Holomorphic function
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. Holomorphic functions are the central objects of study in complex analysis.
Though the term analytic function is often used interchangeably with "holomorphic function", the word "analytic" is defined in a broader sense to denote any function that can be written as a convergent power series in a neighbourhood of each point in its domain. The fact that all holomorphic functions are complex analytic functions, and vice versa, is a major theorem in complex analysis.
Holomorphic functions are also sometimes referred to as regular functions. A holomorphic function whose domain is the whole complex plane is called an entire function. The phrase "holomorphic at a point z0" means not just differentiable at z0, but differentiable everywhere within some neighbourhood of z0 in the complex plane.
Definition
Given a complex-valued function f of a single complex variable, the derivative of f at a point z0 in its domain is defined by the limitThis is the same as the definition of the derivative for real functions, except that all of the quantities are complex. In particular, the limit is taken as the complex number z approaches z0, and must have the same value for any sequence of complex values for z that approach z0 on the complex plane. If the limit exists, we say that f is complex-differentiable at the point z0. This concept of complex differentiability shares several properties with real differentiability: it is linear and obeys the product rule, quotient rule, and chain rule.
If f is complex differentiable at every point z0 in an open set U, we say that f is holomorphic on U. We say that f is holomorphic at the point z0 if f is complex differentiable on some neighbourhood of z0. We say that f is holomorphic on some non-open set A if it is holomorphic in an open set containing A. As a pathological non-example, the function given by f = |z|2 is complex differentiable at exactly one point, and for this reason, it is not holomorphic at 0 because there is no open set around 0 on which f is complex differentiable.
The relationship between real differentiability and complex differentiability is the following. If a complex function is holomorphic, then u and v have first partial derivatives with respect to x and y, and satisfy the Cauchy–Riemann equations:
or, equivalently, the Wirtinger derivative of f with respect to the complex conjugate of z is zero:
which is to say that, roughly, f is functionally independent from the complex conjugate of z.
If continuity is not given, the converse is not necessarily true. A simple converse is that if u and v have continuous first partial derivatives and satisfy the Cauchy–Riemann equations, then f is holomorphic. A more satisfying converse, which is much harder to prove, is the Looman–Menchoff theorem: if f is continuous, u and v have first partial derivatives, and they satisfy the Cauchy–Riemann equations, then f is holomorphic.
Terminology
The word "holomorphic" was introduced by two of Cauchy's students, Briot and Bouquet, and derives from the Greek meaning "entire", and meaning "form" or "appearance".Today, the term "holomorphic function" is sometimes preferred to "analytic function", as the latter is a more general concept. This is also because an important result in complex analysis is that every holomorphic function is complex analytic, a fact that does not follow obviously from the definitions. The term "analytic" is however also in wide use.
Properties
Because complex differentiation is linear and obeys the product, quotient, and chain rules; the sums, products and compositions of holomorphic functions are holomorphic, and the quotient of two holomorphic functions is holomorphic wherever the denominator is not zero.If one identifies C with R2, then the holomorphic functions coincide with those functions of two real variables with continuous first derivatives which solve the Cauchy–Riemann equations, a set of two partial differential equations.
Every holomorphic function can be separated into its real and imaginary parts, and each of these is a solution of Laplace's equation on R2. In other words, if we express a holomorphic function f as both u and v are harmonic functions, where v is the harmonic conjugate of u.
Cauchy's integral theorem implies that the contour integral of every holomorphic function along a loop vanishes:
Here γ is a rectifiable path in a simply connected open subset U of the complex plane C whose start point is equal to its end point, and is a holomorphic function.
Cauchy's integral formula states that every function holomorphic inside a disk is completely determined by its values on the disk's boundary. Furthermore: Suppose U is an open subset of C, is a holomorphic function and the closed disk is completely contained in U. Let γ be the circle forming the boundary of D. Then for every a in the interior of D:
where the contour integral is taken counter-clockwise.
The derivative f′ can be written as a contour integral using Cauchy's differentiation formula:
for any simple loop positively winding once around a, and
for infinitesimal positive loops γ around a.
In regions where the first derivative is not zero, holomorphic functions are conformal in the sense that they preserve angles and the shape of small figures.
Every holomorphic function is analytic. That is, a holomorphic function f has derivatives of every order at each point a in its domain, and it coincides with its own Taylor series at a in a neighbourhood of a. In fact, f coincides with its Taylor series at a in any disk centred at that point and lying within the domain of the function.
From an algebraic point of view, the set of holomorphic functions on an open set is a commutative ring and a complex vector space. Additionally, the set of holomorphic functions in an open set U is an integral domain if and only if the open set U is connected. In fact, it is a locally convex topological vector space, with the seminorms being the suprema on compact subsets.
From a geometric perspective, a function f is holomorphic at z0 if and only if its exterior derivative df in a neighbourhood U of z0 is equal to f′ dz for some continuous function f′. It follows from
that df′ is also proportional to dz, implying that the derivative f′ is itself holomorphic and thus that f is infinitely differentiable. Similarly, the fact that implies that any function f that is holomorphic on the simply connected region U is also integrable on U. is independent of the particular choice of path γ, and thus F
Examples
All polynomial functions in z with complex coefficients are holomorphic on C, and so are sine, cosine and the exponential function.. The principal branch of the complex logarithm function is holomorphic on the set The square root function can be defined asand is therefore holomorphic wherever the logarithm log is. The function 1/z is holomorphic on
As a consequence of the Cauchy–Riemann equations, a real-valued holomorphic function must be constant. Therefore, the absolute value of z, the argument of z, the real part of z and the imaginary part of z are not holomorphic. Another typical example of a continuous function which is not holomorphic is the complex conjugate formed by complex conjugation.
Several variables
The definition of a holomorphic function generalizes to several complex variables in a straightforward way. Let D denote an open subset of Cn, and let. The function f is analytic at a point p in D if there exists an open neighbourhood of p in which f is equal to a convergent power series in n complex variables. Define f to be holomorphic if it is analytic at each point in its domain. Osgood's lemma shows that, for a continuous function f, this is equivalent to f being holomorphic in each variable separately. The much deeper Hartogs' theorem proves that the continuity hypothesis is unnecessary: f is holomorphic if and only if it is holomorphic in each variable separately.More generally, a function of several complex variables that is square integrable over every compact subset of its domain is analytic if and only if it satisfies the Cauchy–Riemann equations in the sense of distributions.
Functions of several complex variables are in some basic ways more complicated than functions of a single complex variable. For example, the region of convergence of a power series is not necessarily an open ball; these regions are Reinhardt domains, the simplest example of which is a polydisk. However, they also come with some fundamental restrictions. Unlike functions of a single complex variable, the possible domains on which there are holomorphic functions that cannot be extended to larger domains are highly limited. Such a set is called a domain of holomorphy.
A complex differential -form α is holomorphic if and only if its antiholomorphic Dolbeault derivative is zero,.