Make a math function like a picture below
I would use array
(and \colon
instead of :
):
\documentclass{letter}
\usepackage{amsmath}
\usepackage{array}
\begin{document}
\[
\begin{array}{l@{$\;$}c@{$\;$}c@{$\;$}c}
\phi\colon & R &\to& R\\
& a+b\sqrt{2}&\mapsto &a+b
\end{array}
\]
\end{document}
The following requires two compilations with any change within the maximum width of every \eqmakebox[<tag>]
using the same <tag>
. A second (optional) argument can be specified for aligning the content to the l
eft or r
ight (c
entred is default).
\documentclass{article}
\usepackage{amsmath,eqparbox}
\begin{document}
\begin{align*}
\phi \colon \eqmakebox[ab2]{$ R $} \to \eqmakebox[ab]{$ R $} \\
\eqmakebox[ab2]{$a + b\sqrt{2}$} \mapsto \eqmakebox[ab]{$a + b$}
\end{align*}
\end{document}