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In mathematics, an **analytic function** is a function that is locally given by a convergent power series. There exist both **real analytic functions** and **complex analytic functions**. Functions of each type are infinitely differentiable, but complex analytic functions exhibit properties that do not generally hold for real analytic functions. A function is analytic if and only if its Taylor series about *x*_{0} converges to the function in some neighborhood for every *x*_{0} in its domain.

Formally, a function is *real analytic* on an open set in the real line if for any one can write

in which the coefficients are real numbers and the series is convergent to for in a neighborhood of .

Alternatively, a real analytic function is an infinitely differentiable function such that the Taylor series at any point in its domain

converges to for in a neighborhood of pointwise.^{ [lower-alpha 1] } The set of all real analytic functions on a given set is often denoted by .

A function defined on some subset of the real line is said to be real analytic at a point if there is a neighborhood of on which is real analytic.

The definition of a *complex analytic function* is obtained by replacing, in the definitions above, "real" with "complex" and "real line" with "complex plane". A function is complex analytic if and only if it is holomorphic i.e. it is complex differentiable. For this reason the terms "holomorphic" and "analytic" are often used interchangeably for such functions.^{ [1] }

Typical examples of analytic functions are:

- All elementary functions:
- All polynomials: if a polynomial has degree
*n*, any terms of degree larger than*n*in its Taylor series expansion must immediately vanish to 0, and so this series will be trivially convergent. Furthermore, every polynomial is its own Maclaurin series. - The exponential function is analytic. Any Taylor series for this function converges not only for
*x*close enough to*x*_{0}(as in the definition) but for all values of*x*(real or complex). - The trigonometric functions, logarithm, and the power functions are analytic on any open set of their domain.

- All polynomials: if a polynomial has degree
- Most special functions (at least in some range of the complex plane):

Typical examples of functions that are not analytic are:

- The absolute value function when defined on the set of real numbers or complex numbers is not everywhere analytic because it is not differentiable at 0. Piecewise defined functions (functions given by different formulae in different regions) are typically not analytic where the pieces meet.
- The complex conjugate function
*z*→*z** is not complex analytic, although its restriction to the real line is the identity function and therefore real analytic, and it is real analytic as a function from to . - Other non-analytic smooth functions, and in particular any smooth function with compact support, i.e. , cannot be analytic on .
^{ [2] }

The following conditions are equivalent:

- is real analytic on an open set .
- There is a complex analytic extension of to an open set which contains .
- is real smooth and for every compact set there exists a constant such that for every and every non-negative integer the following bound holds
^{ [3] }

Complex analytic functions are exactly equivalent to holomorphic functions, and are thus much more easily characterized.

For the case of an analytic function with several variables (see below), the real analyticity can be characterized using the Fourier–Bros–Iagolnitzer transform.

In the multivariable case, real analytic functions satisfy a direct generalization of the third characterization.^{ [4] } Let be an open set, and let .

Then is real analytic on if and only if and for every compact there exists a constant such that for every multi-index the following bound holds^{ [5] }

- The sums, products, and compositions of analytic functions are analytic.
- The reciprocal of an analytic function that is nowhere zero is analytic, as is the inverse of an invertible analytic function whose derivative is nowhere zero. (See also the Lagrange inversion theorem.)
- Any analytic function is smooth, that is, infinitely differentiable. The converse is not true for real functions; in fact, in a certain sense, the real analytic functions are sparse compared to all real infinitely differentiable functions. For the complex numbers, the converse does hold, and in fact any function differentiable
*once*on an open set is analytic on that set (see "analyticity and differentiability" below). - For any open set Ω ⊆
**C**, the set*A*(Ω) of all analytic functions*u*: Ω →**C**is a Fréchet space with respect to the uniform convergence on compact sets. The fact that uniform limits on compact sets of analytic functions are analytic is an easy consequence of Morera's theorem. The set of all bounded analytic functions with the supremum norm is a Banach space.

A polynomial cannot be zero at too many points unless it is the zero polynomial (more precisely, the number of zeros is at most the degree of the polynomial). A similar but weaker statement holds for analytic functions. If the set of zeros of an analytic function ƒ has an accumulation point inside its domain, then ƒ is zero everywhere on the connected component containing the accumulation point. In other words, if (*r _{n}*) is a sequence of distinct numbers such that ƒ(

Also, if all the derivatives of an analytic function at a point are zero, the function is constant on the corresponding connected component.

These statements imply that while analytic functions do have more degrees of freedom than polynomials, they are still quite rigid.

