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Let $K$ be a hyperbolic knot, i.e., $S^3 - K$ is an orientable finite volume cusp hyperbolic 3 manifold. Let $M=S^3 - K$ then $M= \mathbb{H}^3/\Gamma$, where $\Gamma$ (Kleinian group) is discret subgroup of $PSL(2,\mathbb{C})$ and also $\pi_1(M)$ is isomorphic to $\Gamma$.

Most of the hyperbolic knots I have seen above $\Gamma$ are always generated by parabolic elements, like 4_1 knot, twist knots, etc.. So, natural questions arise

My question is

  1. Does there exist a hyperbolic knot such that $\Gamma$ is two generated Kleinian group where the generators are not commutative and not parabolics?
  2. More generally does there exist a cusp hyperbolic $3-$ manifold $M=\mathbb{H}^3/\Gamma$ where $\Gamma$ is Kleinian group of two generators, generators of $\Gamma$ are not commute each other and they also not parabolic elements?

My attempted,

I think the answer is yes for 2nd one,

I am taking two loxodromic elements say $\gamma_1$ and $\gamma_2$ such that whose geodesic axis $l_{\gamma_1}$ and $l_{\gamma_2}$ passing from their fixed points far(they don't share the fixed points) from each other, I believe that $\Gamma= <\gamma_1,\gamma_2>$ from a Kleinian group.

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  • $\begingroup$ Cross posted on math.stackexchange. $\endgroup$
    – Lee Mosher
    Commented Oct 14 at 15:55
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    $\begingroup$ Your writing is unclear. What do you mean by "noncommutative generators"? I know what "noncommuting" means and standard parabolic generators of the figure 8 knot group do not commute. And any nonelementary 2-generated Kleinian group admits a 2-generating set both elements of which are loxodromic. $\endgroup$ Commented Oct 14 at 15:57
  • $\begingroup$ @MoisheKohan Thank you so much pointing out mistakes, I am new in this area, I very recently started learning cusp hyperbolic 3-manifolds. Noncommutative generators: I mean generators do not commute each other. How I will get always loxodromic generators? $\endgroup$
    – T ghosh
    Commented Oct 14 at 17:31
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    $\begingroup$ Did you take an algebraic topology class before trying to learn knot theory and hyperbolic geometry? A standard exercise in algebraic topology is to compute homology of a knot complement. Can you do this? $\endgroup$ Commented Oct 14 at 19:48
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    $\begingroup$ @MoisheKohan now I got your point. So, $\mathbb{Z}\oplus \mathbb{Z}$ can't come as a knot group. Thanks $\endgroup$
    – T ghosh
    Commented Oct 14 at 20:24

1 Answer 1

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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 which is "sufficiently long" compared to the elements $\Gamma$. Then the set $\{ \gamma \} \cup \{\gamma \cdot \gamma_i\}$ generates and consists only of loxodromic elements.

A more delicate argument will give loxodromic generating sets, of size two, of (say) the twist knot groups.

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  • $\begingroup$ I think you mean supose $\Gamma=<a,b>$ is a Kleinian group, where $a,b$ both are parabolic elements elements, if $a,b$ are not commute each other then I can always find a loxodromic element $g$ say, so, $\Gamma=<g,g.a,g.b>$, therefore $\Gamma$ is generated by loxdromic elements. Is it? $\endgroup$
    – T ghosh
    Commented Oct 14 at 17:08
  • $\begingroup$ If $a$ and $b$ are parabolic, commute, and generate a Kleinian group, then the quotient $\mathbb{H}/\Gamma$ has infinite volume. $\endgroup$
    – Sam Nead
    Commented Oct 14 at 19:56

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