32,242 research outputs found
Manifold Learning in MR spectroscopy using nonlinear dimensionality reduction and unsupervised clustering
Purpose To investigate whether nonlinear dimensionality reduction improves unsupervised classification of 1H MRS brain tumor data compared with a linear method. Methods In vivo single-voxel 1H magnetic resonance spectroscopy (55 patients) and 1H magnetic resonance spectroscopy imaging (MRSI) (29 patients) data were acquired from histopathologically diagnosed gliomas. Data reduction using Laplacian eigenmaps (LE) or independent component analysis (ICA) was followed by k-means clustering or agglomerative hierarchical clustering (AHC) for unsupervised learning to assess tumor grade and for tissue type segmentation of MRSI data. Results An accuracy of 93% in classification of glioma grade II and grade IV, with 100% accuracy in distinguishing tumor and normal spectra, was obtained by LE with unsupervised clustering, but not with the combination of k-means and ICA. With 1H MRSI data, LE provided a more linear distribution of data for cluster analysis and better cluster stability than ICA. LE combined with k-means or AHC provided 91% accuracy for classifying tumor grade and 100% accuracy for identifying normal tissue voxels. Color-coded visualization of normal brain, tumor core, and infiltration regions was achieved with LE combined with AHC. Conclusion Purpose To investigate whether nonlinear dimensionality reduction improves unsupervised classification of 1H MRS brain tumor data compared with a linear method. Methods In vivo single-voxel 1H magnetic resonance spectroscopy (55 patients) and 1H magnetic resonance spectroscopy imaging (MRSI) (29 patients) data were acquired from histopathologically diagnosed gliomas. Data reduction using Laplacian eigenmaps (LE) or independent component analysis (ICA) was followed by k-means clustering or agglomerative hierarchical clustering (AHC) for unsupervised learning to assess tumor grade and for tissue type segmentation of MRSI data. Results An accuracy of 93% in classification of glioma grade II and grade IV, with 100% accuracy in distinguishing tumor and normal spectra, was obtained by LE with unsupervised clustering, but not with the combination of k-means and ICA. With 1H MRSI data, LE provided a more linear distribution of data for cluster analysis and better cluster stability than ICA. LE combined with k-means or AHC provided 91% accuracy for classifying tumor grade and 100% accuracy for identifying normal tissue voxels. Color-coded visualization of normal brain, tumor core, and infiltration regions was achieved with LE combined with AHC. Conclusion The LE method is promising for unsupervised clustering to separate brain and tumor tissue with automated color-coding for visualization of 1H MRSI data after cluster analysis
Dynamic Structure Function in 3he-4he Mixtures
Relevant features of the dynamic structure function in
He-He mixtures at zero temperature are investigated starting from known
properties of the ground state. Sum rules are used to fix rigorous constraints
to the different contributions to , coming from He and He
elementary excitations, as well as to explore the role of the cross term
. Both the low- (phonon-roton He excitations and
1p-1h He excitations) and high- (deep inelastic scattering) ranges are
discussed.Comment: 29 pages, Plain TeX, 11 figures available by request from
[email protected]
Extended supersolid phase of frustrated hard-core bosons on a triangular lattice
We study a model of hard-core bosons with frustrated nearest-neighbor hopping
() and repulsion () on the triangular lattice. We argue for a supersolid
ground state in the large repulsion () limit where a dimer
representation applies, by constructing a unitary mapping to the well
understood unfrustrated hopping case. This generalized 'Marshall sign rule'
allows us to establish the precise nature of the supersolid order by utilizing
a recently proposed dimer variational wavefunction, whose correlations can be
efficiently calculated using the Grassman approach. By continuity, a supersolid
is predicted over the wide parameter range, . This also establishes a
simple phase diagram for the triangular lattice spin 1/2 XXZ antiferromagnet.Comment: 5 pages, 4 figure
Thermalized Displaced Squeezed Thermal States
In the coordinate representation of thermofield dynamics, we investigate the
thermalized displaced squeezed thermal state which involves two temperatures
successively. We give the wavefunction and the matrix element of the density
operator at any time, and accordingly calculate some quantities related to the
position, momentum and particle number operator, special cases of which are
consistent with the results in the literature. The two temperatures have
diffenent correlations with the squeeze and coherence components. Moreover,
different from the properties of the position and momentum, the average value
