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2 votes
Accepted

When is a 2-bridge knot hyperbolic?

All two-bridge knots are hyperbolic except for the $(2, k)$ torus links. For an “as simple as possible” (but still fairly difficult) proof, see Theorem 10.17 of Purcell’s book Hyperbolic Knot Theory.
Sam Nead's user avatar
  • 28.2k
3 votes
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Existence of orientable finite volume complete cusp hyperbolic 3-manifolds $\mathbb{H}^3 / \...

Yes, all hyperbolic three-manifold fundamental groups can be generated by loxodromic elements. For, suppose that $\Gamma = \{ \gamma_i \}$ is a generating set. Take $\gamma$, a loxodromic element wh …
Moishe Kohan's user avatar
  • 12.3k
12 votes
Accepted

Can I endow the following 3-manifold with a hyperbolic metric?

This three-manifold can also be constructed by taking a genus two surface $S$, crossing with the interval $I$ to get $S \times I$, and attaching a pair of one-handles both of which connect $S \times \ …
Sam Nead's user avatar
  • 28.2k
1 vote
Accepted

Figure 8 knot incomplete hyperbolic structure

The fixed points of $\begin{pmatrix} a & b \\ c & d \end{pmatrix}$, acting on $\mathbb{CP}^1$, are found by solving, for $z$ and $w$, the equation $azw + bw^2 = cz^2 + dzw$. Much of the time you can …
Sam Nead's user avatar
  • 28.2k
4 votes

Residual finiteness of hyperbolic 3-manifold groups

The answer to the first question is "yes" and the answer to the second is "no", assuming that you are looking for a covering which is a locally isometric. If you do not require a locally isometric co …
Sam Nead's user avatar
  • 28.2k
9 votes
Accepted

Hyperbolic three-manifolds that fiber over the circle

Question: Is the length of homotopically non-trivial loops in $M(f)$ bounded below in terms of the genus $g$ of the fiber? Answer: No. Here is one family of examples. Suppose that $S$ is the closed o …
Will Sawin's user avatar
  • 149k
1 vote

Simple curves on hyperbolic tori

They do not give the definition of conjugate geodesic, but we can make an educated guess. Suppose that $F$ is a free group of rank two. (So, isomorphic to the fundamental group of the once-punctured …
Sam Nead's user avatar
  • 28.2k
4 votes
Accepted

Guts of 3-manifolds for sutured manifolds and pared manifolds

Edited: to reflect the correct definitions. Question 1: Why are the guts well-defined? Answer 1: By the JSJ theory there is a unique collection of $I$-bundles (and Seifert fibered spaces) that contai …
Sam Nead's user avatar
  • 28.2k
5 votes
Accepted

A formula for the cross-ratio in terms of hyperbolic data

Your formula can be found on page 355 (near the end of Section 7.4) of Marden's book Outer circles: an introduction to hyperbolic 3-manifolds. In the second edition of the book, with the title, Hyperb …
Sam Nead's user avatar
  • 28.2k
2 votes

Who first used the cross-ratio to describe shapes in hyperbolic geometry?

Thurston (even before his lecture series in 1979) was probably the first to use cross-ratios to describe the shapes of hyperbolic tetrahedra. He was certainly the first to use these shapes to compute …
Sam Nead's user avatar
  • 28.2k
4 votes
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Geodesic laminations on the 4-punctured sphere

The answer to your question is "no". There are simple geodesics in $S = S_{0, 4}$ that lie in a compact subsurface of $S$, but whose closures contain no simple closed geodesic. Here is the usual "fir …
Sam Nead's user avatar
  • 28.2k
5 votes
Accepted

Volume of the Weeks manifold and of the 5.2 knot complement

One explanation is to look at the "spun triangulation" SnapPy gives to the Weeks manifold. This is composed of two ideal tetrahedra with volumes ~0.83828404504 and ~0.10442331774 adding to the famili …
Sam Nead's user avatar
  • 28.2k
2 votes

Proof of homotopic essential simple close curves are isotopic

There is a great deal of overlap between this question and the following questions asked by the same person: About isotopy and homotopy About isotopy of simple close curve However, the exact questio …
Sam Nead's user avatar
  • 28.2k
2 votes
Accepted

Translation length on annular curve graphs

I think that the answer is “yes, such a constant $K$ exists.” Suppose that $g$ is the given mapping class in the stabiliser of $\gamma$. Replace $g$ by a large power (bounded by the topology of $S$) …
Sam Nead's user avatar
  • 28.2k
1 vote

How the hyperbolic metric changes when we add a puncture?

For question one: one possible qualitative approach is to consider the "thick-thin" decomposition of the given metric on $S$. For question two: the hyperbolic metric on a surface and the "moduli of a …
Sam Nead's user avatar
  • 28.2k

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