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I am trying to code the following;

f:X->Y
  x|->f(x)

where -> is \longrightarrow and |-> is \longmapsto. However, my problem is that I am unable to get the x directly underneath the X. I tried the following code:

\\* $f : X\longrightarrow Y$
\\* $\hphantom{<1>}   $x\longmapsto f(x)$\\

However this doesn't get it directly under the X as desired. Any help will be much appreciated.

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Sorry it didn't appear properly in this post, I wanted the small x directly under the big X –  Eduardo Dec 13 '13 at 17:29
2  
Use the align environment from amsmath package. –  Thruston Dec 13 '13 at 17:31
    
Is this expression supposed to occur in a two-line displaymath context with both lines set in the same font size, or is supposed to occur in inline math and with x\longmapsto f(x) fairly small and set below f:X\longrightarrow Y? Please advise. –  Mico Dec 13 '13 at 18:09

2 Answers 2

\begin{equation*}
  \begin{split}
    f: & X\rightarrow Y ,\\
      & x\mapsto f(x).
  \end{split}
\end{equation*}
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I would personally prefer the alignment offered by a direct use of array:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\[
  \begin{array}{@{}r@{\;}c@{\;}c@{\;}l@{}}
    f: & X & \rightarrow & Y,   \\
       & x & \mapsto     & f(x).
  \end{array}
\]
\end{document}

The first argument of array (i.e. @{}r@{\;}c@{\;}c@{\;}l@{} in this case) is a specification for the alignment of the columns, as found in the tabular environment. The @{...} part specifies the space between the columns and I use it here (as suggested by @egreg in the comments) to make the spacing more uniform with the rest of the document.

Result

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3  
IMO this gives too much space, and the X \to Y part end up looking inconsistent with other X\to Y in the normal text –  daleif Dec 13 '13 at 18:05
1  
\begin{array}{@{}r@{\;}c@{\;}c@{\;}c@{}} –  egreg Dec 13 '13 at 18:06
    
@egreg Thanks that's exactly what I was going to propose in a moment! Spot on as always =) –  Bordaigorl Dec 13 '13 at 18:08
    
Edited as suggested by @egreg –  Bordaigorl Dec 13 '13 at 18:12

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