# Greek character in Beamer document title

I need to include the Greek letter sigma in the title of my Beamer presentation. When I use $\sigma$ to obtain the symbol (which works fine in other LaTeX document titles), I receive the warning messages:

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding):
(hyperref)                removing math shift' on input line 28.

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding):
(hyperref)                removing \sigma' on input line 28.

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding):
(hyperref)                removing math shift' on input line 28.


This appears to be happening whenever I use the math environment within the title, regardless of what text is within the $...$ symbols. Note: the sigma symbol still appears in the PDF, but it worries me that it might cause a problem later on.

Is there any way to resolve these warning messages? Alternatively, is there another way to include the letter sigma without using the math environment?

This happens because in general math mode symbols are not available for inclusion in PDF bookmarks, etc. One approach is to use \texorpdfstring:
\title{Some text \texorpdfstring{$\sigma$}{[sigma]}}

which will use $\sigma$ in TeX and [sigma]` for the bookmark. On the other hand, if you are using sigma for something non-mathematical, then consider using a UTF-8 engine (XeTeX or LuaTeX) and inserting the character directly.