I have an equation in an `align' environment. Over one of the equality sign I would like to draw an arrow (slanted, preferred) with the text on the other end being 'This is always true'. How can I use TikZ package in an equation environment like this?
This link contains what I am looking for:
A possiblity:
\documentclass{article}
\usepackage{tikz}
\def\annotateEquality#1{
\tikz[overlay]
\draw[blue,-latex] (-1.5ex,1.5ex) -- +(.7,.5) node[black,right, draw] {#1};
}
\begin{document}
This is an example:
\[
X =\annotateEquality{Always true} Y
\]
\end{document}
Result:
Note that, due to the overlay option, the tikz picture "does not take" any space in the result, which is Ok for the horizontal aligment (otherwise the Y will be "pushed" to the right), but can be bad for the vertical alignment, since the box can be too close to the previous paragraph. In this case you have to manually insert some \vskip before the equation.
You could also use a tikzmark to mark the appropriate points in the text and then draw the appropriate graphics. The advantage of this is that it separates the content from the annotations. In this specific case the advantage is not huge, but in more complicated situations the same methodology can be applied:
This does require two runs. First one to determine the locations, and the second to do the drawing.
The \tikzmark is from Adding a large brace next to a body of text.
\documentclass{article}
\usepackage{xparse}
\usepackage{tikz}
\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node[baseline] (#1) {};}
\NewDocumentCommand{\DrawBox}{O{} O{} m m}{%
% #1 = line draw options
% #2 = node options
% #3 = tikzmark name
% #4 = node text
\begin{tikzpicture}[remember picture, overlay]
\draw [ultra thick, blue, -latex, shorten >=1pt, #1]
(#3) ++ (-1.5ex,1.5ex) -- +(.7,.5)
node [black, right, draw=black, #2] {#4};
\end{tikzpicture}%
}
\begin{document}
This is an example:
[
X =\tikzmark{Point A} Y
]
and another example
[
X =\tikzmark{Point B} Z
]
\DrawBox{Point A}{Always true}
\DrawBox[draw=red][draw=gray, thin, fill=yellow]{Point B}{Sometimes true}
\end{document}
Such a long text would be unreadable in an equation. I would describe this in the surrounding text. Or you define a new symbol for this and describe it in your nomenclature.
% arara: pdflatex
\documentclass{article}
\usepackage{graphicx}
\usepackage{mathtools}
\newcommand\arroweq[1]{\stackrel{\mathclap{\normalfont\scalebox{0.3}{#1}}}{\mathrel{\vec{=}}}}
\begin{document}
\begin{align}
x=y\\
x\arroweq{this is always right} y
\end{align}
\begin{center}
I would recommend to write $x\mathrel{\mathring{=}}y$\\ where $\mathring =$ stands for \emph{this is always right}.
\end{center}
\end{document}
The mathring is just an example. Use whatever you like. Maybe, there is already a symbol for this.
