# Transcendental Galois Theory

Is there a good reference on transcendental Galois Theory?

More precisely, if $$K/k$$ admits a separating transcendence basis (or maybe if it is a separably generated extension) it seems to me that many of the usual theorems of Galois theory go through. Moreover, the group $$\text{Aut}(K/k)$$ seems to have additional structure; namely it should be an algebraic group over $$k$$.

For example, it seems to me that $$k(x_1, ..., x_n)/k$$ has automorphism group $$GL_n(k)$$. (EDIT: As Qiaochu Yuan points out, this is incorrect; the automorphism group at least must contain $$PGL_{n+1}(k)$$, acting via its action on the function field of $$\mathbb{P}_k^n$$.) This sort of thing must be well-studied; if so, what are the standard references on the subject?

I have seen Pete L. Clark's excellent (rough) notes on related subjects here but they seem not to address quite these sorts of questions.

• Isn't the automorphism group more something like $\text{PGL}_{n+1}(k)$? Apr 13, 2011 at 6:11
• How do you figure? I could just be being silly but I don't even see an action of $PGL_{n+1}$... Apr 13, 2011 at 6:16
• I think the action is by generalized fractional linear transformations, e.g. when $n = 1$ we can send $x_1$ to any $\frac{ax_1 + b}{cx_1 + d}$ and this has inverse the corresponding inverse fractional linear transformation. Apr 13, 2011 at 6:22
• I'm pretty sure you're right; as I've remarked in my edit, this action should arise via the natural action of $PGL_{n+1}$ on the function field of $\mathbb{P}^n_k$. Apr 13, 2011 at 6:45

For every $$n \geq 1$$, there is a natural effective action of $$\operatorname{PGL}_{n+1}(k)$$ on $$k(x_1,\ldots,x_n)$$. In fact $$\operatorname{PGL}_{n+1}(k)$$ is the automorphism group of $$\mathbb{P}^n_{/k}$$, the action being the obvious one induced by the action of $$\operatorname{GL}_{n+1}(k)$$ on the vector space $$k^{n+1}$$ in which $$\mathbb{P}^n$$ is the set of lines.
However, no one said this was the entire automorphism group of $$k(x_1,\ldots,x_n)$$! It is when $$n = 1$$ -- for instance because every rational map from a smooth curve to a projective variety is a morphism ("valuative criterion for properness"). However, $$\operatorname{PGL}_{n+1}(k)$$ is known not to be the entire automorphism group of $$k(x_1,\ldots,x_n)$$ when $$n > 1$$. Rather, the full automorphism group is called the Cremona group. For $$n = 2$$ we have a problem in the geometry of surfaces, and it was shown (by Max Noether when $$k = \mathbb{C}$$) that the automorphism group here is generated by the linear automorphisms described above together with a certain set of simple, well-understood birational maps, called quadratic maps or indeed Cremona transformations. But even when $$n = 2$$ this automorphism group is not an algebraic group: it's bigger than that.
When $$n \geq 3$$ it is further known that the linear automorphisms and the Cremona transformations do not generate the whole automorphism group, and apparently no one has even a decent guess as to what a set of generators might look like. I had the good fortune of hearing a talk by James McKernan on (in part) this subject within the last few months, so I am a bit more up on this than I otherwise would be. Anyway, he gave us the sense that this is a pretty hopeless problem at present. For instance, see this recent preprint in which a rather eminent algebraic geometer works rather hard to prove a seemingly rather weak result about finite subgroups of the three dimensional Cremona group!
• Thanks! I should have realized that we could allow birational automorphisms of $\mathbb{P}^n$ as well; I wonder if there is some way of functorially identifying an algebraic subgroup (e.g. look at automorphisms that fix some basepoint on $\mathbb{P}^n$...). Apr 13, 2011 at 7:12
• Recently I have started updating links containing www.math.uga.edu (which is no longer working) to math.uga.edu. I do not know whether this edit is substantive enough for the author of the post to receive the notification - if yes, I apologize for lost of pings. But I should be able to finish this in a few installments. Anyway, here I changed it not to math.uga.edu/~pete/galois.pdf - instead I have used math.uga.edu/~pete/transgalois.pdf (I suppose that was the intention of giving the "redirect pdf" there.) Dec 8, 2016 at 2:24