Wed, 13 Mar 2013

14:00 - 15:00
OCCAM Common Room (RI2.28)

Exact solutions to the total generalised variation minimisation problem

Konstantinos Papafitsoros
(University of Cambridge)
Abstract

********** PLEASE NOTE THE SPECIAL TIME **********

Total generalised variation (TGV) was introduced by Bredies et al. as a high quality regulariser for variational problems arising in mathematical image processing like denoising and deblurring. The main advantage over the classical total variation regularisation is the elimination of the undesirable stairscasing effect. In this talk we will give a small introduction to TGV and provide some properties of the exact solutions to the L^{2}-TGV model in the one dimensional case.

Mon, 04 Mar 2013

14:15 - 15:15
Oxford-Man Institute

Bond Percolation on Isoradial Graphs

IOAN MANOLESCU
(University of Cambridge)
Abstract
The star-triangle transformation is used to obtain an equivalence extending over a set bond percolation models on isoradial graphs. Amongst the consequences are box-crossing (RSW) inequalities and the universality of alternating arms exponents (assuming they exist) for such models, under some conditions. In particular this implies criticality for these models.

(joint with Geoffrey Grimmett)

Mon, 28 May 2012

12:00 - 13:00
L3

Instanton - a window into physics of M5-branes

Sungjay Lee
(University of Cambridge)
Abstract
Instantons and W-bosons in 5d N=2 Yang-Mills theory arise from a circle

compactification of the 6d (2,0) theory as Kaluza-Klein modes and winding

self-dual strings, respectively. We study an index which counts BPS

instantons with electric charges in Coulomb and symmetric phases. We first

prove the existence of unique threshold bound state of U(1) instantons for

any instanton number. By studying SU(N) self-dual strings in the Coulomb

phase, we find novel momentum-carrying degrees on the worldsheet. The total

number of these degrees equals the anomaly coefficient of SU(N) (2,0) theory.

We finally propose that our index can be used to study the symmetric phase of

this theory, and provide an interpretation as the superconformal index of the

sigma model on instanton moduli space. 

Tue, 29 May 2012
12:00

Solitons from geometry.

Dr M Dunajski
(University of Cambridge)
Abstract
Solitons are localised non-singular lumps of energy which describe particles non perturbatively. Finding the solitons usually involves solving nonlinear differential equations, but I shall show that in some cases the solitons emerge directly from the underlying space-time geometry: certain abelian vortices arise from surfaces of constant mean curvature in Minkowski space, and skyrmions can be constructed from the holonomy of gravitational instantons.
Mon, 20 Feb 2012

12:00 - 13:00
L3

M-theory dualities and generalised geometry

Hadi Godazgar
(University of Cambridge)
Abstract
In this talk we will review M-theory dualities and recent attempts to make these dualities manifest in eleven-dimensional supergravity. We will review the work of Berman and Perry and then outline a prescription, called non-linear realisation, for making larger duality symmetries manifest. Finally, we will explain how the local symmetries are described by generalised geometry, which leads to a duality-covariant constraint that allows one to reduce from generalised space to physical space.
Thu, 25 May 2000

14:00 - 15:00
Comlab

Self-scaled barriers for semidefinite programming

Dr Raphael Hauser
(University of Cambridge)
Abstract
I am going to show that all self-scaled barriers for the

cone of symmetric positive semidefinite matrices are of the form

$X\mapsto -c_1\ln\det X +c_0$ for some constants $c_1$ > $0,c_0 \in$ \RN.

Equivalently one could state say that all such functions may be

obtained via a homothetic transformation of the universal barrier

functional for this cone. The result shows that there is a certain

degree of redundancy in the axiomatic theory of self-scaled barriers,

and hence that certain aspects of this theory can be simplified. All

relevant concepts will be defined. In particular I am going to give

a short introduction to the notion of self-concordance and the

intuitive ideas that motivate its definition.

Thu, 07 Jun 2001

14:00 - 15:00
Rutherford Appleton Laboratory, nr Didcot

Some properties of thin plate spline interpolation

Prof Mike J D Powell
(University of Cambridge)
Abstract
Let the thin plate spline radial basis function method be applied to

interpolate values of a smooth function $f(x)$, $x \!\in\! {\cal R}^d$.

It is known that, if the data are the values $f(jh)$, $j \in {\cal Z}^d$,

where $h$ is the spacing between data points and ${\cal Z}^d$ is the

set of points in $d$ dimensions with integer coordinates, then the

accuracy of the interpolant is of magnitude $h^{d+2}$. This beautiful

result, due to Buhmann, will be explained briefly. We will also survey

some recent findings of Bejancu on Lagrange functions in two dimensions

when interpolating at the integer points of the half-plane ${\cal Z}^2

\cap \{ x : x_2 \!\geq\! 0 \}$. Most of our attention, however, will

be given to the current research of the author on interpolation in one

dimension at the points $h {\cal Z} \cap [0,1]$, the purpose of the work

being to establish theoretically the apparent deterioration in accuracy

at the ends of the range from ${\cal O} ( h^3 )$ to ${\cal O} ( h^{3/2}

)$ that has been observed in practice. The analysis includes a study of

the Lagrange functions of the semi-infinite grid ${\cal Z} \cap \{ x :

x \!\geq\! 0 \}$ in one dimension.

Thu, 28 Nov 2002

14:00 - 15:00
Comlab

On the convergence of interior point methods for linear programming

Dr Coralia Cartis
(University of Cambridge)
Abstract
Long-step primal-dual path-following algorithms constitute the

framework of practical interior point methods for

solving linear programming problems. We consider

such an algorithm and a second order variant of it.

We address the problem of the convergence of

the sequences of iterates generated by the two algorithms

to the analytic centre of the optimal primal-dual set.

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