List of predefined spaces:
(whereas $$a\b$$ is not a valid filter expression since the blank space is missing; it is recommended to use the tilde ~ instead of the simple whitespace)
(second space is 10x2=20px)
Note: Simple blank spaces and tildes (~) are ignored by the TeX filter and don't produce any space. You must use one of the defined math spaces to get a visible (extra) space.
» Math Notation Help