page_title: Figure-eight knot (mathematics)
section_title: Mathematical properties
hierarchical_section_title: Figure-eight knot (mathematics) / Mathematical properties
caption_attribution_description: English: Mathematical surface which the boundaries are a warped cylinder. Português: Superfície ilustrando o bordo de um cilindro deformado, parte do acervo da Matemateca IME-USP
context_page_description: In knot theory, a figure-eight knot is the unique knot with a crossing number of four. This makes it the knot with the third-smallest possible crossing number, after the unknot and the
  trefoil knot. The figure-eight knot is a prime knot.
context_section_description: The figure-eight knot has played an important role historically (and continues to do so) in the theory of 3-manifolds.  Sometime in the mid-to-late 1970s, William Thurston showed that the figure-eight was hyperbolic, by decomposing its complement into two ideal hyperbolic tetrahedra.  (Robert Riley and Troels Jørgensen, working independently of each other, had earlier shown that the figure-eight knot was hyperbolic by other means.) This construction, new at the time, led him to many powerful results and methods.  For example, he was able to show that all but ten Dehn surgeries on the figure-eight knot resulted in non-Haken, non-Seifert-fibered irreducible 3-manifolds; these were the first such examples.  Many more have been discovered by generalizing Thurston's construction to other knots and links.
  The figure-eight knot is also the hyperbolic knot whose complement has the smallest possible volume, 2.02988... according to the work of Chun Cao and Robert Meyerhoff.  From this perspective, the figure-eight knot can be considered the simplest hyperbolic knot.   The figure eight knot complement is a double-cover of the Gieseking manifold, which has the smallest volume among non-compact hyperbolic 3-manifolds.
  The figure-eight knot and the (−2,3,7) pretzel knot are the only two hyperbolic knots known to have more than 6 exceptional surgeries, Dehn surgeries resulting in a non-hyperbolic 3-manifold; they have 10 and 7, respectively.  A theorem of Lackenby and Meyerhoff, whose proof relies on the geometrization conjecture and computer assistance, holds that 10 is the largest possible number of exceptional surgeries of any hyperbolic knot. However, it is not currently known whether the figure-eight knot is the only one that achieves the bound of 10.  A well-known conjecture is that the bound (except for the two knots mentioned) is 6.
caption: Mathematical surface Illustrating Figure-eight knot
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