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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?

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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.

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