A **complex differential equation** is a differential equation whose solutions are functions of a complex variable.

Constructing integrals involves choice of what path to take, which means singularities and branch points of the equation need to be studied. Analytic continuation is used to generate new solutions and this means topological considerations such as monodromy, coverings and connectedness are to be taken into account.

Existence and uniqueness theorems involve the use of majorants and minorants.

Study of rational second order ODEs in the complex plane led to the discovery of new transcendental special functions, which are now known as Painlevé transcendents.

Nevanlinna theory can be used to study complex differential equations. This leads to extensions of Malmquist's theorem.^{ [1] }

Generalizations include partial differential equations in several complex variables, or differential equations on complex manifolds.^{ [2] } Also there are at least a couple of ways of studying complex difference equations: either study holomorphic functions ^{ [3] } which satisfy functional relations given by the difference equation or study discrete analogs^{ [4] } of holomorphicity such as monodiffric functions. Also integral equations can be studied in the complex domain.^{ [5] }

Some of the early contributors to the theory of complex differential equations include:

**Complex analysis**, traditionally known as the **theory of functions of a complex variable**, is the branch of mathematical analysis that investigates functions of complex numbers. It is useful in many branches of mathematics, including algebraic geometry, number theory, analytic combinatorics, applied mathematics; as well as in physics, including the branches of hydrodynamics, thermodynamics, and particularly quantum mechanics. By extension, use of complex analysis also has applications in engineering fields such as nuclear, aerospace, mechanical and electrical engineering.

In the field of complex analysis in mathematics, the **Cauchy–Riemann equations**, named after Augustin Cauchy and Bernhard Riemann, consist of a system of two partial differential equations which, together with certain continuity and differentiability criteria, form a necessary and sufficient condition for a complex function to be complex differentiable, that is, holomorphic. This system of equations first appeared in the work of Jean le Rond d'Alembert. Later, Leonhard Euler connected this system to the analytic functions. Cauchy (1814) then used these equations to construct his theory of functions. Riemann's dissertation on the theory of functions appeared in 1851.

In mathematics, a **holomorphic function** is a complex-valued function of one or more complex variables that is, at every point of its domain, complex differentiable in a neighborhood of the point. The existence of a complex derivative in a neighbourhood is a very strong condition, for it implies that any holomorphic function is actually infinitely differentiable and equal, locally, to its own Taylor series (*analytic*). Holomorphic functions are the central objects of study in complex analysis.

In complex analysis, the **Riemann mapping theorem** states that if *U* is a non-empty simply connected open subset of the complex number plane **C** which is not all of **C**, then there exists a biholomorphic mapping *f* from *U* onto the open unit disk

**Georg Friedrich Bernhard Riemann** was a German mathematician who made contributions to analysis, number theory, and differential geometry. In the field of real analysis, he is mostly known for the first rigorous formulation of the integral, the Riemann integral, and his work on Fourier series. His contributions to complex analysis include most notably the introduction of Riemann surfaces, breaking new ground in a natural, geometric treatment of complex analysis. His famous 1859 paper on the prime-counting function, containing the original statement of the Riemann hypothesis, is regarded as one of the most influential papers in analytic number theory. Through his pioneering contributions to differential geometry, Riemann laid the foundations of the mathematics of general relativity. He is considered by many to be one of the greatest mathematicians of all time.

**Mathematical analysis** is the branch of mathematics dealing with limits and related theories, such as differentiation, integration, measure, infinite series, and analytic functions.

In mathematics, a **partial differential equation** (**PDE**) is an equation which imposes relations between the various partial derivatives of a multivariable function.

In mathematics, the **uniformization theorem** says that every simply connected Riemann surface is conformally equivalent to one of three Riemann surfaces: the open unit disk, the complex plane, or the Riemann sphere. In particular it implies that every Riemann surface admits a Riemannian metric of constant curvature. For compact Riemann surfaces, those with universal cover the unit disk are precisely the hyperbolic surfaces of genus greater than 1, all with non-abelian fundamental group; those with universal cover the complex plane are the Riemann surfaces of genus 1, namely the complex tori or elliptic curves with fundamental group **Z**^{2}; and those with universal cover the Riemann sphere are those of genus zero, namely the Riemann sphere itself, with trivial fundamental group.

The theory of functions of **several complex variables** is the branch of mathematics dealing with complex-valued functions

In mathematics, the **value distribution theory of holomorphic functions** is a division of mathematical analysis. It tries to get quantitative measures of the number of times a function *f*(*z*) assumes a value *a*, as *z* grows in size, refining the Picard theorem on behaviour close to an essential singularity. The theory exists for analytic functions of one complex variable *z*, or of several complex variables.