As noted above, any analytic function (real or complex) is infinitely differentiable (also known as smooth, or *C ^{∞}*). (Note that this differentiability is in the sense of real variables; compare complex derivatives below.) There exist smooth real functions that are not analytic: see non-analytic smooth function. In fact there are many such functions.

The situation is quite different when one considers complex analytic functions and complex derivatives. It can be proved that any complex function differentiable (in the complex sense) in an open set is analytic. Consequently, in complex analysis, the term *analytic function* is synonymous with * holomorphic function *.

Real and complex analytic functions have important differences (one could notice that even from their different relationship with differentiability). Analyticity of complex functions is a more restrictive property, as it has more restrictive necessary conditions and complex analytic functions have more structure than their real-line counterparts.^{ [6] }

According to Liouville's theorem, any bounded complex analytic function defined on the whole complex plane is constant. The corresponding statement for real analytic functions, with the complex plane replaced by the real line, is clearly false; this is illustrated by

Also, if a complex analytic function is defined in an open ball around a point *x*_{0}, its power series expansion at *x*_{0} is convergent in the whole open ball (holomorphic functions are analytic). This statement for real analytic functions (with open ball meaning an open interval of the real line rather than an open disk of the complex plane) is not true in general; the function of the example above gives an example for *x*_{0} = 0 and a ball of radius exceeding 1, since the power series 1 − *x*^{2} + *x*^{4} − *x*^{6}... diverges for |*x*| ≥ 1.

Any real analytic function on some open set on the real line can be extended to a complex analytic function on some open set of the complex plane. However, not every real analytic function defined on the whole real line can be extended to a complex function defined on the whole complex plane. The function ƒ(*x*) defined in the paragraph above is a counterexample, as it is not defined for *x* = ±*i*. This explains why the Taylor series of ƒ(*x*) diverges for |*x*| > 1, i.e., the radius of convergence is 1 because the complexified function has a pole at distance 1 from the evaluation point 0 and no further poles within the open disc of radius 1 around the evaluation point.

One can define analytic functions in several variables by means of power series in those variables (see power series). Analytic functions of several variables have some of the same properties as analytic functions of one variable. However, especially for complex analytic functions, new and interesting phenomena show up in 2 or more complex dimensions:

- Zero sets of complex analytic functions in more than one variable are never discrete. This can be proved by Hartogs's extension theorem.
- Domains of holomorphy for single-valued functions consist of arbitrary (connected) open sets. In several complex variables, however, only some connected open sets are domains of holomorphy. The characterization of domains of holomorphy leads to the notion of pseudoconvexity.

- ↑ This implies uniform convergence as well in a (possibly smaller) neighborhood of .

- ↑ Churchill; Brown; Verhey (1948).
*Complex Variables and Applications*. McGraw-Hill. p. 46. ISBN 0-07-010855-2.A function

*f*of the complex variable*z*is*analytic*at point*z*_{0}if its derivative exists not only at*z*but at each point*z*in some neighborhood of*z*_{0}. It is analytic in a region*R*if it is analytic at every point in*R*. The term*holomorphic*is also used in the literature do denote analyticity - ↑ Strichartz, Robert S. (1994).
*A guide to distribution theory and Fourier transforms*. Boca Raton: CRC Press. ISBN 0-8493-8273-4. OCLC 28890674. - ↑ Krantz & Parks 2002, p. 15.
- ↑ Komatsu, Hikosaburo (1960). "A characterization of real analytic functions".
*Proceedings of the Japan Academy*.**36**(3): 90–93. doi: 10.3792/pja/1195524081 . ISSN 0021-4280. - ↑ "Gevrey class - Encyclopedia of Mathematics".
*encyclopediaofmath.org*. Retrieved 2020-08-30. - ↑ Krantz & Parks 2002.

**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 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 mathematics, **real analysis** is the branch of mathematical analysis that studies the behavior of real numbers, sequences and series of real numbers, and real functions. Some particular properties of real-valued sequences and functions that real analysis studies include convergence, limits, continuity, smoothness, differentiability and integrability.

In calculus, **Taylor's theorem** gives an approximation of a *k*-times differentiable function around a given point by a polynomial of degree *k*, called the *k*th-order **Taylor polynomial**. For a smooth function, the Taylor polynomial is the truncation at the order *k* of the Taylor series of the function. The first-order Taylor polynomial is the linear approximation of the function, and the second-order Taylor polynomial is often referred to as the **quadratic approximation**. There are several versions of Taylor's theorem, some giving explicit estimates of the approximation error of the function by its Taylor polynomial.

