Forthcoming events in this series


Mon, 18 Nov 2019

16:00 - 17:00
C1

Erdős' primitive set conjecture

Jared Duker Lichtman
(Oxford)
Abstract

A subset of the integers larger than 1 is called $\textit{primitive}$ if no member divides another. Erdős proved in 1935 that the sum of $1/(n \log n)$ over $n$ in a primitive set $A$ is universally bounded for any choice of $A$. In 1988, he famously asked if this universal bound is attained by the set of prime numbers. In this talk we shall discuss some recent progress towards this conjecture and related results, drawing on ideas from analysis, probability, & combinatorics.

Mon, 11 Nov 2019

16:00 - 17:00
C1

On Serre's Uniformity Conjecture

Jay Swar
(Oxford)
Abstract

Given a prime p and an elliptic curve E (say over Q), one can associate a "mod p Galois representation" of the absolute Galois group of Q by considering the natural action on p-torsion points of E.

In 1972, Serre showed that if the endomorphism ring of E is "minimal", then there exists a prime P(E) such that for all p>P(E), the mod p Galois representation is surjective. This raised an immediate question (now known as Serre's uniformity conjecture) on whether P(E) can be bounded as E ranges over elliptic curves over Q with minimal endomorphism rings.

I'll sketch a proof of this result, the current status of the conjecture, and (time permitting) some extensions of this result (e.g. to abelian varieties with appropriately analogous endomorphism rings).

Mon, 04 Nov 2019

16:00 - 17:00
C1

What is Arakelov Geometry?

Esteban Gomezllata Marmolejo
(Oxford)
Abstract

Arakelov geometry studies schemes X over ℤ, together with the Hermitian complex geometry of X(ℂ).
Most notably, it has been used to give a proof of Mordell's conjecture (Faltings's Theorem) by Paul Vojta; curves of genus greater than 1 have at most finitely many rational points.
In this talk, we'll introduce some of the ideas behind Arakelov theory, and show how many results in Arakelov theory are analogous—with additional structure—to classic results such as intersection theory and Riemann Roch.

Mon, 28 Oct 2019

16:00 - 17:00
C1

Cartier Operators

Zhenhua Wu
(Oxford)
Abstract

Given a morphism of schemes of characteristic p, we can construct a morphism from the exterior algebra of Kahler differentials to the cohomology of De Rham complex, which is an isomorphism when the original morphism is smooth.

Mon, 21 Oct 2019

16:00 - 17:00
C1

Relative decidability via the tilting correspondence

Konstantinos Kartas
(Oxford University)
Abstract

The goal of the talk is to present a proof of the following statement:
Let (K,v) be an algebraic extension of (Q_p,v_p) whose completion is perfectoid. We show that K is relatively decidable to its tilt K^♭, i.e. if K^♭ is decidable in the language of valued fields, then so is K. 
In the first part [of the talk], I will try to cover the necessary background needed from model theory and the theory of perfectoid fields.

Mon, 14 Oct 2019

16:00 - 17:00
C1

From Chabauty's Method to Kim's Non-Abelian Chabauty's Method

Nadav Gropper
(Archaeology, Oxford)
Abstract

In 1941, Chabauty gave a way to compute the set of rational points on specific curves. In 2004, Minhyong Kim showed how to extend Chabauty's method to a bigger class of curves using anabelian methods. In the talk, I will explain Chabauty's method and give an outline of how Kim extended those methods.

Mon, 10 Jun 2019

16:00 - 17:00
C1

The Golod-Shafarevich Theorem: Endgame

Jay Swar
(Oxford)
Abstract

The principal ideal theorem (1930) guaranteed that any number field K would embed into a finite extension, called the Hilbert class field of K, in which every ideal of the original field became principal -- however the Hilbert class field itself will not necessarily have class number 1. The class field tower problem asked whether iteratively taking Hilbert class fields must stabilize after finitely many steps. In 1964, it was finally answered in the negative by Golod and Shafarevich who produced infinitely many examples and pioneered the framework that is still the most common setting for deciding when a number field will have an infinite class field tower.

In this talk, I will finish the proof of their cohomological result and thus fully justify how it settled the class field tower problem.

