An alternating knot is a Knot which possesses a knot diagram in which crossings alternate between under- and overpasses. Not all knot diagrams of alternating knots need be alternating diagrams.

The Trefoil Knot and Figure-of-Eight Knot are alternating knots. One of Tait's Knot Conjectures states that the number of crossings is the same for any diagram of a reduced alternating knot. Furthermore, a reduced alternating projection of a knot has the least number of crossings for any projection of that knot. Both of these facts were proved true by Kauffman (1988), Thistlethwaite (1987), and Murasugi (1987).

If has a reduced alternating projection of crossings, then the Span of is . Let
be the Crossing Number. Then an alternating knot (a
Knot Sum) satisfies

In fact, this is true as well for the larger class of Adequate Knots and postulated for all Knots. The number of Prime alternating knots of crossing for , 2, ... are 0, 0, 1, 1, 2, 3, 7, 18, 41, 123, 367, ... (Sloane's A002864).

**References**

Adams, C. C. *The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots.* New York:
W. H. Freeman, pp. 159-164, 1994.

Arnold, B.; Au, M.; Candy, C.; Erdener, K.; Fan, J.; Flynn, R.; Muir, J.; Wu, D.; and Hoste, J. ``Tabulating Alternating Knots through 14 Crossings.'' ftp://chs.cusd.claremont.edu/pub/knot/paper.TeX.txt.

Arnold, B.; Au, M.; Candy, C.; Erdener, K.; Fan, J.; Flynn, R.; Muir, J.; Wu, D.; and Hoste, J. ftp://chs.cusd.claremont.edu/pub/knot/AltKnots/.

Erdener, K. and Flynn, R. ``Rolfsen's Table of all Alternating Diagrams through 9 Crossings.'' ftp://chs.cusd.claremont.edu/pub/knot/Rolfsen_table.final.

Kauffman, L. ``New Invariants in the Theory of Knots.'' *Amer. Math. Monthly* **95**, 195-242, 1988.

Murasugi, K. ``Jones Polynomials and Classical Conjectures in Knot Theory.'' *Topology* **26**, 297-307,
1987.

Sloane, N. J. A. Sequence
A002864/M0847
in ``An On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.
*The Encyclopedia of Integer Sequences.* San Diego: Academic Press, 1995.

Thistlethwaite, M. ``A Spanning Tree Expansion for the Jones Polynomial.'' *Topology* **26**, 297-309, 1987.

© 1996-9

1999-05-25