Fibered knot

Figure-eight knot is fibered.

In knot theory, a branch of mathematics, a knot or link $K$ in the 3-dimensional sphere $S^3$ is called fibered or fibred (sometimes Neuwirth knot in older texts, after Lee Neuwirth) if there is a 1-parameter family $F_t$ of Seifert surfaces for $K$, where the parameter $t$ runs through the points of the unit circle $S^1$, such that if $s$ is not equal to $t$ then the intersection of $F_s$ and $F_t$ is exactly $K$.

For example:

• The unknot, trefoil knot, and figure-eight knot are fibered knots.

• The Hopf link is a fibered link.

Fibered knots and links arise naturally, but not exclusively, in complex algebraic geometry. For instance, each singular point of a complex plane curve can be described topologically as the cone on a fibered knot or link called the link of the singularity. The trefoil knot is the link of the cusp singularity $z^2+w^3$; the Hopf link (oriented correctly) is the link of the node singularity $z^2+w^2$. In these cases, the family of Seifert surfaces is an aspect of the Milnor fibration of the singularity.

A knot is fibered if and only if it is the binding of some open book decomposition of $S^3$.

Knots that are not fibered

Stevedore's knot is not fibered

The Alexander polynomial of a fibered knot is monic, i.e. the coefficients of the highest and lowest powers of t are plus or minus 1. Examples of knots with nonmonic Alexander polynomials abound, for example the twist knots have Alexander polynomials qt − (2q + 1) + qt⁻¹, where q is the number of half-twists. In particular the Stevedore's knot is not fibered.

See also

• (−2,3,7) pretzel knot

External links

• http://www.sciencedirect.com/science/article/pii/004093838290009X
• http://www.msp.warwick.ac.uk/gt/2010/14-04/p050.xhtml