488 research outputs found
Geometric versus homotopy theoretic equivariant bordism
By results of Loeffler and Comezana, the Pontrjagin-Thom map from geometric
G-equivariant bordism to homotopy theoretic equivariant bordism is injective
for compact abelian G. If G = S^1 x ... x S^1, we prove that the associated
fixed point square is a pull back square, thus confirming a recent conjecture
of D. Sinha. This is used in order to determine the image of the
Pontrjagin-Thom map for toral G.Comment: 19 pages, minor inaccuracies removed, to appear in Math. An
Oriented Matroids -- Combinatorial Structures Underlying Loop Quantum Gravity
We analyze combinatorial structures which play a central role in determining
spectral properties of the volume operator in loop quantum gravity (LQG). These
structures encode geometrical information of the embedding of arbitrary valence
vertices of a graph in 3-dimensional Riemannian space, and can be represented
by sign strings containing relative orientations of embedded edges. We
demonstrate that these signature factors are a special representation of the
general mathematical concept of an oriented matroid. Moreover, we show that
oriented matroids can also be used to describe the topology (connectedness) of
directed graphs. Hence the mathematical methods developed for oriented matroids
can be applied to the difficult combinatorics of embedded graphs underlying the
construction of LQG. As a first application we revisit the analysis of [4-5],
and find that enumeration of all possible sign configurations used there is
equivalent to enumerating all realizable oriented matroids of rank 3, and thus
can be greatly simplified. We find that for 7-valent vertices having no
coplanar triples of edge tangents, the smallest non-zero eigenvalue of the
volume spectrum does not grow as one increases the maximum spin \jmax at the
vertex, for any orientation of the edge tangents. This indicates that, in
contrast to the area operator, considering large \jmax does not necessarily
imply large volume eigenvalues. In addition we give an outlook to possible
starting points for rewriting the combinatorics of LQG in terms of oriented
matroids.Comment: 43 pages, 26 figures, LaTeX. Version published in CQG. Typos
corrected, presentation slightly extende
Point processes and stochastic displacement fields
The effect of a stochastic displacement field on a statistically independent
point process is analyzed. Stochastic displacement fields can be divided into
two large classes: spatially correlated and uncorrelated. For both cases exact
transformation equations for the two-point correlation function and the power
spectrum of the point process are found, and a detailed study of them with
important paradigmatic examples is done. The results are general and in any
dimension. A particular attention is devoted to the kind of large scale
correlations that can be introduced by the displacement field, and to the
realizability of arbitrary ``superhomogeneous'' point processes.Comment: 17 pages, 7 figure
Diagonal and Low-Rank Matrix Decompositions, Correlation Matrices, and Ellipsoid Fitting
In this paper we establish links between, and new results for, three problems
that are not usually considered together. The first is a matrix decomposition
problem that arises in areas such as statistical modeling and signal
processing: given a matrix formed as the sum of an unknown diagonal matrix
and an unknown low rank positive semidefinite matrix, decompose into these
constituents. The second problem we consider is to determine the facial
structure of the set of correlation matrices, a convex set also known as the
elliptope. This convex body, and particularly its facial structure, plays a
role in applications from combinatorial optimization to mathematical finance.
The third problem is a basic geometric question: given points
(where ) determine whether there is a centered
ellipsoid passing \emph{exactly} through all of the points.
We show that in a precise sense these three problems are equivalent.
Furthermore we establish a simple sufficient condition on a subspace that
ensures any positive semidefinite matrix with column space can be
recovered from for any diagonal matrix using a convex
optimization-based heuristic known as minimum trace factor analysis. This
result leads to a new understanding of the structure of rank-deficient
correlation matrices and a simple condition on a set of points that ensures
there is a centered ellipsoid passing through them.Comment: 20 page
Quantum Control in the Unitary Sphere: Lambda-S1 and its Categorical Model
In a recent paper, a realizability technique has been used to give a
semantics of a quantum lambda calculus. Such a technique gives rise to an
infinite number of valid typing rules, without giving preference to any subset
of those. In this paper, we introduce a valid subset of typing rules, defining
an expressive enough quantum calculus. Then, we propose a categorical semantics
for it. Such a semantics consists of an adjunction between the category of
semi-vector spaces of value distributions (that is, linear combinations of
values in the lambda calculus), and the category of sets of value
distributions.Comment: 26 pages plus appendi
Quantum control theory and applications: A survey
This paper presents a survey on quantum control theory and applications from
a control systems perspective. Some of the basic concepts and main developments
(including open-loop control and closed-loop control) in quantum control theory
are reviewed. In the area of open-loop quantum control, the paper surveys the
notion of controllability for quantum systems and presents several control
design strategies including optimal control, Lyapunov-based methodologies,
variable structure control and quantum incoherent control. In the area of
closed-loop quantum control, the paper reviews closed-loop learning control and
several important issues related to quantum feedback control including quantum
filtering, feedback stabilization, LQG control and robust quantum control.Comment: 38 pages, invited survey paper from a control systems perspective,
some references are added, published versio
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