Compute genus via Riemann-Hurwitz and monodromy

[jackmell]

Hi guys,

I new to Algebraic Geometry and was wondering if someone could help me with this problem:

Homework Statement

Given the algebraic curve w(z) represented by w^5+w^2+z^2=0, show that the genus is one by employing the Riemann-Hurwitz formula.

Homework Equations

For the mapping f:X\to S, the Riemann-Hurwitz formula is given by:
2g(X)-2=\text{deg}(f)\left[2g(S)-2\right)+\sum (e_w-1)

which I assume the expression f:X\to S as applied to my problem can be written as w(z):\mathbb{C}\to S, that is, the function w(z) maps the (five-sheeted) complex plane to the Riemann surface given by S and it is this surface that is topologically equivalent to a Riemann sphere with one handle thus having genus one. If that's not the correct way to state that, could someone correct that for me please?

The Attempt at a Solution

Homework Statement

I believe I understand somewhat well, direct construction of the Riemann surface given by the expression w^2-p(z)=0 and the subsequent determination of its genus directly from the union of two Riemann spheres. However I'm not able to deduce that via Riemann-Hurwitz.

But back to the problem above:

I'm taking g(X) to be the genus of the complex plane which is zero.

The degree of f I'm taking to be the highest degree of f(z,w) in w which is five.

The sum is where I'm having the problem. This I believe can be computed from the monodromy group which I assume represents the ramifications around each branch-point which I take to mean the branching around each branch-point including infinity. Here's my analysis of that:

The function has six finite branch points and the surface S ramifies around each one as a two-valued branch and three single-valued branches. I think each monodromy group about these branch-points can be written in terms of \left\{(n,m)\right\} with n being the sheet number and m, the branch order as:

((1,2),(3,1),(4,1),(5,1))

Maybe though that's not a valid way to write that. However, around the branch-point at infinity, the function ramifies into a single five-valued branch so the monodromy group is ((1,5))

So my (incorrect) analysis of the sum would be \sum (e_w-1)=(2-1)+(1-1)+(1-1)+(1-1) for each finite branch-point and \sum (e_w-1)=5-1 for the point at infinity. Plugging all this into the Riemann-Hurwitz formula I obtain:

-2=5\left[2g-2\right]+10

and solving for g I obtain g=-1/5 which is obviously not correct.

I don't know, maybe I'm not even qualified to even ask the question. Still I would like to see how it's solved at the very least if someone could help me with that and as I study more the subject, I'm sure it will come together eventually for me.
Thanks,
Jack

 

[jackmell]

Let f(z,w)=z^2+w^2+w^5

then it's much easier to compute it via the formula:

p=\sum_{i=1}^M \frac{r_i-1}{2}-n+1

with n the degree of the function and r_i the ramifications over each critical point including infinity.

The finite critical points are zeros to R(f,f_w)=0 where R is the resultant since the function has no poles. There are seven critical points, one of which is the origin. At the origin, the function splits into five single-cycles. At the other six, it ramifies into a 2-cycle and three single cycles. At infinity, it fully ramifies into a 5-cycle. So then we have:

p=\sum_{i=1}^8 \frac{r_i-1}{2}-n+1=1/2(1+1+1+1+1+1+4)-5+1=1