Function theory may refer to:

• Theory of functions of a real variable, the traditional name of real analysis, a branch of mathematical analysis dealing with the real numbers and real-valued functions of a real variable
• Theory of functions of a complex variable, the historical name for complex analysis, the branch of mathematical analysis that investigates functions of complex numbers
• Constructive function theory, the study of the connection between the smoothness of a function and its degree of approximation
• Geometric function theory, the study of geometric properties of analytic functions

Topics referred to by the same term

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