Mon, 03 Jun 2019

16:00 - 17:00
C1

The Golod-Shafarevich Theorem

Jay Swar
(Oxford)
Abstract

The principal ideal theorem (1930) ascertained that any number field K embeds into a finite extension, called the Hilbert class field of K, in which every ideal of the original field became principal -- however the Hilbert class field itself will not necessarily have class number 1. The class field tower problem asked whether iteratively taking Hilbert class fields must stabilize after finitely many steps. In 1964, it was finally answered in the negative by Golod and Shafarevich who produced infinitely many examples and pioneered the framework that is still the most common setting for deciding when a number field will have an infinite class field tower.

In this talk, I will sketch the proof of their cohomological result and explain how it settled the class field tower problem.

Mon, 15 Oct 2018

16:00 - 17:00
C3

Periods and the number Zagier forgot

Adam Keilthy
(Oxford)
Abstract

A particularly active area of research in modern algebraic number theory is the study of a class of numbers, called periods. In their simplest form, periods are integrals of rational functions over domains defined by rational in equations. They form a ring, which encompasses all algebraic numbers, logarithms thereof and \pi. They arise in the study of modular forms, cohomology and quantum field theory, and conjecturally have a sort of Galois theory.

We will take a whirlwind tour of these numbers, before discussing non-periods. In particular, we will sketch the construction of an explicit non-period, often forgotten about.

Mon, 30 Jan 2017

16:00 - 17:00
C3

Cohomology of Varieties

Alex Torzewski
(Dept. Mathematics, University of Warwick)
Abstract

We outline what we expect from a good cohomology theory and introduce some of the most common cohomology theories. We go on to discuss what properties each should encode and detail attempts to fit them into a common framework. We build evidence for this viewpoint through several worked number theoretic examples and explain how many of the key conjectures in number theory fit into this theory of motives.

Mon, 24 Oct 2016

16:00 - 17:00
C3

On sets of irreducible polynomials closed by composition

Giacomo Micheli
(Oxford)
Abstract

Let S be a set of monic degree 2 polynomials over a finite field and let C be the compositional semigroup generated by S. In this talk we establish a necessary and sufficient condition for C to be consisting entirely of irreducible polynomials. The condition we deduce depends on the finite data encoded in a certain graph uniquely determined by the generating set S. Using this machinery we are able both to show examples of semigroups of irreducible polynomials generated by two degree 2 polynomials and to give some non-existence results for some of these sets in infinitely many prime fields satisfying certain arithmetic conditions (this is a joint work with A.Ferraguti and R.Schnyder). Time permitting, we will also describe how to use character sum techniques to bound the size of the graph determined by the generating set (this is a joint work with D.R. Heath-Brown).

Mon, 10 Oct 2016
16:00
C3

The large sieve

Aled Walker
(Oxford)
Abstract

The large sieve is a powerful analytic tool in number theory, with many beautiful and diverse applications. In its most general form it resembles an approximate Bessel's inequality, and this clear modern theory rests on the combined effort of countless mathematicians in the mid-twentieth century -- Linnik, Roth, Selberg, Montgomery, Vaughan, and Bombieri, to name a few. However, it is hardly obvious to the beginner why this rather abstract inequality should be called 'large', or 'sieve'. In this introductory talk, aimed particularly at new graduate students, we discuss the rudimentary theory of the large sieve, some particular applications to sieving problems, and (at least one) proof. 

Mon, 16 May 2016
16:00
C3

Curves and their fundamental groups

Junghwan Lim
((Oxford University))
Abstract

I will describe a sketch of the proof of Grothendieck conjecture on fundamental groups.
 

Mon, 09 May 2016
16:00
C3

TBA

Vandita Patel
(Warwick University)
Mon, 09 May 2016
16:00
C3

Descent of a sum of Consecutive Cubes ... Twice!!

Vandita Patel
(Warwick University)
Abstract

Given an integer $d$ such that $2 \leq d \leq 50$, we want to
answer the question: When is the sum of
$d$ consecutive cubes a perfect power? In other words, we want to find all
integer solutions to the equation
$(x+1)^3 + (x+2)^3 + \cdots + (x+d)^3 = y^p$. In this talk, we present some
of the techniques used to tackle such diophantine problems.

