fix latex warnings

blueprint/src/chapter/FrobeniusProject.tex

@@ -116,7 +116,7 @@ \subsection{Examples}
 their product $a_n\in A$, which becomes $X_n^2$ modulo $Q$, so all
 of the $X_{i}$ will be algebraically independent in $L/K$ and $X_{i}^2\in K$.
 
-\section{The extension $B/A$.}
+\section{The extension \texorpdfstring{$B/A$}{B/A}.}
 
 The precise set-up we'll work in is the following. We fix $G$ a finite group acting
 on $B$ a commutative ring, and we have another commutative ring $A$ such
@@ -203,7 +203,7 @@ \section{The extension $B/A$.}
 \begin{proof} Use $M_b$.
 \end{proof}
 
-\section{The extension $(B/Q)/(A/P)$.}
+\section{The extension \texorpdfstring{$(B/Q)/(A/P)$}{(B/Q)/(A/P)}.}
 
 Note that $Q$ is prime, so $B/Q$ is an integral domain and hence nontrivial.
 Furthermore, all our polynomials are monic and hence nonzero (indeed they
@@ -280,7 +280,7 @@ \section{The extension $(B/Q)/(A/P)$.}
   $X=\beta$ shows that $\beta$ divides $\alpha$.
 \end{proof}
 
-\section{The extension $L/K$.}
+\section{The extension \texorpdfstring{$L/K$}{L/K}.}
 
 \begin{theorem}
   \label{foo1}