In the mathematical field of knot theory, the hyperbolic volume of a hyperbolic link is the volume of the link's complement with respect to its complete hyperbolic metric. The volume is necessarily a finite real number, and is a topological invariant of the link.^{[1]} As a link invariant, it was first studied by William Thurston in connection with his geometrization conjecture.^{[2]}
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Contents
Knot and link invariant
A hyperbolic link is a link in the 3sphere whose complement (the space formed by removing the link from the 3sphere) can be given a complete Riemannian metric of constant negative curvature, giving it the structure of a hyperbolic 3manifold, a quotient of hyperbolic space by a group acting freely and discontinuously on it. The components of the link will become cusps of the 3manifold, and the manifold itself will have finite volume. By Mostow rigidity, when a link complement has a hyperbolic structure, this structure is uniquely determined, and any geometric invariants of the structure are also topological invariants of the link. In particular, the hyperbolic volume of the complement is a knot invariant. In order to make it welldefined for all knots or links, the hyperbolic volume of a nonhyperbolic knot or link is often defined to be zero.
There are only finitely many hyperbolic knots for any given volume.^{[2]} A mutation of a hyperbolic knot will have the same volume,^{[3]} so it is possible to concoct examples with equal volumes; indeed, there are arbitrarily large finite sets of distinct knots with equal volumes.^{[2]} In practice, hyperbolic volume has proven very effective in distinguishing knots, utilized in some of the extensive efforts at knot tabulation. Jeffrey Weeks's computer program SnapPea is the ubiquitous tool used to compute hyperbolic volume of a link.^{[1]}
Examples
The volumes of the following knots are:
 Figureeight knot = 2.0298832 (sequence A091518 in the OEIS)
 Threetwist knot = 2.82812
 Stevedore knot (mathematics) = 3.16396
 6₂ knot = 4.40083
 Endless knot = 5.13794
 Perko pair = 5.63877
 6₃ knot = 5.69302
Arbitrary manifolds
More generally, the hyperbolic volume may be defined for any hyperbolic 3manifold. The Weeks manifold has the smallest possible volume of any closed manifold (a manifold that, unlike link complements, has no cusps); its volume is approximately 0.9427.^{[4]} Thurston and Jørgensen proved that the set of real numbers that are hyperbolic volumes of 3manifolds is wellordered, with order type ω^{ω}.^{[5]}
References
 ^ ^{a} ^{b} Adams, Colin; Hildebrand, Martin; Weeks, Jeffrey (1991), "Hyperbolic invariants of knots and links", Transactions of the American Mathematical Society, 326 (1): 1–56, doi:10.2307/2001854, MR 0994161.
 ^ ^{a} ^{b} ^{c} Wielenberg, Norbert J. (1981), "Hyperbolic 3manifolds which share a fundamental polyhedron", Riemann surfaces and related topics: Proceedings of the 1978 Stony Brook Conference (State Univ. New York, Stony Brook, N.Y., 1978), Ann. of Math. Stud., 97, Princeton, N.J.: Princeton Univ. Press, pp. 505–513, MR 0624835.
 ^ Ruberman, Daniel (1987), "Mutation and volumes of knots in S^{3}", Inventiones Mathematicae, 90 (1): 189–215, Bibcode:1987InMat..90..189R, doi:10.1007/BF01389038, MR 0906585.
 ^ Gabai, David; Meyerhoff, Robert; Milley, Peter (2009), "Minimum volume cusped hyperbolic threemanifolds", Journal of the American Mathematical Society, 22 (4): 1157–1215, arXiv:0705.4325, Bibcode:2009JAMS...22.1157G, doi:10.1090/S0894034709006390, MR 2525782.
 ^ Neumann, Walter D.; Zagier, Don (1985), "Volumes of hyperbolic threemanifolds", Topology, 24 (3): 307–332, doi:10.1016/00409383(85)900047, MR 0815482.