Questions on hyperbolic geometry, the geometry on manifolds with negative curvature.

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8
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1answer
203 views

Lines in upper half-space

I'm teaching a tour-of-classical-geometry class this semester, and we are soon to introduce hyperbolic geometry. I am very inexpert in this subject, and I have a question about a compatibility of a ...
1
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1answer
31 views

saccheri quadrilateral - how does base=summit violate hyperbolic parallel axiom?

I drew diagonals across the quadrilateral and was able to prove that the summit angles are right angles by SSS and CPCTC. Therefore the two congruent triangles creats a quadrilateral with an angle sum ...
0
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1answer
14 views

Given two hermitian matrices of signature (2,1) there exists a Cayley transform between them?

Given a matrix $A\in M_{k\times l}(\mathbb{C})$ we define the hermitian transpose of $A$ as the matrix $A^*=\overline{A}^t\in M_{l\times k}(\mathbb{C})$. We say a matrix $H\in M_k(\mathbb{C})$ is ...
0
votes
2answers
34 views

Distance in Poincaré disk from origin to a point given

Let $C$ circle $x^2+y^2=1$ find the distance (Poincaré disk) from $O=(0,0)$ to $(x,y)$ The distance in Poincaré is $d=ln(AB,PQ)$ where AB are a segment of the curve and P and Q are points in the ...
2
votes
1answer
34 views

Formula for the midpoint in the hyperbolic geometry

I have two questions. First, is there a relatively simple formula for the midpoint of two points $a_1$ and $a_2$ in the disk with respect to the hyperbolic geometry? That is, the point on the ...
2
votes
1answer
38 views

How to visualize the region $\mathbb{H}/\Gamma_0(4)$ and its cusps?

In number theory we learn that $\theta(z) = \sum q^{n^2}$ is a modular form with respect to $\Gamma = \Gamma_0(4)$. This boils down to two properties: $\theta(z)= \theta(z+1)$ this shift symmetry ...
1
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1answer
74 views

Proof that three parallel lines don't be cutted by a transversal in Klein model

How do you prove that three parallel lines don't be cutted by a transversal? By definition parallel are Chords that meet on the boundary circle are limiting parallel lines. Then I built three ...
1
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1answer
15 views

hyperbolic trigonometry when one angle =0

The hyperbolic trigonometry functions don't really help when you have one angle =0 (the remaining lenght of side $AB$ becomes ${\infty}-{\infty}$ ) Given a triangle $\triangle AB \Omega$ with $\...
0
votes
1answer
23 views

quasi-geodesics in hyperbolic space

I've stumbled across a proof of geodesic stability in hyperbolic space, located in the following blog post: https://lamington.wordpress.com/2010/05/19/hyperbolic-geometry-notes-5-mostow-rigidity/ ...
2
votes
1answer
26 views

Horocycle transformation in the Poincare half plane model

I was puzzeling with how to find an easy formula to calculate the length of a horocycle in the Poincare half plane model Then I had the brainwave that I can just use a transformation and then find ...
1
vote
1answer
21 views

About the congruence relation on Poincaré Half-Plane model

I've been studying Hyperbolic Geometry under Hilbert Axiomatization on the Poincaré Half-Plane model. The congruence relation of segments is defined as $AB \equiv CD \Leftrightarrow \exists L \in Lob(\...
0
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0answers
18 views

Hyperbolic isometries preserve hyperbolic paralelism

In the Poicaré half-plane model, under Cayley-Klein metric, i. e., $ d:\mathbb{H}^2\rightarrow\mathbb{R}, d(A,B) = \big{|} log \frac{\bar{AA_{\infty}}.\bar{BB_{\infty}}}{\bar{BA_{\infty}}.\bar{AB_{\...
2
votes
1answer
40 views

Proof of an identity that relates hyperbolic trigonometric function to an expression with euclidean trigonometric functions.

