Link the deferred calculation

tex/complex-ana/quintic.tex

@@ -173,7 +173,8 @@ \section{Step 3: Normal Groups}
 \end{definition}
 
 So all that remains is showing that $S_5$ is not solvable.
-We leave the housekeeping as an exercise.
+This is a calculation that isn't relevant to the topology ideas in this chapter,
+so we defer it to \Cref{prob:A_5_not_solvable}.
 
 \section{Summary}
 While this is indeed a valid proof, it has some pros and cons.
@@ -190,5 +191,6 @@ \section{Summary}
 
 \begin{problem}
 	\gim
+	\label{prob:A_5_not_solvable}
 	Show that $A_5$ is not solvable.
 \end{problem}