- #1
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I decided earlier this week that I was going to compute by hand the genus of an elliptic curve. I've had a miserable (but enlightening!) time!
I eventually stumbled upon the trick in Shafaravich: I should be looking at the rational differential forms, and counting zeroes & poles of things.
But I still feel like there should be a way to do it without resorting to the holomorphic stuff. (Though, I suppose I don't have enough intuition for algebraic geometry to have any right to think so. )
This is what I had been doing:
I decided to consider the complex projective elliptic curve E : y² z = x³ - x z².
Let U be the open affine subset consisting of all of the points of the form (u : v : 1). This is the affine curve defined by v² = u³ - u.
Let V be the open affine subset consisting of all of the points of the form (s : 1 : t). This is the affine curve defined by t = s³ - s t².
Then {U, V} is an open cover of E. Let W be their intersection. On W, the change of variable relations are ut = s and vt = 1.
(Does omega -- Ω -- show up right?)
Ω, the differential forms on U, is simply the C-module generated by {du, dv}, satisfying the relation 2v dv = (3u² - 1) du.
Ω[V] is the C[V]-module generated by {ds, dt}, satisfying the relation (1 + 2st) dt = (3s² - t²) ds
And we have maps from both of these into the C[W]-module Ω[W]. The collection of all of the relevant relations is:
v² = u³ - u
t = s³ - s t²
ut = s
vt = 1
2v dv = (3u² - 1) du
(1 + 2st) dt = (3s² - t²) ds
u dt + t du = ds
v dt + t dv = 0
and I was trying to find the intersection of the images of the two maps. (As C-vector spaces, I suppose) But I just couldn't figure out how to do it. After much work, I eventually stumbled across the global differential form... but I have absolutely no idea how I would go about proving that was the only one (up to a constant).
So, I suppose my problem is that I just don't know how I would compute the intersection of the images of these two maps -- is this a tractable problem at all, in this case or in general?
I eventually stumbled upon the trick in Shafaravich: I should be looking at the rational differential forms, and counting zeroes & poles of things.
But I still feel like there should be a way to do it without resorting to the holomorphic stuff. (Though, I suppose I don't have enough intuition for algebraic geometry to have any right to think so. )
This is what I had been doing:
I decided to consider the complex projective elliptic curve E : y² z = x³ - x z².
Let U be the open affine subset consisting of all of the points of the form (u : v : 1). This is the affine curve defined by v² = u³ - u.
Let V be the open affine subset consisting of all of the points of the form (s : 1 : t). This is the affine curve defined by t = s³ - s t².
Then {U, V} is an open cover of E. Let W be their intersection. On W, the change of variable relations are ut = s and vt = 1.
(Does omega -- Ω -- show up right?)
Ω, the differential forms on U, is simply the C-module generated by {du, dv}, satisfying the relation 2v dv = (3u² - 1) du.
Ω[V] is the C[V]-module generated by {ds, dt}, satisfying the relation (1 + 2st) dt = (3s² - t²) ds
And we have maps from both of these into the C[W]-module Ω[W]. The collection of all of the relevant relations is:
v² = u³ - u
t = s³ - s t²
ut = s
vt = 1
2v dv = (3u² - 1) du
(1 + 2st) dt = (3s² - t²) ds
u dt + t du = ds
v dt + t dv = 0
and I was trying to find the intersection of the images of the two maps. (As C-vector spaces, I suppose) But I just couldn't figure out how to do it. After much work, I eventually stumbled across the global differential form... but I have absolutely no idea how I would go about proving that was the only one (up to a constant).
So, I suppose my problem is that I just don't know how I would compute the intersection of the images of these two maps -- is this a tractable problem at all, in this case or in general?