"TM" == Timothy Murphy writes:

 >> e.g. Shafarevich groups in algebraic geometry are often denoted
 >> with capital cyrillic letter sha - not only in russian literature!
 TM> But my serious point is that it seems to me more sensible to use
 TM> a macro-name like \shaf rather than arranging for a single input
 TM> character to create this output.

if the document input encoding is ascii or some latin-based
(e.g. iso-8859-1), then this is indeed more sensible (no other way),
but if the input encoding is cyrillic (e.g. koi8-r) or UTF-8,
then it may be more natural to just use the keyboard character
(cyrillic letter sha) for that.

(with the mathtext package, it will work)