and variance of the particle number operator as well as the second-order
correlation function are time-independent.Comment: 7 pages, no figures, Revtex fil
Intentionality versus Constructive Empiricism
By focussing on the intentional character of observation in science, we argue that Constructive Empiricism – B.C. van Fraassen’s much debated and explored view of science – is inconsistent. We then argue there are at least two ways out of our Inconsistency Argument, one of which is more easily to square with Constructive Empiricism than the other
Spherical Functions Associated With the Three Dimensional Sphere
In this paper, we determine all irreducible spherical functions \Phi of any K
-type associated to the pair (G,K)=(\SO(4),\SO(3)). This is accomplished by
associating to \Phi a vector valued function H=H(u) of a real variable u, which
is analytic at u=0 and whose components are solutions of two coupled systems of
ordinary differential equations. By an appropriate conjugation involving Hahn
polynomials we uncouple one of the systems. Then this is taken to an uncoupled
system of hypergeometric equations, leading to a vector valued solution P=P(u)
whose entries are Gegenbauer's polynomials. Afterward, we identify those
simultaneous solutions and use the representation theory of \SO(4) to
characterize all irreducible spherical functions. The functions P=P(u)
corresponding to the irreducible spherical functions of a fixed K-type \pi_\ell
are appropriately packaged into a sequence of matrix valued polynomials
(P_w)_{w\ge0} of size (\ell+1)\times(\ell+1). Finally we proved that \widetilde
P_w={P_0}^{-1}P_w is a sequence of matrix orthogonal polynomials with respect
to a weight matrix W. Moreover we showed that W admits a second order symmetric
hypergeometric operator \widetilde D and a first order symmetric differential
operator \widetilde E.Comment: 49 pages, 2 figure
Quantum and Classical Spins on the Spatially Distorted Kagome Lattice: Applications to Volborthite
In Volborthite, spin-1/2 moments form a distorted Kagom\'e lattice, of corner
sharing isosceles triangles with exchange constants on two bonds and
on the third bond. We study the properties of such spin systems, and show that
despite the distortion, the lattice retains a great deal of frustration.
Although sub-extensive, the classical ground state degeneracy remains very
large, growing exponentially with the system perimeter. We consider degeneracy
lifting by thermal and quantum fluctuations. To linear (spin wave) order, the
degeneracy is found to stay intact. Two complementary approaches are therefore
introduced, appropriate to low and high temperatures, which point to the same
ordered pattern. In the low temperature limit, an effective chirality
Hamiltonian is derived from non-linear spin waves which predicts a transition
on increasing , from type order to a new
ferrimagnetic {\em striped chirality} order with a doubled unit cell. This is
confirmed by a large-N approximation on the O() model on this lattice. While
the saddle point solution produces a line degeneracy, corrections
select the non-trivial wavevector of the striped chirality state. The quantum
limit of spin 1/2 on this lattice is studied via exact small system
diagonalization and compare well with experimental results at intermediate
temperatures. We suggest that the very low temperature spin frozen state seen
in NMR experiments may be related to the disconnected nature of classical
ground states on this lattice, which leads to a prediction for NMR line shapes.Comment: revised, section V about exact diagonalization is extensively
rewritten, 17 pages, 11 figures, RevTex 4, accepted by Phys. Rev.
Le coût de production et la compétitivité de la viande ovine Algerienne : Cas de l’agneau de Djelfa
L’objectif de ce travail est de contribuer à la connaissance des coûts liés à l'élevage de l’agneau de la steppe dans la perspective de gagner en compétitivité. Basé sur une enquête auprès des éleveurs de la wilaya de Djelfa, ce travail montre que le coût de l'alimentation est la charge la plus importante quel que soit le type d’éleveurs. Le calcul des indicateurs de compétitivité le coefficient de protection nominale (CPN) qui mesure la protection d’un produit sur le marché local par rapport au marché international, le coefficient de protection effective (CPE) qui prend en compte la protection des intrants échangeables utilisés, et le coût en ressources domestiques (CRD) qui mesure la compétitivité d’un produit locale par rapport au marché international montre que l’Algérie possède un avantage comparatif dans cette filière. Par contre, dans une situation de libre échange, le coût élevé de l’alimentation influe négativement sur le niveau de la compétitivité de la viande ovine algérienne.Mots clés : La viande ovine, Coûts de production, Compétitivité, Avantage comparatif, Djelfa
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