# Upper half-plane

In mathematics, the upper half-plane, ⁠${\mathcal {H}},$⁠ is the set of points ⁠$(x,y)$⁠ in the Cartesian plane with ⁠$y>0.$⁠ The lower half-plane is the set of points ⁠$(x,y)$⁠ with ⁠$y<0$⁠ instead. Arbitrary oriented half-planes can be obtained via a planar rotation. Half-planes are an example of two-dimensional half-space. A half-plane can be split in two quadrants.

## Affine geometry

The affine transformations of the upper half-plane include
* shifts $(x,y)\mapsto (x+c,y)$, $c\in \mathbb {R}$, and
* dilations $(x,y)\mapsto (\lambda x,\lambda y)$, $\lambda >0.$
Proposition: Let ⁠$A$⁠ and ⁠$B$⁠ be semicircles in the upper half-plane with centers on the boundary. Then there is an affine mapping that takes $A$ to $B$.
    Proof: First shift the center of ⁠$A$⁠ to ⁠$(0,0).$⁠ Then take $\lambda =({\text{diameter of}}\ B)/({\text{diameter of}}\ A)$
and dilate. Then shift ⁠$(0,0)$⁠ to the center of ⁠$B.$⁠

## Inversive geometry

Definition: ${\mathcal {Z}}:=\left\{\left(\cos ^{2}\theta ,{\tfrac {1}{2}}\sin 2\theta \right)\mid 0<\theta <\pi \right\}$.
⁠${\mathcal {Z}}$⁠ can be recognized as the circle of radius ⁠${\tfrac {1}{2}}$⁠ centered at ⁠${\bigl{\tfrac {1}{2}},0{\bigr )},$⁠ and as the polar plot of $\rho (\theta )=\cos \theta .$
Proposition: ⁠$(0,0),$⁠ ⁠$\rho (\theta )$⁠ in ⁠${\mathcal {Z}},$⁠ and ⁠$(1,\tan \theta )$⁠ are collinear points.
In fact, ${\mathcal {Z}}$ is the inversion of the line ${\bigl \(1,y)\mid y>0{\bigr \}}$ in the unit circle. Indeed, the diagonal from ⁠$(0,0)$⁠ to ⁠$(1,\tan \theta )$⁠ has squared length $1+\tan ^{2}\theta =\sec ^{2}\theta$, so that $\rho (\theta )=\cos \theta$ is the reciprocal of that length.

## Metric geometry

The distance between any two points ⁠$p$⁠ and ⁠$q$⁠ in the upper half-plane can be consistently defined as follows: The perpendicular bisector of the segment from ⁠$p$⁠ to ⁠$q$⁠ either intersects the boundary or is parallel to it. In the latter case ⁠$p$⁠ and ⁠$q$⁠ lie on a ray perpendicular to the boundary and logarithmic measure can be used to define a distance that is invariant under dilation. In the former case ⁠$p$⁠ and ⁠$q$⁠ lie on a circle centered at the intersection of their perpendicular bisector and the boundary. By the above proposition this circle can be moved by affine motion to ⁠${\mathcal {Z}}.$⁠ Distances on ⁠${\mathcal {Z}}$⁠ can be defined using the correspondence with points on ${\bigl \(1,y)\mid y>0{\bigr \}}$ and logarithmic measure on this ray. In consequence, the upper half-plane becomes a metric space. The generic name of this metric space is the hyperbolic plane. In terms of the models of hyperbolic geometry, this model is frequently designated the Poincaré half-plane model.

## Complex plane

Mathematicians sometimes identify the Cartesian plane with the complex plane, and then the upper half-plane corresponds to the set of complex numbers with positive imaginary part:
    ${\mathcal {H}}:=\{x+iy\mid y>0;\ x,y\in \mathbb {R} \}.$
The term arises from a common visualization of the complex number $x+iy$ as the point $(x,y)$ in the plane endowed with Cartesian coordinates. When the $y$ axis is oriented vertically, the "upper half-plane" corresponds to the region above the $x$ axis and thus complex numbers for which $y>0$.
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 ⁠${\mathcal {D}}$⁠ (the set of all complex numbers of absolute value less than one) is equivalent by a conformal mapping to ⁠${\mathcal {H}}$⁠ (see "Poincaré metric"), meaning that it is usually possible to pass between ⁠${\mathcal {H}}$⁠ and ⁠${\mathcal {D}}.$⁠
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.
The closed upper half-plane is the union of the upper half-plane and the real axis. It is the closure of the upper half-plane.

## Generalizations

One natural generalization in differential geometry is hyperbolic $n$-space ⁠${\mathcal {H}}^{n},$⁠ the maximally symmetric, simply connected, ⁠$n$⁠-dimensional Riemannian manifold with constant sectional curvature $-1$. In this terminology, the upper half-plane is ⁠${\mathcal {H}}^{2}$⁠ 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 ⁠${\mathcal {H}}^{n}$⁠ of ⁠$n$⁠ copies of the upper half-plane. Yet another space interesting to number theorists is the Siegel upper half-space ⁠${\mathcal {H}}_{n},$⁠ which is the domain of Siegel modular forms.