How to write two functions, one on top of the other

Question

How can I write this

in LaTex?

Answer 1

Using the tikz-cd package you have the possibility to use shifts (for example, shift right, shift left with a possible value for fine tuning position) to easily typeset this kind of sets of multiple arrows in commutative diagrams.

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}
X\ar[r,"f"] & Y\ar[r,shift left,"g_{1}"]\ar[r,shift right,"g_{2}",swap] & Z
\end{tikzcd}

\begin{tikzcd}
X\ar[r,"f"] & Y\ar[r,shift left=1ex,"g_{1}"]\ar[r,shift right=0.4ex,"g_{2}",swap] & Z
\end{tikzcd}

\end{document}

Answer 2

A pstricks solution, with the psmatrix environment:

\documentclass{article}
\usepackage{pstricks-add}
\usepackage{auto-pst-pdf} 

\begin{document}

\[ \psset{arrows=->, arrowinset=0.15 , linewidth=0.6pt, labelsep=1.5pt, nodesep=3pt, colsep=1.2cm, shortput=nab}
\everypsbox{\scriptstyle}
\begin{psmatrix}
X & Y & Z
%%%
 \ncline{1,1}{1,2}^{f}\ncline[offset=2pt]{1,2}{1,3}^{g_1}\ncline[offset =-2pt]{1,2}{1,3}_{g_2}
\end{psmatrix} \]

 \end{document}