# Category Archives: maths

## We’re hiring (2023 edition)

We’re hiring!

Two permanent positions in pure maths. One in geometry/topology, one open for all areas of pure mathematics. Applications are through mathjobs (link to Job 1, link to Job 2), and are due December 18 (AEDT).

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## The Jacobson Density Theorem

I’ve never been at ease with the Jacobson density theorem, every proof that I’d seen felt somewhat weird and never had any stickiness in my mind. So I came up with my own proof, I hope you like it.

The Theorem (Jacobson Density Theorem}
Let be a field and a -algebra (yes unital). Let be a finite dimensional simple -module and (which is a division algebra by Schur’s Lemma). Then the canonical homomorphism from to is surjective.

Proof
First, without loss of generality (replacing by a quotient), we can assume that is faithful.

Let us first deal with the case when is a division algebra. Then . And as well, acting by right multiplication. And we are done.

Since is not a division algebra it has a non-zero element which is not invertible.

If is not nilpotent, then the minimal polynomial of factors as where . In particular it has two coprime factors, so by the Chinese remainder theorem has a non-trivial idempotent. In particular, contains a non-trivial idempotent.

If on the other hand is nilpotent, then we can find with , and . Since is irreducible, there exists with . Then and . So is not nilpotent, and we can run the argument of the previous paragraph again to conclude that contains a non-trivial idempotent.

Let be this non-trivial idempotent. We will use the fact that is an irreducible -module. This is part of a general fact, that is a bijection betwen simple -modules with and simple -modules. Furthermore, from the nature of the constructions of both directions of this bijection, we can conclude that

Now we can perform an induction on dimension. The inductive hypothesis tells us that and are both surjective. To finish, given the symmetry between and , it suffices to show that given any -hyperplane with and any -line , there is a nonzero element in with kernel and image . (I thought about this step myself in terms of matrices, but actually writing it up like that feels a little gauche).

The inductive hypothesis applied to allows us to find a nonzero element with . Pick nonzero and . Since is simple there exists with . Then does the trick and our proof is complete.

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## Tension in teaching

The following quote is by Matt Emerton (in a comment on MathOverflow)

I think there is a genuine tension between proofs that a professional will like (where professional here may mean professional algebraist!) and ones that are elementary. For professionals, reductions and devissages are easy, natural, and we don’t even think of them as real landmarks in the proof; they are just serve as passages between the key points and ideas. But in writing things out, they can take a lot of words, and seem (as you wrote) mysterious and difficult. I don’t know the best way to deal with this tension.

Interestingly, Matt posted it as part of a discussion about exactly what I wanted to talk about in this post, the teaching of the structure theorem for finitely generated abelian groups, or more generally, of finitely generated modules over a PID.

My personal connection is that I taught this as part of our third-year algebra course this year at the University of Melbourne, and am slated to do so again next year. I think that I did not do a particularly good job of teaching it in 2022, primarily because I got distracted by the reductions and devissages and tried to proceed along those lines as much as possible, when what I have learned is more appropriate for one of these courses is the more prosaic approach involving matrix manipulations. It is with the matrix manipulations (directly proving Smith Normal Form) that I plan to teach this part of the course in 2023 (and beyond, if necessary).

For completeness, allow me to state the professionals’ proof: Split off the quotient by the torsion subgroup to reduce to the torsion case. Then canonically decompose the module into a direct sum of its p-primary components. Then use the fact that is injective over itself to manually split the remaining short exact sequences needed to complete the classification.

While it may not be reasonable to expect a third-year student to follow this proof, I think it is fair to expect any PhD student of mine to be able to understand and execute this proof.

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## SMMC 2022 A4

The Simon Marais Mathematics Competition happened last weekend. It is a maths competition for undergraduate students across Europe, Asia, Africa and Oceania. This post is about problem A4, which I submitted. I’ll talk a bit about where the problem came from, a generalisation, a conjecture and also provide a solution. The entire paper is available on the Marais website, and solutions should be put up there at some time in the near future.

Problem (SMMC 2022 A4)
Let be a positive integer, and let be an odd integer such that every prime factor of is larger than . Prove that

is an integer that has no prime factor in common with .

Origins

Let and let be the subgroup of monomial matrices (a matrix is a monomial matrix if and only if it has exactly one nonzero entry in each row and column). I show below in my solution that this question is equivalent to the fact that the integer is coprime to . Now why would I ever care about that?

This coprimality fact implies that the cohomology of with mod coefficients is isomorphic to the cohomology of with mod coefficients. And I was interested in these cohomology groups because the second cohomology group classifies central extensions, which is what I used to think about back in my PhD days. The group feels somewhat more “combinatorial” than , so it is nice to be able to pass information from to for free.

Generalisations (known and conjectural)

Let be a split reductive group over , which I conflate with its -points below in an abuse of notation. Let be a maximal split torus and its normaliser in . Then

Here is the set of positive roots and the collection of integers are the exponents of the Weyl group. Then the same argument as in my proof below shows that this fraction is an integer, relatively prime to .

