Random Permutations

Postdoctoral research fellow

I am advertising a postdoctoral position at Queen’s University Belfast (QUB). The start date would be Oct 2023 or later, and the duration is up to four years. The ideal candidate would have a background and strong research track record in algebra and combinatorics, with particular interest in growth in groups or permutation groups, or some other area of algebraic combinatorics or additive combinatorics. If this sounds like you, please consider applying!

The application window closes unfortunately rather soon: 17th of April. Please do not hesitate to get in touch with any questions.

PhD students

I am also open to PhD applications in principle. If you are interested please get in touch with a CV and a list of interests. I am not sure starting Oct 2023 is feasible at this point but certainly Oct 2024 is.

1. Boston–Shalev for conjugacy classes

Last week Daniele Garzoni and I uploaded to the arxiv a preprint on the Boston–Shalev conjecture for the conjugacy class weighting. The Boston–Shalev conjecture in its original form predicts that, in any finite simple group $G$ , in any transitive action, the proportion of elements acting as derangements is at least some universal constant $c > 0$ . This conjecture was proved by Fulman and Guralnick in a long series of papers. Daniele and I looked at conjugacy classes instead, and we found an analogous result to be true: the proportion of conjugacy classes containing derangements is at least some universal constant $c' > 0$ .

Our proof depends on the correspondence between semisimple conjugacy classes in a group of Lie type and polynomials over a finite field possibly with certain restrictions: either symmetry or conjugate-symmetry. We studied these sets of polynomials from an “anatomical” perspective, and we needed to prove several nontrivial estimates, e.g., for

the number of polynomials with a factor of a given degree (which is closely related the “multiplication table problem”),
the number of polynomials with an even or odd number of irreducible factors,
the number of polynomials with no factors of small degree,
or the number of polynomials factorizing in a certain way (e.g., as $f = gg^*$ , $g$ irreducible, $g^*$ the reciprocal polynomial).

For a particularly neat example, we found that, if the order of the ground field is odd, exactly half the self-reciprocal polynomials have an even number of irreducible factors — is there a simple proof of this fact?

2. Growth in Linear Groups

Yesterday Brendan Murphy, Endre Szabo, Laci Pyber, and I uploaded a substantial update to our preprint Growth in Linear Groups, in which we prove one general form of the “Helfgott–Lindenstrauss conjecture”. This conjecture asserts that if a symmetric subset $A$ of a general linear group $\mathrm{GL}_n(F)$ ( $n$ bounded, $F$ an arbitrary field) exhibits bounded tripling, $|A^3| \le K|A|$ , then $A$ suffers a precise structure: there are subgroup $H \trianglelefteq \Gamma \le \langle A \rangle$ such that $\Gamma / H$ is nilpotent of class at most $n-1$ , $H$ is contained in a bounded power $A^{O_n(1)}$ , and $A$ is covered by $K^{O_n(1)}$ cosets of $\Gamma$ . Following prodding by the referee and others, we put a lot more work in and proved one additional property: $\Gamma$ can be taken to be normal in $\langle A \rangle$ . This seemingly technical additional point is actually very subtle, and I strongly doubted whether it was true late into the project, more-or-less until we actually proved it.

We also added another significant “application”. This is not exactly an application of the result, but rather of the same toolkit. We showed that if $G \le \mathrm{GL}_n(F)$ (again $F$ an arbitrary field) is any finite subgroup which is $K(n)$ -quasirandom, for some quantity $K(n)$ depending only on $n$ , then the diameter of any Cayley graph of $G$ is polylogarithmic in the order of $|G|$ (that is, Babai’s conjecture holds for $G$ ). This was previously known for $G$ simple (Breuillard–Green–Tao, Pyber–Szabo, 2010). Our result establishes that it is only necessary that $G$ is sufficiently quasirandom. (There is a strong trend in asymptotic group theory of weakening results requiring simplicity to only requiring quasirandomness.)

The intention of our paper is more-or-less to “polish off” the theory of growth in bounded rank. By contrast, growth in high-rank simple groups is still poorly understood.

3. The Kelley–Meka result

Not my own work, but it cannot go unmentioned. There was a spectacular breakthrough in additive combinatorics last week. Kelley and Meka proved a Behrend-like upper bound for the density of a subset $A \subset \{1, \dots, n\}$ free of three-term arithmetic progressions (Roth’s theorem): the density of $A$ is bounded by $\exp(-c (\log n)^\beta)$ for some constants $c, \beta > 0$ . Already there are other expositions of the method which are also worth looking at: see the notes by Bloom and Sisask and Green (to appear, possibly).

Until this work, density $1 /\log n$ was the “logarithmic barrier”, only very recently and barely overcome by Bloom and Sisask. Now that the logarithmic barrier has been completely smashed, it seems inevitable that the new barometer for progress on Roth’s theorem is the exponent $\beta$ . Kelley and Meka obtain $\beta = 1/11$ , while the Behrend construction shows $\beta \le 1/2$ .