# Some notes on local/global Galois groups

While I’m posting things.

Here are some notes that I wrote for a Galois representations learning seminar. I was tasked with giving the first talk about local fields, global fields, their Galois groups, and their connection.

Since most participants were seasoned veterans (at least insofar as basic definitions/results go) I tried to sail towards slightly more interesting waters. Thus, these notes, while containing (basically) the bare-bones technical information, have a slightly different goal then a standard introductory talk on the subject. Namely, they focus on two things:

1. Trying to establish, via multiple analogies, a ‘geometric understanding’ of what $G_{\mathbb{Q}_p}$ is measuring with regards to $G_\mathbb{Q}$–how it is studying the ‘local arithmetico-geometric data of a (punctured) $\text{Spec}(\mathbb{Z}$) at $p$‘.
2. Trying to emphasize the ‘credo’ that the hard part of something like $G_{\mathbb{Q}_p}$ is the wild ramification group $P_{\mathbb{Q}_p}$. This is done by explaining how $G_{\mathbb{Q}_p}/P_{\mathbb{Q}_p}$ is ‘simple’ and explaining how one can understand geometrically (by thinking about the geometry of curves over finite fields) why wild ramification is hard.This, for people that know some Galois representations, should not be a shocking focus since the oomph of big results like Grothendieck’s $\ell$-adic monodromy theorem is that $P_{\mathbb{Q}_p}$ is ‘almost killed’ when discussing $\ell$-adic representations and, combining this with our credo, explains why $\ell$-adic representations are ‘simpler’ than $p$-adic ones.

There should be two warnings though:

1. I proof read these even less than I usually do for posts. So, please take the contents with an extra large grain of salt. Please let me know if any mistakes are present and I will (attempt to) correct them.
2. Apparently there is a phrase ‘simple’ in group theory, which is kind of a big deal. I kind of, perhaps, maybe forgot this while writing these notes. So the phrase ‘simple group’ should be translated to ‘not very complicated group’ in these notes.

Here are the notes: rachel-seminar-talk-3.

# An update of sorts

I get an email every week or so which says something to the effect of “When are you going to make another blog post?” I am continually amazed by this–it’s absolutely shocking to me to see so many people interested in the things I write, and I find the emails and comments that I get heartening to no end.

So, for those who have been asking, I have not been posting that many things for a combination of two reasons. First, I’ve just been super busy. I’ve been running many seminars here at Berkeley and, of course more importantly, been working on my thesis. But, perhaps even more of the issue, I’ve started multiple, multiple very long posts that are approaching completion, but (for semi-perfectionist reasons) I’d rather not post them quite yet.

For those curious, a sampler of these in-preparation posts are:

• A post on why the tilting functor of Scholze et al. is a reasonable thing to do. Sort of in the same mindset of You could have invented spectral sequences.
• A post on stacks, with a focus on understanding the statement “$H^1(S,\text{Aut}(\mathcal{F}))$ (i.e. $\text{Aut}(\mathcal{F})$-torsors) classify objects on $S$ locally isomorphic to $\mathcal{F}$” or, equivalently, with a focus on the “theory of twists”.
• A high-level discussion of the Eichler-Shimura construction and how it fits into the larger picture of the Langlands program. I find that using more advanced ideas (such as etale cohomology) not only makes the whole construction incredibly more natural, but leads onequite readily into the general idea of why Shimura varieties are important–why they should realize something like the global Langlands correspondence.
• A rambling discussion of motives, the Weight-Monodromy conjecture, and Galois representations.
• A semi-thorough discussion of modular curves, modular forms, and their relationship from an algebro-geometric standpoint (something like a ‘what I’ve needed to know from Katz-Mazur’).

Essentially all of these are ‘mostly done’, and I hope to post (some) of them soon–probably in the order they were listed above.

Anyways, thanks again for your continued support! A special thanks to Dr. Woit whose undeservedly kind words brought quite a bit of attention to my small blog.

# A computation a day: a pullback pushforward

In this post we compute the Galois representation $i^\ast R^m j_\ast\mathbb{Q}_\ell$ where $j:\mathbb{G}_{m,\overline{k}}\hookrightarrow\mathbb{A}^1_k$ is the natural inclusion and $i:\text{Spec}(k)\hookrightarrow \mathbb{A}^1_k$ is the inclusion of the origin.