# Cusp neighborhood

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In mathematics, a cusp neighborhood is defined as a set of points near a cusp.

## Cusp neighborhood for a Riemann surface

The cusp neighborhood for a hyperbolic Riemann surface can be defined in terms of its Fuchsian model.

Suppose that the Fuchsian group G contains a parabolic element g. For example, the element t ∈ SL(2,Z) where

$t(z)={\begin{pmatrix}1&1\\0&1\end{pmatrix}}:z={\frac {1\cdot z+1}{0\cdot z+1}}=z+1$ is a parabolic element. Note that all parabolic elements of SL(2,C) are conjugate to this element. That is, if g ∈ SL(2,Z) is parabolic, then $g=h^{-1}th$ for some h ∈ SL(2,Z).

The set

$U=\{z\in \mathbf {H} :\Im z>1\}$ where H is the upper half-plane has

$\gamma (U)\cap U=\emptyset$ for any $\gamma \in G-\langle g\rangle$ where $\langle g\rangle$ is understood to mean the group generated by g. That is, γ acts properly discontinuously on U. Because of this, it can be seen that the projection of U onto H/G is thus

$E=U/\langle g\rangle$ .

Here, E is called the neighborhood of the cusp corresponding to g.

Note that the hyperbolic area of E is exactly 1, when computed using the canonical Poincaré metric. This is most easily seen by example: consider the intersection of U defined above with the fundamental domain

$\left\{z\in H:\left|z\right|>1,\,\left|\,{\mbox{Re}}(z)\,\right|<{\frac {1}{2}}\right\}$ of the modular group, as would be appropriate for the choice of T as the parabolic element. When integrated over the volume element

$d\mu ={\frac {dxdy}{y^{2}}}$ the result is trivially 1. Areas of all cusp neighborhoods are equal to this, by the invariance of the area under conjugation.