystyle}

% percusse
\tikzset{execute at begin node={\everymath{\displaystyle}}} 

% egreg
\makeatletter
\renewcommand\donotcoloroutermaths{%
  \everymath{\everymath{\displaystyle\beamer@setmathcolor}}%
}
\makeatother

\begin{document}
\begin{gather}
  \tikz \node {$\sum_i$ tikz in gather};    % works with either solution
  \intertext{                               % works with percusse's solution
    \tikz \node {
      $\sum_i$ tikz in intertext
    };
  }
\end{gather}

\begin{tabular}{l}
  $\sum_i$                                  % works with egreg's solution
\end{tabular}
\end{document}

So in the end, I am using both solutions combined.