For every
the complex value can be split into its real and imaginary parts and , respectively, which can be considered as real functions of two real variables:
The functions and are called the real part and the imaginary part of the complex function , respectively. Following are the notations for and that are used most commonly (the parentheses around may be omitted):
Re
Im
The branch of mathematics concerning differentiable complex functions is called function theory or complex analysis.