I would suggest you tu use the tikz-cd package for your commutative diagrams; also, since the problematic label is not really the name of a map, but rather its action, I would also suggest you to describe the map outside the diagram (otherwise, the diagram might result too crowded):

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

\DeclareMathOperator{\Ker}{Ker}

\begin{document}

\[
\begin{tikzcd}
G\arrow[twoheadrightarrow]{r}{\varphi}\arrow{d}{\pi} &amp; G' \\
G/\Ker(\varphi)\arrow[sloped,swap]{ur}[near start]{\tau}[swap]{\cong}
\end{tikzcd}
\]
where $\tau:G/\Ker(\varphi)\rightarrow G'$ is defined by $g\Ker({\varphi}){\mapsto}{\varphi(G)}$.

\end{document}




Of course, you can draw the diagram with the originally requested labels, perhaps increasing the distance between nodes:

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

\DeclareMathOperator{\Ker}{Ker}

\begin{document}

\[
\begin{tikzcd}[row sep=60pt,column sep=huge]
G\arrow[twoheadrightarrow]{r}{\varphi}\arrow{d}{\pi} &amp; G' \\
G/\Ker(\varphi)\arrow[sloped,swap]{ur}[pos=0.15]{g\Ker({\varphi}){\mapsto}{\varphi(G)}}[swap]{\cong}
\end{tikzcd}
\]

\end{document}




Notice the use of

\DeclareMathOperator{\Ker}{Ker}


(requires the amsmath package) to produce the correct shape for the Ker operator; (I initially overlooked this, but  Ethan Bolker signaled it out in a comment).

Tikz: Text along top and bottom of arrow path
As I do not use Libre.fm, I'd like to remove it from the sidepane in Rhythmbox (Ubuntu Precise). I did it some time ago on of my machines. But now I cannot recall how I did it. :-( 
I could not find a plugin to deactivate, or a package to uninstall. 
