Van der Grinten projection: Difference between revisions
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In the mathematical field of [[knot theory]], the '''Arf invariant''' of a knot, named after [[Cahit Arf]], is a [[knot invariant]] obtained from a quadratic form associated to a [[Seifert surface]]. If ''F'' is a Seifert surface of a knot, then the homology group H<sub>1</sub>(''F'', '''Z'''/2'''Z''') has a quadratic form whose value is the number of full twists mod 2 in a neighborhood of an imbedded circle representing an element of the homology group. The [[Arf invariant]] of this quadratic form is the Arf invariant of the knot. | |||
==Definition by Seifert matrix== | |||
Let <math>V = v_{i,j}</math> be a [[Seifert matrix]] of the knot, constructed from a set of curves on a [[Seifert surface]] of genus ''g'' which represent a basis for the first [[homology (mathematics)|homology]] of the surface. This means that ''V'' is a 2''g'' × 2''g'' matrix with the property that ''V'' − ''V''<sup>T</sup> is a [[symplectic matrix]]. The ''Arf invariant'' of the knot is the residue of | |||
:<math>\sum\limits^g_{i=1}v_{2i-1,2i-1}v_{2i,2i} \pmod 2.</math> | |||
Specifically, if <math>\{a_i,b_i\}, i=1...g</math>, is a symplectic basis for the intersection form on the Seifert surface, then | |||
:<math>Arf(K) = \sum\limits^g_{i=1}lk(a_i, a_i^{+})lk(b_i,b_i^{+}) \pmod 2.</math> | |||
where <math>a^i</math> denotes the positive pushoff of ''a''. | |||
==Definition by pass equivalence== | |||
This approach to the Arf invariant is due to [[Louis Kauffman]]. | |||
We define two knots to be '''pass equivalent''' if they are related by a finite sequence of pass-moves,<ref>Kaufmann (1987) p.74</ref> which are illustrated below: (no figure right now) | |||
Every knot is pass-equivalent to either the [[unknot]] or the [[Trefoil knot|trefoil]]; these two knots are not pass-equivalent and additionally, the right- and left-handed trefoils are pass-equivalent.<ref>Kaufmann (1987) pp.75–78</ref> | |||
Now we can define the Arf invariant of a knot to be 0 if it is pass-equivalent to the unknot, or 1 if it is pass-equivalent to the trefoil. This definition is equivalent to the one above. | |||
==Definition by partition function== | |||
[[Vaughan Jones]] showed that the Arf invariant can be obtained by taking the partition function of a signed planar graph associated to a [[knot diagram]]. | |||
==Definition by Alexander polynomial== | |||
This approach to the Arf invariant is by Raymond Robertello.<ref name="Robertello">Robertello, Raymond, [[Communications on Pure and Applied Mathematics]], Volume 18, pp. 543–555, 1965</ref> Let | |||
: <math>\Delta(t) = c_0 + c_1 t + \cdots + c_n t^n + \cdots + c_0 t^{2n}</math> | |||
be the Alexander polynomial of the knot. Then the Arf invariant is the residue of | |||
: <math> c_{n-1} + c_{n-3} + \cdots + c_r</math> | |||
modulo 2, where ''r'' = 0 for ''n'' odd, and ''r'' = 1 for ''n'' even. | |||
Kunio Murasugi<ref name="Murasugi">Murasugi, Kunio, The Arf Invariant for Knot Types, Proceedings of the American Mathematical Society, Vol. 21, No. 1. (Apr., 1969), pp. 69–72</ref> proved that the Arf invariant is zero if and only if Δ(−1) <math>\equiv</math> ±1 modulo 8. | |||
==Notes== | |||
<references/> | |||
==References== | |||
* {{cite book | last=Kauffman | first=Louis H. | authorlink = Louis Kauffman | title = Formal knot theory | year = 1983 | series=Mathematical notes | volume=30 | publisher=Princeton University Press | isbn = 0-691-08336-3 }} | |||
* {{cite book | last = Kauffman | first= Louis H. | authorlink = Louis Kauffman | title = On knots | volume=115 | series=Annals of Mathematics Studies | publisher=Princeton University Press | year=1987 | isbn=0-691-08435-1 }} | |||
* {{cite book | last=Kirby | first=Robion | authorlink = Robion Kirby | title = The topology of 4-manifolds | year = 1989 | series=Lecture Notes in Mathematics | volume=1374, | publisher =[[Springer-Verlag]] | isbn=0-387-51148-2 }} | |||
{{Knot theory|state=collapsed}} | |||
{{DEFAULTSORT:Arf invariant of a knot}} | |||
[[Category:Knot invariants]] | |||
Revision as of 19:30, 22 December 2013
In the mathematical field of knot theory, the Arf invariant of a knot, named after Cahit Arf, is a knot invariant obtained from a quadratic form associated to a Seifert surface. If F is a Seifert surface of a knot, then the homology group H1(F, Z/2Z) has a quadratic form whose value is the number of full twists mod 2 in a neighborhood of an imbedded circle representing an element of the homology group. The Arf invariant of this quadratic form is the Arf invariant of the knot.
Definition by Seifert matrix
Let be a Seifert matrix of the knot, constructed from a set of curves on a Seifert surface of genus g which represent a basis for the first homology of the surface. This means that V is a 2g × 2g matrix with the property that V − VT is a symplectic matrix. The Arf invariant of the knot is the residue of
Specifically, if , is a symplectic basis for the intersection form on the Seifert surface, then
where denotes the positive pushoff of a.
Definition by pass equivalence
This approach to the Arf invariant is due to Louis Kauffman.
We define two knots to be pass equivalent if they are related by a finite sequence of pass-moves,[1] which are illustrated below: (no figure right now)
Every knot is pass-equivalent to either the unknot or the trefoil; these two knots are not pass-equivalent and additionally, the right- and left-handed trefoils are pass-equivalent.[2]
Now we can define the Arf invariant of a knot to be 0 if it is pass-equivalent to the unknot, or 1 if it is pass-equivalent to the trefoil. This definition is equivalent to the one above.
Definition by partition function
Vaughan Jones showed that the Arf invariant can be obtained by taking the partition function of a signed planar graph associated to a knot diagram.
Definition by Alexander polynomial
This approach to the Arf invariant is by Raymond Robertello.[3] Let
be the Alexander polynomial of the knot. Then the Arf invariant is the residue of
modulo 2, where r = 0 for n odd, and r = 1 for n even.
Kunio Murasugi[4] proved that the Arf invariant is zero if and only if Δ(−1) ±1 modulo 8.
Notes
- ↑ Kaufmann (1987) p.74
- ↑ Kaufmann (1987) pp.75–78
- ↑ Robertello, Raymond, Communications on Pure and Applied Mathematics, Volume 18, pp. 543–555, 1965
- ↑ Murasugi, Kunio, The Arf Invariant for Knot Types, Proceedings of the American Mathematical Society, Vol. 21, No. 1. (Apr., 1969), pp. 69–72
References
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