159 research outputs found
Stationary Kernels and Gaussian Processes on Lie Groups and their Homogeneous Spaces II: non-compact symmetric spaces
Gaussian processes are arguably the most important class of spatiotemporal
models within machine learning. They encode prior information about the modeled
function and can be used for exact or approximate Bayesian learning. In many
applications, particularly in physical sciences and engineering, but also in
areas such as geostatistics and neuroscience, invariance to symmetries is one
of the most fundamental forms of prior information one can consider. The
invariance of a Gaussian process' covariance to such symmetries gives rise to
the most natural generalization of the concept of stationarity to such spaces.
In this work, we develop constructive and practical techniques for building
stationary Gaussian processes on a very large class of non-Euclidean spaces
arising in the context of symmetries. Our techniques make it possible to (i)
calculate covariance kernels and (ii) sample from prior and posterior Gaussian
processes defined on such spaces, both in a practical manner. This work is
split into two parts, each involving different technical considerations: part I
studies compact spaces, while part II studies non-compact spaces possessing
certain structure. Our contributions make the non-Euclidean Gaussian process
models we study compatible with well-understood computational techniques
available in standard Gaussian process software packages, thereby making them
accessible to practitioners
Kernel density estimation on the Siegel space applied to radar processing
Main techniques of probability density estimation on Riemannian manifolds are reviewed in the case of the Siegel space. For computational reasons we chose to focus on the kernel density estimation. The main result of the paper is the expression of Pelletier's kernel density estimator. The method is applied to density estimation of reflection coefficients from radar observations
Iwasawa N=8 Attractors
Starting from the symplectic construction of the Lie algebra e_7(7) due to
Adams, we consider an Iwasawa parametrization of the coset E_7(7)/SU(8), which
is the scalar manifold of N=8, d=4 supergravity. Our approach, and the manifest
off-shell symmetry of the resulting symplectic frame, is determined by a
non-compact Cartan subalgebra of the maximal subgroup SL(8,R) of E_7(7). In
absence of gauging, we utilize the explicit expression of the Lie algebra to
study the origin of E_7(7)/SU(8) as scalar configuration of a 1/8-BPS extremal
black hole attractor. In such a framework, we highlight the action of a U(1)
symmetry spanning the dyonic 1/8-BPS attractors. Within a suitable
supersymmetry truncation allowing for the embedding of the Reissner-Nordstrom
black hole, this U(1) is interpreted as nothing but the global R-symmetry of
pure N=2 supergravity. Moreover, we find that the above mentioned U(1) symmetry
is broken down to a discrete subgroup Z_4, implying that all 1/8-BPS Iwasawa
attractors are non-dyonic near the origin of the scalar manifold. We can trace
this phenomenon back to the fact that the Cartan subalgebra of SL(8,R) used in
our construction endows the symplectic frame with a manifest off-shell
covariance which is smaller than SL(8,R) itself. Thus, the consistence of the
Adams-Iwasawa symplectic basis with the action of the U(1) symmetry gives rise
to the observed Z_4 residual non-dyonic symmetry.Comment: 1+26 page
Peripheral separability and cusps of arithmetic hyperbolic orbifolds
For X = R, C, or H it is well known that cusp cross-sections of finite volume
X-hyperbolic (n+1)-orbifolds are flat n-orbifolds or almost flat orbifolds
modelled on the (2n+1)-dimensional Heisenberg group N_{2n+1} or the
(4n+3)-dimensional quaternionic Heisenberg group N_{4n+3}(H). We give a
necessary and sufficient condition for such manifolds to be diffeomorphic to a
cusp cross-section of an arithmetic X-hyperbolic (n+1)-orbifold. A principal
tool in the proof of this classification theorem is a subgroup separability
result which may be of independent interest.Comment: Published by Algebraic and Geometric Topology at
http://www.maths.warwick.ac.uk/agt/AGTVol4/agt-4-32.abs.htm
Spherical representations of Lie supergroups
The classical Cartan-Helgason theorem characterises finite-dimensional
spherical representations of reductive Lie groups in terms of their highest
weights. We generalise the theorem to the case of a reductive symmetric
supergroup pair of even type. Along the way, we compute the
Harish-Chandra -function of the symmetric superspace . By way of an
application, we show that all spherical representations are self-dual in type
AIII|AIII.Comment: 37 pages; title changed; substantially revised version; accepted for
publication, J. Func. Anal. (2014
Cusps of arithmetic orbifolds
This thesis investigates cusp cross-sections of arithmetic real, complex, and
quaternionic hyperbolic --orbifolds. We give a smooth classification of
these submanifolds and analyze their induced geometry. One of the primary tools
is a new subgroup separability result for general arithmetic lattices.Comment: 76 pages; Ph.D. thesi
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