# Seifert matrices — Figure 8 knot

I've just learnt about Seifert matrices and thought it might be a good idea to compute some. Can you tell me if this is right: Here $x_1^+$ denotes the push off of $x_1$. I have omitted the diagram for $x_2$ since I think that it's the same as the one for $x_1$. For the linking number of $x_1$ and $x_2^+$ I seem to get $\mathrm{lk} (x_1, x_2^+) = 0$ and similarly for $x_1^+$ and $x_2$. I think that's wrong but I'm not sure. The Seifert matrix I get is $$S = \begin{pmatrix} 1 & 0 \\ 0 & 1\end{pmatrix}$$

I found an example on the internet that gets Seifert matrix $$S = \begin{pmatrix} 1 & -1 \\ 0 & -1\end{pmatrix}$$

But since a knot can have many Seifert surfaces there can be many different Seifert matrices so this doesn't tell me much.

Question 1: In general, how can I check that my Seifert matrix is correct?

Question 2: I think the process of constructing the Seifert surface is fiddly and hence error prone. Is there a neater way to compute the Seifert matrix of a knot? (I want to use the matrix to compute the Arf invariant)

Question 3: In the picture above I'm particularly unsure about picture 3. How do I know that I attached the bands correctly?

Edit:

Assuming the other Seifert matrix is correct, my sums are wrong since mine gets me Arf invariant $1$ whereas the other matrix gives me Arf invariant $0$. Nonetheless, I'd be very grateful if someone could point out where my mistake is.

• I just noticed that picture 3 is actually not quite right. But the idea behind it is, as you can see from picture 4 which should be right. – Rudy the Reindeer Sep 4 '12 at 14:45 Then you will see that you get $$S = \begin{pmatrix} -1 & 0 \\ 1 & 1\end{pmatrix}$$ which gives you $\mathcal{A} (q) = 0$ which is what you want.