193 research outputs found
Double Greedy Algorithms: Reduced Basis Methods for Transport Dominated Problems
The central objective of this paper is to develop reduced basis methods for
parameter dependent transport dominated problems that are rigorously proven to
exhibit rate-optimal performance when compared with the Kolmogorov -widths
of the solution sets. The central ingredient is the construction of
computationally feasible "tight" surrogates which in turn are based on deriving
a suitable well-conditioned variational formulation for the parameter dependent
problem. The theoretical results are illustrated by numerical experiments for
convection-diffusion and pure transport equations. In particular, the latter
example sheds some light on the smoothness of the dependence of the solutions
on the parameters
Computing generators of the unit group of an integral abelian group ring
We describe an algorithm for obtaining generators of the unit group of the
integral group ring ZG of a finite abelian group G. We used our implementation
in Magma of this algorithm to compute the unit groups of ZG for G of order up
to 110. In particular for those cases we obtained the index of the group of
Hoechsmann units in the full unit group. At the end of the paper we describe an
algorithm for the more general problem of finding generators of an arithmetic
group corresponding to a diagonalizable algebraic group
Fast recognition of alternating groups of unknown degree
We present a constructive recognition algorithm to decide whether a given
black-box group is isomorphic to an alternating or a symmetric group without
prior knowledge of the degree. This eliminates the major gap in known
algorithms, as they require the degree as additional input.
Our methods are probabilistic and rely on results about proportions of
elements with certain properties in alternating and symmetric groups. These
results are of independent interest; for instance, we establish a lower bound
for the proportion of involutions with small support.Comment: 31 pages, submitted to Journal of Algebr
The virtual Haken conjecture: Experiments and examples
A 3-manifold is Haken if it contains a topologically essential surface. The
Virtual Haken Conjecture says that every irreducible 3-manifold with infinite
fundamental group has a finite cover which is Haken. Here, we discuss two
interrelated topics concerning this conjecture.
First, we describe computer experiments which give strong evidence that the
Virtual Haken Conjecture is true for hyperbolic 3-manifolds. We took the
complete Hodgson-Weeks census of 10,986 small-volume closed hyperbolic
3-manifolds, and for each of them found finite covers which are Haken. There
are interesting and unexplained patterns in the data which may lead to a better
understanding of this problem.
Second, we discuss a method for transferring the virtual Haken property under
Dehn filling. In particular, we show that if a 3-manifold with torus boundary
has a Seifert fibered Dehn filling with hyperbolic base orbifold, then most of
the Dehn filled manifolds are virtually Haken. We use this to show that every
non-trivial Dehn surgery on the figure-8 knot is virtually Haken.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol7/paper12.abs.htm
On cohomology of crystallographic groups with cyclic holonomy of split type
We disprove a conjecture stating that the integral cohomology of any
crystallographic group Z^n \rtimes Z_m is given by the cohomology of Z_m with
coefficients in the cohomology of the group Z^n, by providing a complete list
of counterexamples up to dimension 5. We also find a counterexample with odd
order holonomy, m=9, in dimension 8 and finish the computations of the
cohomology of 6-dimensional crystallographic groups arising as orbifold
fundamental groups of certain Calabi-Yau toroidal orbifolds.Comment: 13 page
The Effect of Commitment, Communication and Participation on Resistance to Change: The Role of Change Readiness
There is growing concern surrounding the effect of resistance to change on organisational change success. The main purpose of the present research was to clarify the relationships between important contextual variables highlighted in the literature, and resistance to change and readiness for change. Participants completed an online survey while their organisation was about to or already going through a change. As predicted, the results show the importance that participant perception of the adequacy of communication had on resistance to change and that this relationship was mediated by the readiness dimension of viewing the change as appropriate. The relationship between other contextual variables of perceived opportunities for participation and affective organisational commitment, and resistance to change were not found to be mediated by readiness for change dimensions. Affective commitment however, showed a direct negative relationship with resistance to change. These findings highlight the importance of a planned approached to change-related communications, and its potential to reduce resistance to change by effectively creating readiness for change in an organisation. Implications of these results and suggestions for future research are discussed
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