What operators and functions can I use in a Views "Global: Math expression" field?
Views Math expression fields use a custom expression parser in the Chaos Tool Suite (ctools) module. The code for this is contained in ctools/includes/math-expr.inc.

As of mid-July 2012, the built-in functions are:

'sin','sinh','arcsin','asin','arcsinh','asinh',
'cos','cosh','arccos','acos','arccosh','acosh',
'tan','tanh','arctan','atan','arctanh','atanh',
'pow', 'exp',
'sqrt','abs','ln','log',
'time', 'ceil', 'floor', 'min', 'max', 'round'


Most of these are PHP's own math functions. The trigonometric functions beginning with "arc" are aliases to the ones beginning with "a". "ln" is an alias for "log".

The built-in operators are + - * / ^

The first four act just as they do in PHP. The fifth is the exponent operator; x ^ y is the same as pow(x,y). 

You can create your own functions using this syntax, and such functions can use more than one variable. You can use this to create your own modulo function: 

mod(x,y) = x - (floor(x/y) * y);


(Beware: The above function does not work with negative numbers.)