let tikzpicture environment cut into equation

Question

Im new to the tikz package how can I have the arrow closer to the equal sign and can I have it align automatically?

\documentclass{article}

\usepackage{tikz}
\usepackage{amsmath}

\begin{document}
$$\left(\int_{-\infty}^\infty e^{-x^2}\text{d}x\right)^2=\int_{-\infty}^\infty\int_{-\infty}^\infty e^{-x^2-y^2}\text{d}x\text{d}y$$

\hspace{3cm}
\begin{tikzpicture}
        \draw[<-](0.9,0.5) -- (0.9,0) -- (0,0) -- (0,-.5) node[anchor=north] {explanation};
\end{tikzpicture}
\end{document}

Answer 1

This can be done using \tikzmarknode from the tikzmark TikZ library. This command creates a TikZ node around basically whatever you want. Here, the equal sign is a good candidate given what you wish to do. The node can be accessed from a tikzpicture that uses the remember picture and overlay options. You need to compile twice because this technique relies on the .aux file.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

% From egreg: https://tex.stackexchange.com/a/84308/73317
\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\begin{document}
\[ \left( \int_{-\infty}^{+\infty} e^{-x^2} \diff x \right)^{\!2}
   \tikzmarknode{eqsign}{=}
   \int_{-\infty}^{+\infty} \!\!\!
     \int_{-\infty}^{+\infty} \! e^{-x^2-y^2} \diff x \diff y \]

\begin{tikzpicture}[remember picture, overlay]
  \draw[<-] (eqsign) ++ (0,-1ex) -- ++(0,-0.5) -- ++(-0.9,0) -- ++(0,-.5)
            node[anchor=north] {explanation};
\end{tikzpicture}
\end{document}

Answer 2

For an explanation of the \tikznode command see my answer to "How to add arrow in equations and matrix?".

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows} % for arrow tip stealth'xpdf 
\newcommand\tikznode[3][]{%
  \tikz[remember picture,baseline=(#2.base)]
      \node[minimum size=0pt,inner sep=0pt,#1](#2){#3};%
}
\usepackage{amsmath}

\begin{document}
\[\left(\int_{-\infty}^\infty e^{-x^2}\text{d}x\right)^2\mathrel{\tikznode{eq}{$=$}}\int_{-\infty}^\infty\int_{-\infty}^\infty e^{-x^2-y^2}\text{d}x\text{d}y
\]
\begin{tikzpicture}[remember picture,overlay]
  \draw[stealth'-,shorten <=2pt,rounded corners]
    (eq) |- ++(-0.9,-0.5) -- ++(0,-.5) node[below] {explanation};
\end{tikzpicture}
\end{document}

Answer 3

As mentioned in frougon's answer, tikzmark can be used here. However, I think one should also make sure that the annotation does not overlap with the text. So one may want to put the tikzpicture inside the equation, measure the relevant distance and add it.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc,tikzmark}

\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\begin{document}
Some text before the equation.
\[ \left( \,\int\limits_{-\infty}^\infty \mathrm{e}^{-x^2} \diff x \right)^{2}
   \tikzmarknode{eqsign}{=}
   \int\limits_{-\infty}^\infty  \int\limits_{-\infty}^\infty 
   \mathrm{e}^{-x^2-y^2} \diff x \diff y 
\begin{tikzpicture}[remember picture,baseline={(tmp.base)}]
  \node(tmp){\vphantom{1}};
  \draw[latex-, overlay] (eqsign.south)  |- ++(-1,-0.7) -- ++ (0,-0.5)
            node[below] (expl) {explanation};
  % add point to tikzpicture to make it take the right size, subtract inner sep     
  \path ([yshift=2pt]expl.south-|tmp);
\end{tikzpicture}
\]
Some more text after the equation.
\end{document}