536 research outputs found
The degree of approximation by Hausdorff means of a conjugate Fourier series
The purpose of this paper is to analyze the degree of approximation of a function that is a conjugate of a function belonging to the Lipschitz class by Hausdorff means of a conjugate series of the Fourier series
Multivariate Davenport series
We consider series of the form , where
and is the sawtooth function. They are the natural multivariate
extension of Davenport series. Their global (Sobolev) and pointwise regularity
are studied and their multifractal properties are derived. Finally, we list
some open problems which concern the study of these series.Comment: 43 page
Spectral Triples on Carnot Manifolds
We analyze whether one can construct a spectral triple for a Carnot manifold
, which detects its Carnot-Carath\'{e}odory metric and its graded dimension.
Therefore we construct self-adjoint horizontal Dirac operators and show
that each horizontal Dirac operator detects the metric via Connes' formula, but
we also find that in no case these operators are hypoelliptic, which means they
fail to have a compact resolvent.
First we consider an example on compact Carnot nilmanifolds in detail, where
we present a construction for a horizontal Dirac operator arising via pullback
from the Dirac operator on the torus. Following an approach by Christian B\"ar
to decompose the horizontal Clifford bundle, we detect that this operator has
an infinite dimensional kernel. But in spite of this, in the case of Heisenberg
nilmanifolds we will be able to discover the graded dimension from the
asymptotic behavior of the eigenvalues of this horizontal Dirac operator.
Afterwards we turn to the general case, showing that any horizontal Dirac
operator fails to be hypoelliptic. Doing this, we develop a criterion from
which hypoellipticity of certain graded differential operators can be excluded
by considering the situation on a Heisenberg manifold, for which a complete
characterization of hypoellipticity in known by the Rockland condition.
Finally, we show how spectral triples can be constructed from horizontal
Laplacians via the Heisenberg pseudodifferential calculus developed by Richard
Beals and Peter Greiner. We suggest a few of these constructions, and discuss
under which assumptions it may be possible to get an equivalent metric to the
Carnot-Carath\'{e}odory metric from these operators. In addition, we mention a
formula by which the Carnot-Carath\'{e}odory metric can be detected from
arbitrary horizontal Laplacians.Comment: This is my PhD thesis, written at Leibniz Universitaet Hannover and
submitted on May 14th, 2013. Email: [email protected]
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