Hopf link
 |
|
| Braid length | 2 |
|---|---|
| Braid no. | 2 |
| Crossing no. | 2 |
| Hyperbolic volume | 0 |
| Linking no. | 1 |
| Stick no. | 6 |
| Unknotting no. | 1 |
| Conway notation | [2] |
| A-B notation | 22 1 |
| Thistlethwaite | L2a1 |
| Last /Next | L0 / L4a1 |
| Other | |
| alternating, torus, fibered | |
In mathematical knot theory, the Hopf link is the simplest nontrivial link with more than one component. It consists of two circles linked together exactly once, and is named after Heinz Hopf.
A concrete model consists of two unit circles in perpendicular planes, each passing through the center of the other. This model minimizes the ropelength of the link and until 2002 the Hopf link was the only link whose ropelength was known. The convex hull of these two circles forms a shape called an oloid.
Depending on the relative orientations of the two components the linking number of the Hopf link is ±1.
The Hopf link is a (2,2)-torus link with the braid word
The knot complement of the Hopf link is R × S1 × S1, the cylinder over a torus. This space has a locally Euclidean geometry, so the Hopf link is not a hyperbolic link. The knot group of the Hopf link (the fundamental group of its complement) is Z2 (the free abelian group on two generators), distinguishing it from an unlinked pair of loops which has the free group on two generators as its group.
The Hopf-link is not tricolorable. This is easily seen from the fact that the link can only take on two colors that leads it to fail the second part of the definition of tricoloribility. At each crossing, it will take a maximum of 2 colors. Thus, if it satisfies the rule of having more than 1 color, it fails the rule of having 1 or 3 color at each crossing. If it satisfies the rule of having 1 or 3 colors at each crossing, it will fail the rule of having more than 1 color.
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