[OS X TeX] math in running head

[Alain Schremmer]

\documentclass[11pt]{book} uses caps in the running head and prints,  
say, F(X) instead of f(x) although not in either the section titles  
themselves or in the table of contents or in the chapter toc's as  
generated by titletoc.

I understand that this is "the rule" but would not mind an (easy) way  
to print f(x) in the running head as everywhere else. Can the running  
head be all in lower case which would be fine?

I have searched around, e.g. "math in running head", but to no avail.  
Seems like, "it's just not done".

Hopeful regards
--schremmer