 

Mon, 02 May 2016
16:00
C3

Explicit Kummer coordinates for higher genus curves

Christopher Nicholls
((Oxford University))
Abstract

I will explain how to find an explicit embedding of the Kummer variety of a higher genus curve into projective space and discuss applications of such an embedding to the study of rational points on Jacobians of curves, as well as the original curves.

Mon, 29 Feb 2016
16:30
C1

Torelli and Borel-Tits theorems via trichotomy

Carlos Alfonso Ruiz Guido
((Oxford University))
Abstract

Using the "trichotomy principle" by Boris Zilber I will give model theoretic proofs of appropriate versions of Torelli theorem and Borel-Tits theorem. The first one has interesting applications to anabelian geometry, I won't assume any prior knowledge in model theory.

Mon, 22 Feb 2016
16:30
C1

Congruence and non-congruence level structures on elliptic curves: a hands-on tour of the modular tower

Alexander Betts
((Oxford University))
Abstract
Classically, one puts an algebraic structure on certain "congruence" quotients of the upper half plane by interpreting them as spaces parametrising elliptic curves with certain level structures on their torsion subgroups. However, the non-congruence quotients don't admit such a straightforward description.
 
We will sketch the classical theory of congruence modular curves and level structures, and then discuss a preprint by W. Chen which extends the above notions to non-congruence modular curves by considering so-called Teichmueller level structures on the fundamental groups of punctured elliptic curves.
Mon, 15 Feb 2016
16:30
C1

Partition regularity of $x+y=z^2$ over $\mathbb{Z}/p\mathbb{Z}$

Sofia Lindqvist
((Oxford University))
Abstract

Consider the following question. Given a $k$-colouring of the positive integers, must there exist a solution to $x+y=z^2$ with $x,y,z$ all the same colour (and not all equal to 2)? Using $10$ colours a counterexample can be given to show that the answer is "no". If one instead asks the same question over $\mathbb{Z}/p\mathbb{Z}$ for some prime $p$, the answer turns out to be "yes", provided $p$ is large enough in terms of the number of colours used.  I will talk about how to prove this using techniques developed by Ben Green and Tom Sanders. The main ingredients are a regularity lemma, a counting lemma and a Ramsey lemma.

Mon, 08 Feb 2016
16:30
C1

The degree zero part of the motivic polylogarithm and the Deligne-Beilinson cohomology

Danny Scarponi
(Univ.Toulouse)
Abstract

Last year, G. Kings and D. Rossler related the degree zero part of the polylogarithm
on abelian schemes pol^0 with another object previously defined by V. Maillot and D.
Rossler. More precisely, they proved that the canonical class of currents constructed
by Maillot and Rossler provides us with the realization of pol^0 in analytic Deligne
cohomology.
I will show that, adding some properness conditions, it is possible to give a
refinement of Kings and Rossler’s result involving Deligne-Beilinson cohomology
instead of analytic Deligne cohomology.

 

Mon, 01 Feb 2016
16:30
C1

Linear (in)equalities in primes

Aled Walker
((Oxford University))
Abstract

Many theorems and conjectures in prime number theory are equivalent to finding solutions to certain linear equations in primes -- witness Goldbach's conjecture, the twin prime conjecture, Vinogradov's theorem, finding k-term arithmetic progressions, etcetera. Classically these problems were attacked using Fourier analysis -- the 'circle' method -- which yielded some success, provided that the number of variables was sufficiently large. More recently, a long research programme of Ben Green and Terence Tao introduced two deep and wide-ranging techniques -- so-called 'higher order Fourier analysis' and the 'transference principle' -- which reduces the number of required variables dramatically. In particular, these methods give an asymptotic formula for the number of k-term arithmetic progressions of primes up to X. In this talk we will give a brief survey of these techniques, and describe new work of the speaker, partially ongoing, which applies the Green-Tao machinery to count prime solutions to certain linear inequalities in primes -- a 'higher order Davenport-Heilbronn method'. 

Mon, 25 Jan 2016
16:30
C1

Iterating the algebraic étale-Brauer obstruction

Francesca Balestrieri
((Oxford University))
Abstract

A question by Poonen asks whether iterating the étale-Brauer set can give a finer obstruction set. We tackle the algebraic version of Poonen's question and give, in many cases, a negative answer.

Mon, 30 Nov 2015

16:00 - 17:00
C2

TBA

Simon Rydin Myerson
(Oxford)