Lissajous knot

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In knot theory, a Lissajous knot is a knot defined by parametric equations of the form

A Lissajous 821 knot

where , , and are integers and the phase shifts , , and may be any real numbers.[1]

The projection of a Lissajous knot onto any of the three coordinate planes is a Lissajous curve, and many of the properties of these knots are closely related to properties of Lissajous curves.

Replacing the cosine function in the parametrization by a triangle wave transforms every Lissajous knot isotopically into a billiard curve inside a cube, the simplest case of so-called billiard knots. Billiard knots can also be studied in other domains, for instance in a cylinder.[2]

Form[edit]

Because a knot cannot be self-intersecting, the three integers must be pairwise relatively prime, and none of the quantities

may be an integer multiple of pi. Moreover, by making a substitution of the form , one may assume that any of the three phase shifts , , is equal to zero.

Examples[edit]

Here are some examples of Lissajous knots,[3] all of which have :

There are infinitely many different Lissajous knots,[4] and other examples with 10 or fewer crossings include the 74 knot, the 815 knot, the 101 knot, the 1035 knot, the 1058 knot, and the composite knot 52* # 52,[1] as well as the 916 knot, 1076 knot, the 1099 knot, the 10122 knot, the 10144 knot, the granny knot, and the composite knot 52 # 52.[5] In addition, it is known that every twist knot with Arf invariant zero is a Lissajous knot.[6]

Symmetry[edit]

Lissajous knots are highly symmetric, though the type of symmetry depends on whether or not the numbers , , and are all odd.

Odd case[edit]

If , , and are all odd, then the point reflection across the origin is a symmetry of the Lissajous knot which preserves the knot orientation.

In general, a knot that has an orientation-preserving point reflection symmetry is known as strongly plus amphicheiral.[7] This is a fairly rare property: only three prime knots with twelve or fewer crossings are strongly plus amphicheiral prime knot, the first of which has crossing number ten.[8] Since this is so rare, ′most′ prime Lissajous knots lie in the even case.

Even case[edit]

If one of the frequencies (say ) is even, then the 180° rotation around the x-axis is a symmetry of the Lissajous knot. In general, a knot that has a symmetry of this type is called 2-periodic, so every even Lissajous knot must be 2-periodic.

Consequences[edit]

The symmetry of a Lissajous knot puts severe constraints on the Alexander polynomial. In the odd case, the Alexander polynomial of the Lissajous knot must be a perfect square.[9] In the even case, the Alexander polynomial must be a perfect square modulo 2.[10] In addition, the Arf invariant of a Lissajous knot must be zero. It follows that:

References[edit]

  1. ^ a b M.G.V. Bogle, J.E. Hearst, V.F.R. Jones, L. Stoilov, "Lissajous knots", Journal of Knot Theory and Its Ramifications, 3(2), 1994, 121–140.
  2. ^ C. Lamm, D. Obermeyer. "Billiard knots in a cylinder", Journal of Knot Theory and Its Ramifications, 8(3), 1999, 353–366.
  3. ^ Cromwell, Peter R. (2004). Knots and links. Cambridge, UK: Cambridge University Press. p. 13. ISBN 0-521-54831-4. 
  4. ^ C. Lamm. "There are infinitely many Lissajous knots." Manuscripta Math., 93:29–37, 1997, Springerlink
  5. ^ A. Boocher; J. Daigle; J. Hoste; W. Zheng (2007). "Sampling Lissajous and Fourier knots". arXiv:0707.4210Freely accessible. 
  6. ^ Hoste, Jim; Zirbel, Laura (2006). "Lissajous knots and knots with Lissajous projections". arXiv:math.GT/0605632Freely accessible. 
  7. ^ Przytycki, Jozef H. (2004). "Symmetric knots and billiard knots". arXiv:math/0405151Freely accessible. 
  8. ^ Jim Hoste, Morwen Thistlethwaite, and Jeff Weeks. "The first 1,701,936 knots." Math. Intelligencer, 20(4):33–48, 1998.
  9. ^ R. Hartley and A Kawauchi. "Polynomials of amphicheiral knots." Math. Ann., 243:63–70, 1979.
  10. ^ K. Murasugi. "On periodic knots." Comment. Math.Helv., 46:162–174, 1971.