Given a line $r$ and a (superior) semicircle perpendicular to $r$, and an arc $[AB]$ in the semicircle, I need to prove that $$ \sinh(m(AB)) = \frac{\cos(\alpha)+\cos(\beta)}{\sin(\alpha)\sin(\beta)} ...
0
votes
2answers
37 views

Length of parametrized path

Can someone guide me through how to solve this problem? Let $P = (0,1)$ and $Q = (1,1)$, and let $\gamma$ be the following parametrized path in $\mathbb H^2$ from $P$ to $Q$: $\gamma(t) = (t,1)$. ...
1
vote
1answer
44 views

Going from Metric to Distance Function in the Poincaré Half Plane

Let the Poincaré Half Plane be the set $\{(x, y) \in \mathbb{R}^2 : y > 0\}$. It is a known result that the the metric $ds^2 = \frac{dx^2 + dy^2}{y^2}$ yields a distance function $f$ such that ...
2
votes
1answer
115 views

Hyperbolic circles are euclidean circles in the Poincaré Half Plane Model

Consider the metric space $(\mathbb{H}²,d_{\mathbb{H}^2})$, where $d_{\mathbb{H}^2}$ is the hyperbolic Cayley Klein metric, i.e., $ d_{\mathbb{H}^2}(A,B) = |log ((AA_{\infty}. BB_{\infty}) / (BA_{\...
0
votes
0answers
25 views

Formula for length of diagonal in a Lambert quadrilateral

Given a Lambert quadrilateral $AOBF$ where the angles $ \angle FAO , \angle AOB , \angle OBF $ are right, and $F$ is opposite $O , \angle AFB$ is the acute angle , and the Gaussian curvature = -1 (so ...
0
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0answers
27 views

The action of an S-arithmetic group on the hyerbolic plane

I have a really quick question. I am interested in $G=SL_2(\mathbb{Z}[1/p_1,...,1/p_n])$, where $p_1,..., p_n$ are prime numbers. Since $G$ is a subgroup of $SL_2(\mathbb{R})$, it acts in the ...
0
votes
1answer
28 views

How to build a hexagon according to Poincaré model?

Given a side, I know how to build a hexagon in the euclidean geometry. How can i build it in the hyperbolic geometry according to the Poincaré model? By translating every step using hyperbolic circle ...
0
votes
1answer
37 views

Conics and Loci Question (Hyperbolae and Circles)

A circle has the equation $x^2 + y^2 = r^2$. Tangents are drawn from a point $P(x_1,y_1)$ to the circle and these touch the circle at points $A$ and $B$. If the position of $P$ can vary and the locus ...
0
votes
1answer
58 views

Parallelism preservation of hyperbolic rigid motions on the half plane model

I need to proof (Under Hilbert axiomatization) that hyperbolic rigid motions, with respect to the metric $ d:\mathbb{H}^2 \times\mathbb{H}^2\rightarrow\mathbb{R}: d(A,B) =\left| \log \left( \frac{|AA_{...
0
votes
1answer
19 views

Parametrizaction of a Hyperboloid

I do not understand why when you revolve a hyperbola around a circle the respective parameters (cosh (v) and cos (u)) are multiplied by each other to get the parametric form of the hyperboloid. I ...
1
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1answer
44 views

Hyperbolic area

Define Hyperbolic area of a subset $E$ of the unit disk $D$ to be $\displaystyle 4\int \int_E \frac{dx dy}{(1-|z|^2)^2}$. Show that the hyperbolic area is invariant under conformal self maps of $D$. ...
0
votes
1answer
100 views

Rulings of One Sheet Hyperboloid

Let $M$ be a hyperboloid of one sheet satisfying $x^2+y^2-z^2=1$. Show that $x(u,v)=(\frac{uv+1}{uv-1},\frac{u-v}{uv-1},\frac{u+v}{uv-1})$ gives a parametrization of $M$ where both sets of parameter ...
0
votes
1answer
39 views

Rotate a point around another point in the Poincaré Hyperbolic Disk

Suppose I have a point $P = (x_1,y_1)$ in the Poincaré disk model. How do I rotate it about another point $Q = (x_2,y_2) \neq(0,0)$ by a Euclidean angle $\alpha$? If $Q = (0,0)$ this is simple, just ...
2
votes
0answers
44 views

