575 research outputs found
The Geometry of Almost Einstein (2,3,5) Distributions
We analyze the classic problem of existence of Einstein metrics in a given
conformal structure for the class of conformal structures inducedf Nurowski's
construction by (oriented) (2,3,5) distributions. We characterize in two ways
such conformal structures that admit an almost Einstein scale: First, they are
precisely the oriented conformal structures that are induced by at
least two distinct oriented (2,3,5) distributions; in this case there is a
1-parameter family of such distributions that induce . Second, they
are characterized by the existence of a holonomy reduction to ,
, or a particular semidirect product , according to the sign of the Einstein constant of the
corresponding metric. Via the curved orbit decomposition formalism such a
reduction partitions the underlying manifold into several submanifolds and
endows each ith a geometric structure. This establishes novel links between
(2,3,5) distributions and many other geometries - several classical geometries
among them - including: Sasaki-Einstein geometry and its paracomplex and
null-complex analogues in dimension 5; K\"ahler-Einstein geometry and its
paracomplex and null-complex analogues, Fefferman Lorentzian conformal
structures, and para-Fefferman neutral conformal structures in dimension 4; CR
geometry and the point geometry of second-order ordinary differential equations
in dimension 3; and projective geometry in dimension 2. We describe a
generalized Fefferman construction that builds from a 4-dimensional
K\"ahler-Einstein or para-K\"ahler-Einstein structure a family of (2,3,5)
distributions that induce the same (Einstein) conformal structure. We exploit
some of these links to construct new examples, establishing the existence of
nonflat almost Einstein (2,3,5) conformal structures for which the Einstein
constant is positive and negative
Shortest path embeddings of graphs on surfaces
The classical theorem of F\'{a}ry states that every planar graph can be
represented by an embedding in which every edge is represented by a straight
line segment. We consider generalizations of F\'{a}ry's theorem to surfaces
equipped with Riemannian metrics. In this setting, we require that every edge
is drawn as a shortest path between its two endpoints and we call an embedding
with this property a shortest path embedding. The main question addressed in
this paper is whether given a closed surface S, there exists a Riemannian
metric for which every topologically embeddable graph admits a shortest path
embedding. This question is also motivated by various problems regarding
crossing numbers on surfaces.
We observe that the round metrics on the sphere and the projective plane have
this property. We provide flat metrics on the torus and the Klein bottle which
also have this property.
Then we show that for the unit square flat metric on the Klein bottle there
exists a graph without shortest path embeddings. We show, moreover, that for
large g, there exist graphs G embeddable into the orientable surface of genus
g, such that with large probability a random hyperbolic metric does not admit a
shortest path embedding of G, where the probability measure is proportional to
the Weil-Petersson volume on moduli space.
Finally, we construct a hyperbolic metric on every orientable surface S of
genus g, such that every graph embeddable into S can be embedded so that every
edge is a concatenation of at most O(g) shortest paths.Comment: 22 pages, 11 figures: Version 3 is updated after comments of
reviewer
Density of Spherically-Embedded Stiefel and Grassmann Codes
The density of a code is the fraction of the coding space covered by packing
balls centered around the codewords. This paper investigates the density of
codes in the complex Stiefel and Grassmann manifolds equipped with the chordal
distance. The choice of distance enables the treatment of the manifolds as
subspaces of Euclidean hyperspheres. In this geometry, the densest packings are
not necessarily equivalent to maximum-minimum-distance codes. Computing a
code's density follows from computing: i) the normalized volume of a metric
ball and ii) the kissing radius, the radius of the largest balls one can pack
around the codewords without overlapping. First, the normalized volume of a
metric ball is evaluated by asymptotic approximations. The volume of a small
ball can be well-approximated by the volume of a locally-equivalent tangential
ball. In order to properly normalize this approximation, the precise volumes of
the manifolds induced by their spherical embedding are computed. For larger
balls, a hyperspherical cap approximation is used, which is justified by a
volume comparison theorem showing that the normalized volume of a ball in the
Stiefel or Grassmann manifold is asymptotically equal to the normalized volume
of a ball in its embedding sphere as the dimension grows to infinity. Then,
bounds on the kissing radius are derived alongside corresponding bounds on the
density. Unlike spherical codes or codes in flat spaces, the kissing radius of
Grassmann or Stiefel codes cannot be exactly determined from its minimum
distance. It is nonetheless possible to derive bounds on density as functions
of the minimum distance. Stiefel and Grassmann codes have larger density than
their image spherical codes when dimensions tend to infinity. Finally, the
bounds on density lead to refinements of the standard Hamming bounds for
Stiefel and Grassmann codes.Comment: Two-column version (24 pages, 6 figures, 4 tables). To appear in IEEE
Transactions on Information Theor
Antipodally invariant metrics for fast regression-based super-resolution
Dictionary-based super-resolution (SR) algorithms usually select dictionary atoms based on the distance or similarity metrics. Although the optimal selection of the nearest neighbors is of central importance for such methods, the impact of using proper metrics for SR has been overlooked in literature, mainly due to the vast usage of Euclidean distance. In this paper, we present a very fast regression-based algorithm, which builds on the densely populated anchored neighborhoods and sublinear search structures. We perform a study of the nature of the features commonly used for SR, observing that those features usually lie in the unitary hypersphere, where every point has a diametrically opposite one, i.e., its antipode, with same module and angle, but the opposite direction. Even though, we validate the benefits of using antipodally invariant metrics, most of the binary splits use Euclidean distance, which does not handle antipodes optimally. In order to benefit from both the worlds, we propose a simple yet effective antipodally invariant transform that can be easily included in the Euclidean distance calculation. We modify the original spherical hashing algorithm with this metric in our antipodally invariant spherical hashing scheme, obtaining the same performance as a pure antipodally invariant metric. We round up our contributions with a novel feature transform that obtains a better coarse approximation of the input image thanks to iterative backprojection. The performance of our method, which we named antipodally invariant SR, improves quality (Peak Signal to Noise Ratio) and it is faster than any other state-of-the-art method.Peer ReviewedPostprint (author's final draft
Exploring de Sitter Space and Holography
We explore aspects of the physics of de Sitter (dS) space that are relevant
to holography with a positive cosmological constant. First we display a
nonlocal map that commutes with the de Sitter isometries, transforms the
bulk-boundary propagator and solutions of free wave equations in de Sitter onto
the same quantities in Euclidean anti-de Sitter (EAdS), and takes the two
boundaries of dS to the single EAdS boundary via an antipodal identification.
Second we compute the action of scalar fields on dS as a functional of boundary
data. Third, we display a family of solutions to 3d gravity with a positive
cosmological constant in which the equal time sections are arbitrary genus
Riemann surfaces, and compute the action of these spaces as a functional of
boundary data from the Einstein gravity and Chern-Simons gravity points of
view. These studies suggest that if de Sitter space is dual to a Euclidean
conformal field theory (CFT), this theory should involve two disjoint, but
possibly entangled factors. We argue that these CFTs would be of a novel form,
with unusual hermiticity conditions relating left movers and right movers.
After exploring these conditions in a toy model, we combine our observations to
propose that a holographic dual description of de Sitter space would involve a
pure entangled state in a product of two of our unconventional CFTs associated
with the de Sitter boundaries. This state can be constructed to preserve the de
Sitter symmetries and and its decomposition in a basis appropriate to antipodal
inertial observers would lead to the thermal properties of static patch.Comment: LaTeX, v2: references adde
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