# Riemann surface

In mathematics, particularly in complex analysis, a **Riemann surface** is a connected one-dimensional complex manifold. These surfaces were first studied by and are named after Bernhard Riemann. Riemann surfaces can be thought of as deformed versions of the complex plane: locally near every point they look like patches of the complex plane, but the global topology can be quite different. For example, they can look like a sphere or a torus or several sheets glued together.

The main interest in Riemann surfaces is that holomorphic functions may be defined between them. Riemann surfaces are nowadays considered the natural setting for studying the global behavior of these functions, especially multi-valued functions such as the square root and other algebraic functions, or the logarithm.

Every Riemann surface is a two-dimensional real analytic manifold (i.e., a surface), but it contains more structure (specifically a complex structure) which is needed for the unambiguous definition of holomorphic functions. A two-dimensional real manifold can be turned into a Riemann surface (usually in several inequivalent ways) if and only if it is orientable and metrizable. So the sphere and torus admit complex structures, but the Möbius strip, Klein bottle and real projective plane do not.

Geometrical facts about Riemann surfaces are as "nice" as possible, and they often provide the intuition and motivation for generalizations to other curves, manifolds or varieties. The Riemann–Roch theorem is a prime example of this influence.

A complex structure gives rise to a conformal structure by choosing the standard Euclidean metric given on the complex plane and transporting it to *X* by means of the charts. Showing that a conformal structure determines a complex structure is more difficult.^{[1]}

As with any map between complex manifolds, a function *f*: *M* → *N* between two Riemann surfaces *M* and *N* is called *holomorphic* if for every chart *g* in the atlas of *M* and every chart *h* in the atlas of *N*, the map *h* ∘ *f* ∘ *g*^{−1} is holomorphic (as a function from **C** to **C**) wherever it is defined. The composition of two holomorphic maps is holomorphic. The two Riemann surfaces *M* and *N* are called *biholomorphic* (or *conformally equivalent* to emphasize the conformal point of view) if there exists a bijective holomorphic function from *M* to *N* whose inverse is also holomorphic (it turns out that the latter condition is automatic and can therefore be omitted). Two conformally equivalent Riemann surfaces are for all practical purposes identical.

Each Riemann surface, being a complex manifold, is orientable as a real manifold. For complex charts *f* and *g* with transition function *h* = *f*(*g*^{−1}(*z*)), *h* can be considered as a map from an open set of **R**^{2} to **R**^{2} whose Jacobian in a point *z* is just the real linear map given by multiplication by the complex number *h*'(*z*). However, the real determinant of multiplication by a complex number *α* equals |*α*|^{2}, so the Jacobian of *h* has positive determinant. Consequently, the complex atlas is an oriented atlas.

Every non-compact Riemann surface admits non-constant holomorphic functions (with values in **C**). In fact, every non-compact Riemann surface is a Stein manifold.

The existence of non-constant meromorphic functions can be used to show that any compact Riemann surface is a projective variety, i.e. can be given by polynomial equations inside a projective space. Actually, it can be shown that every compact Riemann surface can be embedded into complex projective 3-space. This is a surprising theorem: Riemann surfaces are given by locally patching charts. If one global condition, namely compactness, is added, the surface is necessarily algebraic. This feature of Riemann surfaces allows one to study them with either the means of analytic or algebraic geometry. The corresponding statement for higher-dimensional objects is false, i.e. there are compact complex 2-manifolds which are not algebraic. On the other hand, every projective complex manifold is necessarily algebraic, see Chow's theorem.

where the coefficients *g*_{2} and *g*_{3} depend on τ, thus giving an elliptic curve *E*_{τ} in the sense of algebraic geometry. Reversing this is accomplished by the j-invariant *j*(*E*), which can be used to determine *τ* and hence a torus.

In complex analytic terms, the Poincaré–Koebe uniformization theorem (a generalization of the Riemann mapping theorem) states that every simply connected Riemann surface is conformally equivalent to one of the following:

For example, hyperbolic Riemann surfaces are ramified covering spaces of the sphere (they have non-constant meromorphic functions), but the sphere does not cover or otherwise map to higher genus surfaces, except as a constant.

The isometry group of a uniformized Riemann surface (equivalently, the conformal automorphism group) reflects its geometry:

The classification scheme above is typically used by geometers. There is a different classification for Riemann surfaces which is typically used by complex analysts. It employs a different definition for "parabolic" and "hyperbolic". In this alternative classification scheme, a Riemann surface is called *parabolic* if there are no non-constant negative subharmonic functions on the surface and is otherwise called *hyperbolic*.^{[4]}^{[5]} This class of hyperbolic surfaces is further subdivided into subclasses according to whether function spaces other than the negative subharmonic functions are degenerate, e.g. Riemann surfaces on which all bounded holomorphic functions are constant, or on which all bounded harmonic functions are constant, or on which all positive harmonic functions are constant, etc.

To avoid confusion, call the classification based on metrics of constant curvature the *geometric classification*, and the one based on degeneracy of function spaces *the function-theoretic classification*. For example, the Riemann surface consisting of "all complex numbers but 0 and 1" is parabolic in the function-theoretic classification but it is hyperbolic in the geometric classification.