Math in the title changes to lowercase automatically (and math symbols render error)

Question

Say, you need upper case math symbols in a chapter title. If one uses classicthesis, the pdf shows a right title with, unfortunately, wrong headings in the next page.

How to fix this unwished case-change? If one uses e.g. \hat{A}, one even gets an error. (this next MWE shows for instance the ea-genus in the headings, and an error).

\documentclass[]{scrreprt}

\PassOptionsToPackage{
 pdfspacing,
                     subfig,beramono,eulermath,parts}{classicthesis}                                        

% \PassOptionsToPackage{fleqn}{amsmath}        
    \usepackage{amsmath}
\usepackage{xspace} % to get the spacing after macros right  
\usepackage{subfig}  
\usepackage{classicthesis} 

\begin{document}


\chapter{The $ \hat{A} $-genus } 

 ...is defined somewhere in next pages

 \newpage

 blah blah....


\end{document}

How to protect the math-part in the title, so that it is also shown as such in the page's headings?

Answer 1

A protected command helps:

\DeclareRobustCommand*{\hatA}{\ensuremath{\hat{A}}}
\begin{document}
\chapter{The \hatA-genus}

And with some bookmark support:

\DeclareRobustCommand*{\hatA}{%
  \texorpdfstring{%
    \ensuremath{\hat{A}}%
  }{%
    \^A%
  }%
}