Tue, 07 May 2019
12:00
L4

Single-valued integration and superstring amplitudes

Clement Dupont
(Montpellier)
Abstract

The classical theory of integration concern integrals of differential forms over domains of integration. In geometric terms, this corresponds to a canonical pairing between de Rham cohomology and singular homology. For varieties defined over the reals, one can make use of complex conjugation to define a real-valued pairing between de Rham cohomology and its dual, de Rham homology. The corresponding theory of integration, that we call single-valued integration, pairs a differential form with a `dual differential form’. We will explain how single-valued periods are computed and give an application to superstring amplitudes in genus zero. This is joint work with Francis Brown.
 

Tue, 07 May 2019

14:00 - 14:30
L5

Sharp error bounds for Ritz vectors and approximate singular vectors

Yuji Nakatsukasa
(Oxford)
Abstract

We derive sharp bounds for the accuracy of approximate eigenvectors (Ritz vectors) obtained by the Rayleigh-Ritz process for symmetric eigenvalue problems. Using information that is available or easy to estimate, our bounds improve the classical Davis-Kahan sin-theta theorem by a factor that can be arbitrarily large, and can give nontrivial information even when the sin-theta theorem suggests that a Ritz vector might have no accuracy at all. We also present extensions in three directions, deriving error bounds for invariant subspaces, singular vectors and subspaces computed by a (Petrov-Galerkin) projection SVD method, and eigenvectors of self-adjoint operators on a Hilbert space.

Tue, 18 Jun 2019

14:30 - 15:30
L6

Enumerating graphs and other discrete structures by degree sequence

Anita Liebenau
Further Information

How many d-regular graphs are there on n vertices? What is the probability that G(n,p) has a specific given degree sequence? 

Asymptotic formulae for the first question are known when d=o(\sqrt(n)) and when d= \Omega(n). More generally, asymptotic formulae are known for 
the number of graphs with a given degree sequence, for a range of degree sequences that is wide enough to deduce asymptotic formulae for the second 
question for p =o(1/o(\sqrt(n))) and p = Theta(1).  

McKay and Wormald showed that the formulae for the sparse case and the 
dense case can be cast into a common form, and then conjectured in 1990 and 1997 that the same formulae should hold for the gap range. A particular consequence of both conjectures is that the degree sequence of the random graph G(n,p) can be approximated by a sequence of n independent 
binomial variables Bin(n − 1, p'). 

In 2017, Nick Wormald and I proved both conjectures. In this talk I will describe the problem and survey some of the earlier methods to showcase the differences to our new methods. I shall also report on enumeration results of other discrete structures, such as bipartite graphs and hypergraphs, that are obtained by adjusting our methods to those settings. 

Tue, 04 Jun 2019

14:30 - 15:30
L6

Non-concentration of the chromatic number of G(n, 1/2)

Annika Heckel
Further Information

A classic result of Shamir and Spencer states that for any function $p=p(n)$, the chromatic number of $G(n,p)$ is whp concentrated on a sequence of intervals of length about $\sqrt{n}$. For $p<n^{-\frac{1}{2} -\epsilon}$, much more is known: here, the chromatic number is concentrated on two consecutive values.

Until now, there have been no non-trivial cases where $\chi(G(n,p))$ is known not to be extremely narrowly concentrated. In 2004, Bollob\'as asked for any such examples, particularly in the case $p=\frac{1}{2}$, in a paper in the problem section of CPC. 

In this talk, we show that the chromatic number of $G(n, 1/2)$ is not whp concentrated on $n^{\frac{1}{4}-\epsilon}$ values

Tue, 07 May 2019

14:30 - 15:30
L6

Around Brooks' theorem

Marthe Bonamy
Further Information

In this talk, we will discuss various results around Brooks' theorem: a graph has chromatic number at most its maximum degree, unless it is a clique or an odd cycle. We will consider stronger variants and local versions, as well as the structure of the solution space of all corresponding colorings.

Tue, 30 Apr 2019

14:30 - 15:30
L6

Erdős-Rothschild problem for five and six colours

Jozef Skokan
Further Information

Given positive integers n,r,k, the Erdős-Rothschild problem asks to determine the largest number of r-edge-colourings without monochromatic k-cliques a graph on n vertices can have. In the case of triangles, i.e. when k=3, the solution is known for r = 2,3,4. We investigate the problem for five and six colours.

Tue, 30 Apr 2019

14:30 - 15:00
L3

Exponential integrators for stiff PDEs

Lloyd Nick Trefethen
(Oxford)
Abstract

Many time-dependent PDEs -- KdV, Burgers, Gray-Scott, Allen-Cahn, Navier-Stokes and many others -- combine a higher-order linear term with a lower-order nonlinear term.  This talk will review the method of exponential integrators for solving such problems with better than 2nd-order accuracy in time.

Mon, 08 Jul 2019 09:00 -
Wed, 10 Jul 2019 17:00
L2

NetMob 2019

NetMob 2019
(University of Oxford and others)
Further Information

NetMob is the primary conference in the analysis of mobile phone datasets in social, urban, societal and industrial problems. Previous editions in Boston and Milano brought together more than 250 researchers, practitioners and decision-makers from more than 140 institutions and 30 countries.

The 2019 edition of NetMob will take place at the Mathematical Institute of Oxford University in a conference format similar to that of the previous editions: one track of short contributed talks, a simplified submission procedure, no proceedings (except for a book of abstracts), and the possibility to present recent results or results submitted elsewhere.

For more information and how to join click here

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