173 research outputs found
Convergence of linearized and adjoint approximations for discontinuous solutions of conservation laws. Part 2: adjoint approximations and extensions
This paper continues the convergence analysis in [M. Giles and S. Ulbrich, SIAM J. Numer. Anal., 48 (2010), pp. 882–904] of discrete approximations to the linearized and adjoint equations arising from an unsteady one-dimensional hyperbolic equation with a convex flux function. We consider a simple modified Lax–Friedrichs discretization on a uniform grid, and a key point is that the numerical smoothing increases the number of points across the nonlinear discontinuity as the grid is refined. It is proved that there is convergence in the discrete approximation of linearized output functionals even for Dirac initial perturbations and pointwise convergence almost everywhere for the solution of the adjoint discrete equations. In particular, the adjoint approximation converges to the correct uniform value in the region in which characteristics propagate into the discontinuity. Moreover, it is shown that the results of [M. Giles and S. Ulbrich, SIAM J. Numer. Anal., 48 (2010), pp. 882–904] and the present paper hold also for quite general nonlinear initial data which contain multiple shocks and for which shocks form at a later time and/or merge
Multilevel Monte Carlo methods
The author's presentation of multilevel Monte Carlo path simulation at the
MCQMC 2006 conference stimulated a lot of research into multilevel Monte Carlo
methods. This paper reviews the progress since then, emphasising the
simplicity, flexibility and generality of the multilevel Monte Carlo approach.
It also offers a few original ideas and suggests areas for future research
An integral method for solving nonlinear eigenvalue problems
We propose a numerical method for computing all eigenvalues (and the
corresponding eigenvectors) of a nonlinear holomorphic eigenvalue problem that
lie within a given contour in the complex plane. The method uses complex
integrals of the resolvent operator, applied to at least column vectors,
where is the number of eigenvalues inside the contour. The theorem of
Keldysh is employed to show that the original nonlinear eigenvalue problem
reduces to a linear eigenvalue problem of dimension .
No initial approximations of eigenvalues and eigenvectors are needed. The
method is particularly suitable for moderately large eigenvalue problems where
is much smaller than the matrix dimension. We also give an extension of the
method to the case where is larger than the matrix dimension. The
quadrature errors caused by the trapezoid sum are discussed for the case of
analytic closed contours. Using well known techniques it is shown that the
error decays exponentially with an exponent given by the product of the number
of quadrature points and the minimal distance of the eigenvalues to the
contour
Variable response of nirK and nirS containing denitrifier communities to long term pH manipulation and cultivation
Denitrification is a key process responsible for the majority of soil nitrous oxide (N2O) emissions but the influences of pH and cultivation on the soil denitrifier community remain poorly understood. We hypothesised that the abundance and community structure of the total bacterial community and bacterial denitrifiers would be pH sensitive and that nirK and nirS containing denitrifiers would differ in their responses to change in pH and cultivation. We investigated the effect of long-term pH adjusted soils (ranging from pH 4.2 to pH 6.6) under different lengths of grass cultivation (one, two and three years of ley grass) on the general bacterial and denitrifier functional communities using 16S rRNA, nirK and nirS genes as markers. Denitrifier abundance increased with pH, and at pH below 4.7 there was a greater loss in nirS abundance per unit drop in pH than soils above this threshold pH. All community structures responded to changes in soil pH whilst cultivation only influenced the community structure of nirK. These differences in denitrifier responses highlight the importance of considering both nirK and nirS gene markers for estimating denitrifier activity. Identifying such thresholds in response of the microbial community to changes in pH is essential to understanding impacts of management or environmental change
Tracing the String: BMN correspondence at Finite J^2/N
Employing the string bit formalism of hep-th/0209215, we identify the basis
transformation that relates BMN operators in N=4 gauge theory to string states
in the dual string field theory at finite g_2=J^2/N. In this basis, the
supercharge truncates at linear order in g_2, and the mixing amplitude between
1 and 2-string states precisely matches with the (corrected) answer of
hep-th/0206073 for the 3-string amplitude in light-cone string field theory.
Supersymmetry then predicts the order g_2^2 contact term in the string bit
Hamiltonian. The resulting leading order mass renormalization of string states
agrees with the recently computed shift in conformal dimension of BMN operators
in the gauge theory.Comment: 11 pages, 1 figur
An introduction to multilevel Monte Carlo for option valuation
Monte Carlo is a simple and flexible tool that is widely used in
computational finance. In this context, it is common for the quantity of
interest to be the expected value of a random variable defined via a stochastic
differential equation. In 2008, Giles proposed a remarkable improvement to the
approach of discretizing with a numerical method and applying standard Monte
Carlo. His multilevel Monte Carlo method offers an order of speed up given by
the inverse of epsilon, where epsilon is the required accuracy. So computations
can run 100 times more quickly when two digits of accuracy are required. The
multilevel philosophy has since been adopted by a range of researchers and a
wealth of practically significant results has arisen, most of which have yet to
make their way into the expository literature.
In this work, we give a brief, accessible, introduction to multilevel Monte
Carlo and summarize recent results applicable to the task of option evaluation.Comment: Submitted to International Journal of Computer Mathematics, special
issue on Computational Methods in Financ
- …