Definitions

# Jones polynomial

In the mathematical field of knot theory, the Jones polynomial is a knot polynomial discovered by Vaughan Jones in 1983. Specifically, it is an invariant of an oriented knot or link which assigns to each oriented knot or link a Laurent polynomial in the variable $t^\left\{1/2\right\}$ with integer coefficients.

## Definition by the bracket

Suppose we have an oriented link $L$, given as a knot diagram. We will define the Jones polynomial, $V\left(L\right)$, using Kauffman's bracket polynomial, which we denote by $langle~rangle$. Note that here the bracket polynomial is a Laurent polynomial in the variable $A$ with integer coefficients.

First, we define the auxiliary polynomial (also known as the normalized bracket polynomial) $X\left(L\right) = \left(-A^3\right)^\left\{-w\left(L\right)\right\}langle L rangle$, where $w\left(L\right)$ denotes the writhe of $L$ in its given diagram. The writhe of a diagram is the number of positive crossings ($L_\left\{+\right\}$ in the figure below) minus the number of negative crossings ($L_\left\{-\right\}$). The writhe is not a knot invariant.

$X\left(L\right)$ is a knot invariant since it is invariant under changes of the diagram of $L$ by the three Reidemeister moves. Invariance under type II and III Reidemeister moves follows from invariance of the bracket under those moves. The bracket polynomial is known to change by multiplication by $-A^\left\{pm 3\right\}$ under a type I Reidemeister move. The definition of the $X$ polynomial given above is designed to nullify this change, since the writhe changes appropriately by +1 or -1 under type I moves.

Now make the substitution $A = t^\left\{-1/4\right\}$ in $X\left(L\right)$ to get the Jones polynomial $V\left(L\right)$. This results in a Laurent polynomial with integer coefficients in the variable $t^\left\{1/2\right\}$.

## Definition by braid representation

Jones' original formulation of his polynomial came from his study of operator algebras. In Jones' approach, it resulted from a kind of "trace" of a particular braid representation into an algebra which originally arose while studying certain models, e.g. the Potts model, in statistical mechanics.

Let a link L be given. A theorem of Alexander's states that it is the trace closure of a braid, say with n strands. Now define a representation $rho$ of the braid group on n strands, Bn, into the Temperley-Lieb algebra TLn with coefficients in $mathbb Z \left[A, A^\left\{-1\right\}\right]$ and $delta = -A^2 - A^\left\{-2\right\}$. A standard braid generator $sigma_i$ gets sent to $A e_i + A^\left\{-1\right\} 1$, where $1, e_1, dots, e_\left\{n-1\right\}$ are the standard generators of the Temperley-Lieb algebra. It can be checked easily that this defines a representation.

Take the braid word $sigma$ obtained previously from L and compute $delta^\left\{n-1\right\} tr rho\left(sigma\right)$ where tr is the Markov trace. This gives $< L >$, where < > is the bracket polynomial. This can be seen by considering, as Kauffman did, the Temperley-Lieb algebra as a particular diagram algebra.

An advantage of this approach is that one can pick similar representations into other algebras, such as the R-matrix representations, leading to "generalized Jones invariants".

## Properties

The Jones polynomial is characterized by the fact that it takes the value 1 on any diagram of the unknot and satisfies the following skein relation:
$\left(t^\left\{1/2\right\} - t^\left\{-1/2\right\}\right)V\left(L_0\right) = t^\left\{-1\right\}V\left(L_\left\{+\right\}\right) - tV\left(L_\left\{-\right\}\right) ,$
where $L_\left\{+\right\}$, $L_\left\{-\right\}$, and $L_\left\{0\right\}$ are oriented link diagrams that are identical except in a small region where they differ by crossing change or smoothing as in the figure below:

The definition of the Jones polynomial by the bracket makes it simple to show that for a knot $K$, the Jones polynomial of its mirror image is given by substitution of $t^\left\{-1\right\}$ for $t$ in $V\left(K\right)$. Thus, an amphicheiral knot, a knot equivalent to its mirror image, has palindromic entries in its Jones polynomial.

See the page on skein relation for an example of a computation using these relations.

As first shown by Edward Witten, the Jones polynomial of a given knot γ, can be obtained by considering Chern-Simons theory on the three-sphere with gauge group SU(2), and computing the vacuum expectation value of a Wilson loop WF(γ), associated to γ, and the fundamental representation F of SU(2).

## Open problems

• Is there a nontrivial knot with Jones polynomial equal to that of the unknot? It is known that there are nontrivial links with Jones polynomial equal to that of the corresponding unlinks by the work of Morwen Thistlethwaite.