In mathematics, mathematical physics and the theory of stochastic processes, a **harmonic function** is a twice continuously differentiable function *f* : *U* → **R**, where *U* is an open subset of **R**^{n}, that satisfies Laplace's equation, that is,

In mathematics, a **power series** is an infinite series of the form

In mathematics, the **Laurent series** of a complex function *f*(*z*) is a representation of that function as a power series which includes terms of negative degree. It may be used to express complex functions in cases where a Taylor series expansion cannot be applied. The Laurent series was named after and first published by Pierre Alphonse Laurent in 1843. Karl Weierstrass may have discovered it first in a paper written in 1841, but it was not published until after his death.

In complex analysis, a branch of mathematics, **analytic continuation** is a technique to extend the domain of definition of a given analytic function. Analytic continuation often succeeds in defining further values of a function, for example in a new region where an infinite series representation in terms of which it is initially defined becomes divergent.

In mathematics, **complex geometry** is the study of complex manifolds, complex algebraic varieties, and functions of several complex variables. Application of transcendental methods to algebraic geometry falls in this category, together with more geometric aspects of complex analysis.

In mathematics, a **modular form** is a (complex) analytic function on the upper half-plane satisfying a certain kind of functional equation with respect to the group action of the modular group, and also satisfying a growth condition. The theory of modular forms therefore belongs to complex analysis but the main importance of the theory has traditionally been in its connections with number theory. Modular forms appear in other areas, such as algebraic topology, sphere packing, and string theory.

In calculus, a **differentiable function** of one real variable is a function whose derivative exists at each point in its domain. In other words, the graph of a differentiable function has a non-vertical tangent line at each interior point in its domain. A differentiable function is smooth and does not contain any break, angle, or cusp.

The theory of **functions of several complex variables** is the branch of mathematics dealing with complex-valued functions. The function is n-tuples of complex numbers, classically considered on the complex coordinate space . The name of the field dealing with the properties of **Function of several complex variables** is called **several complex variables**, that has become a common name for that whole field of study and Mathematics Subject Classification has, as a top-level heading.

In complex analysis, a branch of mathematics, **Morera's theorem**, named after Giacinto Morera, gives an important criterion for proving that a function is holomorphic.

In mathematics, smooth functions and analytic functions are two very important types of functions. One can easily prove that any analytic function of a real argument is smooth. The converse is not true, as demonstrated with the counterexample below.

In mathematical analysis, and applications in geometry, applied mathematics, engineering, and natural sciences, a **function of a real variable** is a function whose domain is the real numbers , or a subset of that contains an interval of positive length. Most real functions that are considered and studied are differentiable in some interval. The most widely considered such functions are the **real functions**, which are the real-valued functions of a real variable, that is, the functions of a real variable whose codomain is the set of real numbers.

In mathematics, **infinite-dimensional holomorphy** is a branch of functional analysis. It is concerned with generalizations of the concept of holomorphic function to functions defined and taking values in complex Banach spaces, typically of infinite dimension. It is one aspect of nonlinear functional analysis.

In real analysis and complex analysis, a branch of mathematics, the **identity theorem** for analytic functions states: given functions *f* and *g* analytic on a domain *D*, if *f* = *g* on some , where has an accumulation point, then *f* = *g* on *D*.

Affine geometry, broadly speaking, is the study of the geometrical properties of lines, planes, and their higher dimensional analogs, in which a notion of "parallel" is retained, but no metrical notions of distance or angle are. Affine spaces differ from linear spaces in that they do not have a distinguished choice of origin. So, in the words of Marcel Berger, "An affine space is nothing more than a vector space whose origin we try to forget about, by adding translations to the linear maps." Accordingly, a **complex affine space**, that is an affine space over the complex numbers, is like a complex vector space, but without a distinguished point to serve as the origin.

In mathematics and in particular the field of complex analysis, **Hurwitz's theorem** is a theorem associating the zeroes of a sequence of holomorphic, compact locally uniformly convergent functions with that of their corresponding limit. The theorem is named after Adolf Hurwitz.

- Conway, John B. (1978).
*Functions of One Complex Variable I*. Graduate Texts in Mathematics 11 (2nd ed.). Springer-Verlag. ISBN 978-0-387-90328-6. - Krantz, Steven; Parks, Harold R. (2002).
*A Primer of Real Analytic Functions*(2nd ed.). Birkhäuser. ISBN 0-8176-4264-1.

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