Improve functions

Algebra.tex

@@ -103,7 +103,7 @@
   \newcommand\abs[1]{\left\lvert #1 \right\rvert}
   \newcommand\real{\mathfrak{Re}}
   \newcommand\inv[1]{#1^{-1}}
-  \newcommand\fun[4]{\underset{\textstyle #3}{#1} \underset{\textstyle \mapsto}{\to} \underset{\textstyle #4}{#2}}
+  \newcommand\fun[4]{\begin{aligned}{#1} &\to {#2} \\ {#3} &\mapsto {#4} \end{aligned}}
   \newcommand\mc\mathcal
   \newcommand\nt\trianglelefteq
   \newcommand\ideal\triangleleft