1,004 research outputs found
Noneuclidean Tessellations and their relation to Reggie Trajectories
The coefficients in the confluent hypergeometric equation specify the Regge
trajectories and the degeneracy of the angular momentum states. Bound states
are associated with real angular momenta while resonances are characterized by
complex angular momenta. With a centrifugal potential, the half-plane is
tessellated by crescents. The addition of an electrostatic potential converts
it into a hydrogen atom, and the crescents into triangles which may have
complex conjugate angles; the angle through which a rotation takes place is
accompanied by a stretching. Rather than studying the properties of the wave
functions themselves, we study their symmetry groups. A complex angle indicates
that the group contains loxodromic elements. Since the domain of such groups is
not the disc, hyperbolic plane geometry cannot be used. Rather, the theory of
the isometric circle is adapted since it treats all groups symmetrically. The
pairing of circles and their inverses is likened to pairing particles with
their antiparticles which then go one to produce nested circles, or a
proliferation of particles. A corollary to Laguerre's theorem, which states
that the euclidean angle is represented by a pure imaginary projective
invariant, represents the imaginary angle in the form of a real projective
invariant.Comment: 27 pages, 4 figure
Inversion of the Momenta Doppler Transform in two dimensions
We introduce an analytic method which stably reconstructs both components of
a (sufficiently) smooth, real valued, vector field compactly supported in the
plane from knowledge of its Doppler transform and its first moment Doppler
transform. The method of proof is constructive. Numerical inversion results
indicate robustness of the method.Comment: 18 pages, 12 figure
Histogram Tomography
In many tomographic imaging problems the data consist of integrals along
lines or curves. Increasingly we encounter "rich tomography" problems where the
quantity imaged is higher dimensional than a scalar per voxel, including
vectors tensors and functions. The data can also be higher dimensional and in
many cases consists of a one or two dimensional spectrum for each ray. In many
such cases the data contain not just integrals along rays but the distribution
of values along the ray. If this is discretized into bins we can think of this
as a histogram. In this paper we introduce the concept of "histogram
tomography". For scalar problems with histogram data this holds the possibility
of reconstruction with fewer rays. In vector and tensor problems it holds the
promise of reconstruction of images that are in the null space of related
integral transforms. For scalar histogram tomography problems we show how bins
in the histogram correspond to reconstructing level sets of function, while
moments of the distribution are the x-ray transform of powers of the unknown
function. In the vector case we give a reconstruction procedure for potential
components of the field. We demonstrate how the histogram longitudinal ray
transform data can be extracted from Bragg edge neutron spectral data and
hence, using moments, a non-linear system of partial differential equations
derived for the strain tensor. In x-ray diffraction tomography of strain the
transverse ray transform can be deduced from the diffraction pattern the full
histogram transverse ray transform cannot. We give an explicit example of
distributions of strain along a line that produce the same diffraction pattern,
and characterize the null space of the relevant transform.Comment: Small corrections from last versio
Consistent Inversion of Noisy Non-Abelian X-Ray Transforms
For a simple surface, the non-linear statistical inverse problem of
recovering a matrix field from discrete, noisy
measurements of the -valued scattering data of a solution of a
matrix ODE is considered (). Injectivity of the map was established by [Paternain, Salo, Uhlmann; Geom.Funct.Anal. 2012].
A statistical algorithm for the solution of this inverse problem based on
Gaussian process priors is proposed, and it is shown how it can be implemented
by infinite-dimensional MCMC methods. It is further shown that as the number
of measurements of point-evaluations of increases, the statistical
error in the recovery of converges to zero in -distance at a
rate that is algebraic in , and approaches for smooth matrix
fields . The proof relies, among other things, on a new stability
estimate for the inverse map .
Key applications of our results are discussed in the case to
polarimetric neutron tomography, see [Desai et al., Nature Sc.Rep. 2018] and
[Hilger et al., Nature Comm. 2018
Analytic Inversion of a Conical Radon Transform Arising in Application of Compton Cameras on the Cylinder
Single photon emission computed tomography (SPECT) is a well-established clinical tool for functional imaging. A limitation of current SPECT systems is the use of mechanical collimation, where only a small fraction of the emitted photons are actually used for image reconstruction. This results in a large noise level and finally in a limited spatial resolution. In order to decrease the noise level and to increase the imaging resolution, Compton cameras have been proposed as an alternative to mechanical collimators. Image reconstruction in SPECT with Compton cameras yields the problem of recovering a marker distribution from integrals over conical surfaces. Due to this and other applications, such conical Radon transforms recently got significant attention. In the current paper we consider the case where the cones of integration have vertices on a circular cylinder and axis pointing to the symmetry axis of the cylinder. Our setup does not use all emitted photons but a much larger fraction than systems based on mechanical collimation. Further, it may be simpler to be fabricated than a Compton camera system collecting full five-dimensional data. As main theoretical results in this paper we derive analytic reconstruction methods for the considered transform. We also investigate the V-line transform with vertices on a circle and symmetry axis orthogonal to the circle, which arises in the special case where the absorber distribution is located in a horizontal plane
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