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# Upper half-plane

In mathematics, the upper half-plane H is the set of complex numbers with positive imaginary part y:

\( \mathbb{H} = \{x + iy \;| y > 0; x, y \in \mathbb{R} \}. \)

The term is associated with a common visualization of complex numbers with points in the plane endowed with Cartesian coordinates, with the Y-axis pointing upwards: the "upper half-plane" corresponds to the half-plane above the X-axis.

It is the domain of many functions of interest in complex analysis, especially modular forms. The lower half-plane, defined by y < 0, is equally good, but less used by convention. The open unit disk D (the set of all complex numbers of absolute value less than one) is equivalent by a conformal mapping (see "Poincaré metric"), meaning that it is usually possible to pass between H and D.

It also plays an important role in hyperbolic geometry, where the Poincaré half-plane model provides a way of examining hyperbolic motions. The Poincaré metric provides a hyperbolic metric on the space.

The uniformization theorem for surfaces states that the upper half-plane is the universal covering space of surfaces with constant negative Gaussian curvature.

Generalizations

One natural generalization in differential geometry is hyperbolic n-space H^{n}, the maximally symmetric, simply connected, n-dimensional Riemannian manifold with constant sectional curvature −1. In this terminology, the upper half-plane is H^{2} since it has real dimension 2.

In number theory, the theory of Hilbert modular forms is concerned with the study of certain functions on the direct product Hn of n copies of the upper half-plane. Yet another space interesting to number theorists is the Siegel upper half-space H^{n}, which is the domain of Siegel modular forms.

See also

Cusp neighborhood

Extended complex upper-half plane

Fuchsian group

Fundamental domain

Hyperbolic geometry

Kleinian group

Modular group

Riemann surface

Schwarz-Ahlfors-Pick theorem

References

Weisstein, Eric W., "Upper Half-Plane", MathWorld.

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