Past events

To numerical analysts and other applied mathematicians Jacobians and Hessians

are matrices, i.e. rectangular arrays of numbers or algebraic expressions.

Possibly taking account of their sparsity such arrays are frequently passed

into library routines for performing various computational tasks.

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A central goal of an activity called automatic differentiation has been the

accumulation of all nonzero entries from elementary partial derivatives

according to some variant of the chainrule. The elementary partials arise

in the user-supplied procedure for evaluating the underlying vector- or

scalar-valued function at a given argument.

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We observe here that in this process a certain kind of structure that we

call "Jacobian scarcity" might be lost. This loss will make the subsequent

calculation of Jacobian vector-products unnecessarily expensive.

Instead we advocate the representation of the Jacobian as a linear computational

graph of minimal complexity. Many theoretical and practical questions remain unresolved.

Condition numbers are a central concept in numerical analysis.

They provide a natural parameter for studying the behavior of

algorithms, as well as sensitivity and geometric properties of a problem.

The condition number of a problem instance is usually a measure

of the distance to the set of ill-posed instances. For instance, the

classical Eckart and Young identity characterizes the condition

number of a square matrix as the reciprocal of its relative distance

to singularity.

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We present concepts of conditioning for optimization problems and

for more general variational problems. We show that the Eckart and

Young identity has natural extension to much wider contexts. We also

discuss conditioning under the presence of block-structure, such as

that determined by a sparsity pattern. The latter has interesting

connections with the mu-number in robust control and with the sign-real

spectral radius.

I present a calculation of the moduli Kahler potential for M-theory

on a G_2 manifold in a large radius approximation. The result is used to

analyze moduli dynamics and moduli stabilization in the context of the

associated four-dimensional effective theory.

Take any region omega and let function u defined inside omega be the

distance from the boundary, u solves the iconal equation \lt|Du\rt|=1 with

boundary condition zero. Functional u is also conjectured (in some cases

proved) to be the "limiting minimiser" of various functionals that

arise models of blistering and micro magnetics. The precise formulation of

these problems involves the notion of gamma convergence. The Aviles Giga

functional is a natural "second order" generalisation of the Cahn

Hilliard model which was one of the early success of the theory of gamma

convergence. These problems turn out to be surprisingly rich with connections

to a number of areas of pdes. We will survey some of the more elementary

results, describe in detail of one main problems in field and state some

partial results.