Harmonic maps between Riemann surfaces

In 'Compact Riemann surfaces' Jost defines harmonic maps between surfaces $S_1,S_2$, with local coordinates z on $S_1$ and metric $\rho^2|du\,d\overline{u}|$ on $S_2$ as $u\in C^2$ solving the ...
1
vote
2answers
80 views

Geodesics in Poincare Disk

I would like to find the geodesics in the Poincare disk. I know that the metric is $$\frac{dx^2+dy^2}{(1-x^2-y^2)^2}$$ so $$s=\int \frac{\sqrt{1+y'^2}}{1-x^2-y^2}\, dx$$ Then I try to find y(x) using ...
0
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0answers
32 views

uniquess of hyperbolic numbers

I'm trying to prove, the uniqueness of hyperbolic numbers, like the complex numbers are unique, but since hyperbolic numbers aren't a field, I can't use the ideas of this. Are there theorems of ...
6
votes
0answers
109 views

How to parameterize these pretty hyperbolic (Amsler) surfaces?

I've seen the attached images describing surfaces of negative curvature. I was wondering if there exist such surfaces with constant Gaussian negative curvature. To this end, I attempted to model the ...
3
votes
0answers
110 views

Zeta functions for flows on hyperbolic surfaces

The Riemann zeta function is defined by $$ \zeta(s)=\sum_{n=1}^\infty \frac1{n^s} $$ and can be written in the form $$ \zeta(s)=\prod_p\frac1{1-p^{-s}}, $$ where the product is over all prime numbers $...
1
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1answer
54 views

This map is an isometry (in the Riemannian sense) of the hyperbolic plane. Why is the following a proof of it?

I'm making my way through a textbook on elementary undergraduate geometry. The author has defined the notion of an isometry between two subsets of $\mathbb{R}^2$ equipped with a Riemannian metric. It ...
4
votes
1answer
26 views

Examples of Lattices in $\operatorname{Isom}(H^n)$ for all $n \geq 2$?

I just had an exam today where I was asked to give an example of a lattice in $\operatorname{Isom}(H^n)$ for all $n \geq 2$, and with bonus points if I could give cocompact and noncocompact examples. ...
2
votes
1answer
61 views

Triangle inequality for hyperbolic metric of logarithm of cross ratio.

Consider the Poincaré Half-Plane model of the Hyperbolic Space $ \mathbb{H}^2 $. I need to proof that the following d function is a metric.$ d:\mathbb{H}^2\rightarrow\mathbb{R}, d(A,B) = \big{|} log \...
1
vote
1answer
38 views

Length of the arc of a hypercycle

I am still puzzeling to get a nice equation for the arclength of an hypercycle. (I asked a similar question (less developed) about a year ago that was never answered, now i am a bit further, i ...
1
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2answers
57 views

Number of inscribed triangles in a rectangular hyperbola touching a parabola [closed]

How many triangles can be incribed in the rectangular hyperbola $xy= c^2$ whose sides all touch the parabola $y^2 =4ax$. How can we start the question . Please help.
0
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0answers
48 views

How do I solve for the volume of a hyperboloid using a double integral in polar coordinates?

Here is the problem text, with my attempts at solving it at the bottom: Suppose you are part of a team designing a water tank in the shape of a hyperboloid. The tank is to have a top radius a of 2 ...
0
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0answers
80 views

problem with proof 'horoball QI extension theorem'

I'm reading the book of Drutu and Kapovich "Lectures on geometric group theory". In the proof of Mostow rigidity theorem, they say that they can extend an $\rho$-equivariant function $f:\Omega\to\...
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0answers
49 views

Fundamental domain of a Fuchsian group is non-compact if the group contains parabolic element

Suppose a discrete subgroup $\Gamma$ of $PSL(2,\mathbb{R})$ acts on $\mathbb{H}^2$. Why is the fundamental domain non-compact if $\Gamma$ contains a parabolic element? Thanks in advance.
0
votes
2answers
26 views

