The signature of a Knot can be defined using the Skein Relationship
Many Unknotting Numbers can be determined using a knot's signature.
See also Skein Relationship, Unknotting Number
References
Gordon, C. McA.; Litherland, R. A.; and Murasugi, K. ``Signatures of Covering Links.'' Canad. J. Math. 33, 381-394, 1981.
Murasugi, K. ``On the Signature of Links.'' Topology 9, 283-298, 1970.
Murasugi, K. ``Signatures and Alexander Polynomials of Two-Bridge Knots.'' C. R. Math. Rep. Acad. Sci. Canada 5,
133-136, 1983.
Murasugi, K. ``On the Signature of a Graph.'' C. R. Math. Rep. Acad. Sci. Canada 10, 107-111, 1988.
Murasugi, K. ``On Invariants of Graphs with Applications to Knot Theory.'' Trans. Amer. Math. Soc. 314, 1-49, 1989.
Rolfsen, D. Knots and Links. Wilmington, DE: Publish or Perish Press, 1976.
Stoimenow, A. ``Signatures.'' http://www.informatik.hu-berlin.de/~stoimeno/ptab/sig10.html.