If we remove the assumption that is split, then I suspect the same conclusion is satisfied, but there is an additional argument needed as the formula for the quotient has additional factors. I have not worked out this argument and really don’t want to resort to case by case arguments, so there is your conjecture (I expect we now need to say is a maximal torus containing a maximal split torus).

Solution

First we show that the fraction in the question is an integer. Since divides as a polynomial for all , the statement only depends on the residue class of modulo . Since every prime factor of is greater than , is relatively prime to . So by Dirichlet’s theorem on primes in arithmetic progressions, we may assume without loss of generality that is prime.

Let and let be the subgroup of monomial matrices. Then

By Lagrange’s theorem is an integer. Since is relatively prime to , we can further divide by the largest power of in and deduce that

is an integer.

Now let be a prime dividing and let be a positive integer. To conclude, it suffices to show that the fraction

has zero -adic valuation. Write , then by the binomial theorem,

Let and . Since , we get

We have the inequality , so

For , we therefore get

as from our assumption that divides .
Thus in our sum, the term with has a strictly smaller -adic valuation than every other term, so determines the -adic valuation of the sum, and we get

completing the proof.

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## Infinite dimensional vector spaces

This is a quick note to prove that two bases of an infinite dimensional vector space have the same cardinality. We freely use the axiom of choice and other standard facts about cardinalities of infinite sets. We will in fact prove the following:

Theorem: Let be a vector space with basis with an infinite set. Let be a linearly independent subset of . (e.g. a basis of a subspace). Then .

To prove this, WLOG is a basis of (by extending to a basis of if necessary). For all , write

Let be the set of pairs with . Then has finite fibres, since the sum above is finite, and is surjective, since the lie in the span of the with in the image of , but also the generate . Since is assumed infinite, this is enough to prove that , as required.

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## Jucys-Murphy elements and induction

This post concerns the representation theory of the symmetric group over the complex numbers. Recall that the irreducible representations of the symmetric group are indexed by partitions of . Let be the irreducible representation indexed by . I want to say some words about the theorem that the decomposition of the induced module is given by the decomposition into eigenspaces under the action of the Jucys-Murphy element.

First, the relevant Jucys-Murphy element is

The way it acts on is not as an element of but by This is well-defined since commutes with .

What this action defines is a natural transformation from the functor to itself. The induction functor is (bi)-adjoint to the restriction functor and this natural transformation is even simpler to construct on the adjoint side. Recall that if and are adjoint functors, then there is an isomorphism

Here refers to the natural transformations from to itself, and the map in this isomorphism is given by pre- and post-composition by the unit and counit of the adjunction.

And the way that yields a natural transformation from to itself is very simple, it’s just by its usual action as an element of . If you transport this natural transformation to a natural transformation of the induction functor via the method I just mentioned, then you get the formula mentioned above.

Now given a pair of adjoint functors and , a natural transformation from to (and hence from to ) and a complex number , we can define a functor by

and similarly for (this requires some linearity assumptions, but they’re satisfied here. Also you could take generalised eigenspaces if you wanted to, but in our application there is no difference).

When you do this, the functors and are adjoint:

Proof: Both and are the -eigenspace of the action of on .

Now apply this to our situation. We also use the following standard fact about the action of the Jucys-Murphy element (as developed e.g. in the Vershik-Okounkov approach):

Consider the decomposition

Then the Jucys-Murphy element acts by the scalar on , where is the content of the box added to to get .

Now translating this statement via the above yoga onto the adjoint side, we get

In the decomposition

the Jucys-Murphy element acts by the scalar on , where is the content of the box added to to get .

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## Singularities of Schubert varieties within a right cell

Martina Lanini and I recently posted our preprint Singularities of Schubert varieties within a right cell to the arXiv. In it, we show that every singularity which appears in a type A Schubert variety appears between two permutations lying in the same right cell. This shows that any behaviour controlled by the singularities of Schubert varieties manifests itself within a Specht module. Some exmples are discussed.

The work was conducted during our recent visit to the thematic trimester program on representation theory at the Institut Henri PoincarĂ© in Paris. I spent an enjoyable first month there before returning to Australia. Originally I was scheduled to be on a plane right now to return to Paris for the end of the program, but alas this is no longer possible. Oh well.

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## We’re hiring

The University of Melbourne maths department is hiring. All our ads can be found on mathjobs.

In pure maths, we have a continuing position in analysis. Applications should be submitted by the earlier deadline of 25 October to ensure full consideration.

There are also other positions available in applied and statistics, as well as a temporary position open to all areas.

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## Sydney Mathematical Research Institute International Visitor Program

There is a new (established in the last couple of years) mathematical institute based at the University of Sydney. I want to call attention to a visitor program they run, for mathematical visits to Australia.

They fund research visits of at least one month’s duration. At least two weeks must be spent at the University of Sydney, but the rest can be taken anywhere in Australia.

Funding rounds for these research visits happen periodically. Currently there is one open for visits within the period April-December 2020, with applications closing on 28 July.

Anyone interested should check out the research intstitute’s webpage.