© | Dror Bar-Natan: Knot Atlas: KnotTheory`:               This page is passe. Go here instead!

7.3 The Alexander-Conway Polynomial

In[1]

In[2]:= ?Alexander
Alexander[K][t] computes the Alexander polynomial of a knot K as a function of the variable t. Alexander[K, r][t] computes a basis of the r'th Alexander ideal of K in Z[t].

In[3]:= Alexander::about
The program Alexander[K, r] to compute Alexander ideals was written by Jana Archibald at the University of Toronto in the summer of 2005.

In[4]:= ?Conway
Conway[K][z] computes the Conway polynomial of a knot K as a function of the variable z.

Figure 4: The knot $8_{18}$.

The Alexander polynomial $A(K)$ and the Conway polynomial $C(K)$ of a knot $K$ always satisfy $A(K)(t)=C(K)(\sqrt{t}-1/\sqrt{t})$. Let us verify this relation for the knot $8_{18}$:

In[5]:=	alex = Alexander[Knot[8, 18]][t]
Out[5]=	-3 5 10 2 3 13 - t + -- - -- - 10 t + 5 t - t 2 t t

In[6]:=	Expand[Conway[Knot[8, 18]][Sqrt[t] - 1/Sqrt[t]]]
Out[6]=	-3 5 10 2 3 13 - t + -- - -- - 10 t + 5 t - t 2 t t

The determinant of a knot $K$ is $\vert A(K)(-1)\vert$. Hence for $8_{18}$ it is

In[7]:=	Abs[alex /. t -> -1]
Out[7]=	45

Alternatively (see Section 7.4):

In[8]:=	KnotDet[Knot[8, 18]]
Out[8]=	45

$V_2(K)$, the (standardly normalized) type 2 Vassiliev invariant of a knot $K$ is the coefficient of $z^2$ in its Conway polynomial

In[9]:=	Coefficient[Conway[Knot[8, 18]][z], z^2]
Out[9]=	1

Alternatively (see Section 7.10),

In[10]:=	Vassiliev[2][Knot[8, 18]]
Out[10]=	0

Sometimes two knots have the same Alexander polynomial but different Alexander ideals. An example is the pair K11a99 and K11a277. They have the same Alexander polynomial, but the second Alexander ideal of the first knot is the whole ring ${\mathbb{Z}}[t]$ while the second Alexander ideal of the second knot is the smaller ideal generated by $3$ and by $1+t$:

In[11]:=

{K1, K2} = {Knot[11, Alternating, 99], Knot[11, Alternating, 277]};

In[12]:=	Alexander[K1] == Alexander[K2]
Out[12]=	True

In[13]:=	Alexander[K1, 2][t]
Out[13]=	{1}

In[14]:=	Alexander[K2, 2][t]
Out[14]=	{3, 1 + t}

Finally, the Alexander polynomial attains 551 values on the 802 knots known to KnotTheory`:

In[15]:=	Length /@ {Union[Alexander[#]& /@ AllKnots[]], AllKnots[]}
Out[15]=	{551, 802}