In the mathematical field of complex analysis, **Nevanlinna theory** is part of the theory of meromorphic functions. It was devised in 1925, by Rolf Nevanlinna. Hermann Weyl has called it "one of the few great mathematical events of century." The theory describes the asymptotic distribution of solutions of the equation *f*(*z*) = *a*, as *a* varies. A fundamental tool is the Nevanlinna characteristic *T*(*r*, *f*) which measures the rate of growth of a meromorphic function.

**Complex dynamics** is the study of dynamical systems defined by iteration of functions on complex number spaces. **Complex analytic dynamics** is the study of the dynamics of specifically analytic functions.

In mathematics, **Painlevé transcendents** are solutions to certain nonlinear second-order ordinary differential equations in the complex plane with the **Painlevé property**, but which are not generally solvable in terms of elementary functions. They were discovered by Émile Picard (1889), Paul Painlevé , Richard Fuchs (1906), and Bertrand Gambier (1910).

In mathematics, **Riemann–Hilbert problems**, named after Bernhard Riemann and David Hilbert, are a class of problems that arise in the study of differential equations in the complex plane. Several existence theorems for Riemann–Hilbert problems have been produced by Mark Krein, Israel Gohberg and others.

In mathematics, the **Riemann–Hilbert correspondence** is a generalization of Hilbert's twenty-first problem to higher dimensions. The original setting was for the Riemann sphere, where it was about the existence of regular differential equations with prescribed monodromy groups. First the Riemann sphere may be replaced by an arbitrary Riemann surface and then, in higher dimensions, Riemann surfaces are replaced by complex manifolds of dimension > 1. There is a correspondence between certain systems of partial differential equations and possible monodromies of their solutions.

In mathematics, the equations governing the **isomonodromic deformation** of meromorphic linear systems of ordinary differential equations are, in a fairly precise sense, the most fundamental exact nonlinear differential equations. As a result, their solutions and properties lie at the heart of the field of exact nonlinearity and integrable systems.

In mathematics, precisely in the theory of functions of several complex variables, **Hartogs's extension theorem** is a statement about the singularities of holomorphic functions of several variables. Informally, it states that the support of the singularities of such functions cannot be compact, therefore the singular set of a function of several complex variables must 'go off to infinity' in some direction. More precisely, it shows that an isolated singularity is always a removable singularity for any analytic function of *n* > 1 complex variables. A first version of this theorem was proved by Friedrich Hartogs, and as such it is known also as **Hartogs's lemma** and **Hartogs's principle**: in earlier Soviet literature, it is also called **Osgood–Brown theorem**, acknowledging later work by Arthur Barton Brown and William Fogg Osgood. This property of holomorphic functions of several variables is also called **Hartogs's phenomenon**: however, the locution "Hartogs's phenomenon" is also used to identify the property of solutions of systems of partial differential or convolution equations satisfying Hartogs type theorems.

**Steven George Krantz** is an American scholar, mathematician, and writer. He has authored more than 280 research papers and more than 135 books. Additionally, Krantz has edited journals such as the *Notices of the American Mathematical Society* and *The Journal of Geometric Analysis*.

In complex analysis of one and several complex variables, **Wirtinger derivatives**, named after Wilhelm Wirtinger who introduced them in 1927 in the course of his studies on the theory of functions of several complex variables, are partial differential operators of the first order which behave in a very similar manner to the ordinary derivatives with respect to one real variable, when applied to holomorphic functions, antiholomorphic functions or simply differentiable functions on complex domains. These operators permit the construction of a differential calculus for such functions that is entirely analogous to the ordinary differential calculus for functions of real variables.

- ↑ Eremenko, A. (1982). "Meromorphic solutions of algebraic differential equations" (PDF).
*Russian Mathematical Surveys*.**37**(4): 61–94. CiteSeerX 10.1.1.139.8499 . doi:10.1070/RM1982v037n04ABEH003967. - ↑ So-Chin Chen; Mei-Chi Shaw (2002).
*Partial Differential Equations in Several Complex Variables*. American Mathematical Society. ISBN 978-0-8218-2961-5. - ↑ Complex Difference Equations of Malmquist Type Archived 2005-08-25 at the Wayback Machine
- ↑ An Introduction to complex functions on product of two time scales
- ↑ Analytic solutions to integral equations in the complex domain

- Einar Hille (1976).
*Ordinary Differential Equations in the Complex Domain*. Wiley. ISBN 978-0-471-39964-3., reprinted by Dover, 1997. - E. Ince (1926).
*Ordinary Differential Equations*. Dover., reprinted by Dover, 2003. - Gromak, Laine, Shimomura (2002).
*Painlevé Differential Equations in the Complex Plane*. de Gruyter. ISBN 978-3-11-017379-6.CS1 maint: multiple names: authors list (link) - Ilpo Laine (1992).
*Nevanlinna Theory and Complex Differential Equations*. de Gruyter. ISBN 978-3-11-013422-3. - Niels Erik Nörlund (1924).
*Vorlesungen uber Differenzenrechnung*. Springer., reprinted by Chelsea 1954

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