Need to change variables in equations with cosh.

i have these five functions: $x=\tau \cosh(s)$ $q=\tau \sinh(s)$ $y= \sinh(s)$ $p= \cosh(s)$ $u= 1/2*\tau*\cosh(2s)+1/2*\tau$ I need to write $u$ in terms of $x$ and $y$ I know the answer is $u=...
2
votes
1answer
33 views

Non-equivalent metrics on $PSL_2(\mathbb{R})$

I am reading a paper on continued fractions and it uses the following result on Lie Groups: Fix an arbitrary left-invariant metric $d$ on $PSL_2(\mathbb{R})$ ... This phrase really throws me off....
-1
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1answer
57 views

How to analytically derive the geometric properties of a hyperbola and programatically use them to graph?

I have the following equation: $$ \left(10^{\left\lfloor\frac{\ln\sqrt k}{\ln10}\right\rfloor-1}+x\right) \left(10^{\left\lfloor\frac{\ln\sqrt k}{\ln10}\right\rfloor-1}+y\right) = k $$ Entering the ...
0
votes
1answer
18 views

Finding the second point of intersection from a normal on a hyperbola

The question is phrased as follows: "A rectangular hyperbola, W, has equation xy = 12" a) Show that the gradient of the normal, N, to W at the point P(2,6), is 1/3. b) Hence find an equation for N/...
2
votes
0answers
27 views

congruency of triangles in hyperbolic and spherical geometry

In Euclidean geometry, we have the following congruencies of triangles: side-side-side, side-angle-side, angle-angle-side = angle-side-angle (because of the angle sum) and side-side-angle (only if the ...
1
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1answer
59 views

Equalities and inequalities for quadrilaterals in hyperbolic space

In euclidean space any quadrilateral satisfies equalities and inequalities $$a^2 + b^2 + c^2 + d^2 = p^2 + q^2 + 4x^2$$ $$a^2 + b^2 + c^2 + d^2 \ge p^2 + q^2$$ where $a,b,c,d$ are the side lenghts, $...
2
votes
0answers
32 views

Subgroups of $\text{PSL}(2, \mathbb{R})$ Closed under Transposition

I am wondering, does anyone know if there is a classification of transposition-closed (Fuchsian) subgroups of $\text{PSL}(2, \mathbb{R})$? I can't read French, so for all I know it's sitting in the ...
0
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0answers
49 views

Hartshorne Exercise 41.2 - Altitudes of a triangle have a common perpendicular [hyperbolic]

This is my first post here ever, so don't be too rude, if i missed something. My question refers to exercise 41.2 "GEOMETRY:EUCLID AND BEYOND" from Robin Hartshorne. You can easily find the book as ...
4
votes
1answer
40 views

Discrete group of isometries of a finitely compact metric space is countable.

This question comes from Ratcliffe's Foundations of Hyperbolic Manifolds. Let $X$ be a finitely compact metric space (i.e. all closed metric balls are compact). Prove that a discrete group $\Gamma$...
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0answers
61 views

Kobayashi distance on the Siegel upper half space

Let $\mathbb{H}_{g}$ be the Siegel upper half space, i.e., the set of complex symmetric $g\times g$ matrices with positive-definite imaginary part. Royden in his article Intrinsic Metrics on ...
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0answers
44 views

Christoffel symbols for the Poincaré ball model

The metric tensor $g_{ij}$ of the Poincaré ball model is $$ g_{ij} = \frac{\delta_{ij}}{(1 - x_k x^k)^2} $$ where $\delta_{ij}$ is the Kronecker delta and $x^k$ are the ambient Cartesian coordinates....
1
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1answer
58 views

Relationship between regular tessellations in general space

Using the notation for regular tessellations $\{p,q\}$ denoting the tessellation consisting of p-gons , q of which meet at each vertex [e.g. $\{3,6\}$ in $\mathbb{R}^{2}$ is the equilateral triangle ...