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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

11 votes

Who first noticed that the Hilbert symbol is a Steinberg symbol ?

EDIT: After looking into the history more closely, I think it's fairly certain that the correct answer to the question is Calvin Moore. (See my added text below.) The question is interesting and l …
Jim Humphreys's user avatar
7 votes

What are the maximal subgroups of GSp(2g,l)?

There has been a lot of systematic study in recent decades of the maximal subgroups of finite simple groups, especially groups of Lie type. This goes far beyond the special case here, but is written …
Jim Humphreys's user avatar
14 votes

how to find the varieties whose cohomology realizes certain representations?

This is an extended comment, certainly not an answer to the very broad and perhaps unanswerable multiple part question being asked here ("How did people find those varieties?"). A facetious response …
Jim Humphreys's user avatar
10 votes

Cohomology of $SL_2(\mathbb{F}_p)$ acting on trace zero matrices over $\mathbb{F}_p$

As Olivier's answer indicates, a direct approach to the computation using classical techniques of Sah and others is possible in a small isolated case like this. But it's a good idea to be aware of s …
Jim Humphreys's user avatar
7 votes

Number of points of the nilpotent cone over a finite field and its cohomology

Concerning your Q1, it may be of interest to fill in some of the background due to Steinberg (which in turn had a lot of influence on Springer's work). In his 1966 ICM talk, Steinberg formulated quit …
Jim Humphreys's user avatar
1 vote

Fixed Points of the Weyl Group action on a Maximal Torus and the Center of a Reductive Group

To me the question itself (and the answers) are out of focus, starting with the claim that the ring of Weyl group invariants is somehow central. Chevalley's 1955 argument does show that this ring is …
Jim Humphreys's user avatar
5 votes

Inverse Galois problem for simple Lie type groups

It's hard to keep track of all relevant literature on the inverse Galois problem for finite groups of Lie type, but at this point many special cases have been worked out while others remain open. On …
Jim Humphreys's user avatar
6 votes

Which compact groups have nonisomorphic irreducible representations of the same dimension?

Without offering a complete answer to the stated question, I'd first ask what significance the answer would have (one way or the other) in terms of Lie theory? I'd also want to extract the essential …
Jim Humphreys's user avatar
5 votes

Explicit construction of an element of ${\rm GL}(2, p)$ of order $p+1$

As the discussion shows, there really isn't any explicit construction which works for all primes $p$. (Here you really want to specify that $p$ is odd, however, to avoid trivialities.) It may or ma …
Jim Humphreys's user avatar
6 votes
3 answers
534 views

Source for embedding multiplicative group of an algebraic closure of a finite field?

It's easy to embed the (cyclic) multiplicative group of a finite field into the multiplicative group of $\mathbb{C}$ (or other algebraically closed field of characteristic 0): assign to a generator of …
Jim Humphreys's user avatar
9 votes

Representations of complex semi-simple algebraic group "defined over $\mathbf{Z}$"?

Let me add a couple of things to what grghxy has said. 1) The study of these groups over $\mathbb{Z}$ has been complicated, going back to Chevalley's work and Borel's lecture notes (which aren't quit …
Jim Humphreys's user avatar
10 votes

Etymology of cuspidal representations

To reinforce what's already been said and add some references, I'd emphasize first that Harish-Chandra's "philosophy of cusp forms" was indeed a driving force in the study of representations over both …
Jim Humphreys's user avatar
13 votes
Accepted

Borel's Paris Lectures

The Paris lectures, along with others he gave later, were spliced together into a publication: Introduction aux groupes arithmetiques (softcover, Hermann, Paris, 1969). As his nominal assistant at …
Jim Humphreys's user avatar
2 votes

Product of two algebraic subgroups of a (solvable) group = another algebraic subgroup?

Maybe it's worth pointing out that the question contained in the header has an obvious negative answer (and is not the main question being asked). The easiest counterexample would be the product of t …
Jim Humphreys's user avatar
18 votes
4 answers
2k views

Origin of symbol *l* for a prime different from a fixed prime?

I've never seen an authoritative explanation for the choice of the lower case letter $\ell$ or $l$ to denote an arbitrary prime different from a given prime $p$. This now has its own LaTeX command \e …
Jim Humphreys's user avatar

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