Complex Analysis, traditionally known as the theory of functions of a complex variable, is a significant branch of mathematics, investigating functions of complex numbers. It is useful in many branches of mathematics, including number theory and applied mathematics; as well as in physics, including hydrodynamics, thermodynamics, and electrical engineering. Complex analysis is particularly concerned with the analytic functions of complex variables or, more generally, meromorphic functions. Complex analysis is widely applicable to two-dimensional problems in physics. Complex analysis is one of the classical branches in mathematics.