444 research outputs found
Self-similar Bianchi models: II. Class B models
In a companion article (referred hearafter as paper I) a detailed study of
the simply transitive Spatially Homogeneous (SH) models of class A concerning
the existence of a simply transitive similarity group has been given. The
present work (paper II) continues and completes the above study by considering
the remaining set of class B models. Following the procedure of paper I we find
all SH models of class B subjected only to the minimal geometric assumption to
admit a proper Homothetic Vector Field (HVF). The physical implications of the
obtained geometric results are studied by specialising our considerations to
the case of vacuum and law perfect fluid models. As a result we
regain all the known exact solutions regarding vacuum and non-tilted perfect
fluid models. In the case of tilted fluids we find the \emph{general
}self-similar solution for the exceptional type VI model and we
identify it as equilibrium point in the corresponding dynamical state space. It
is found that this \emph{new} exact solution belongs to the subclass of models
, is defined for and
although has a five dimensional stable manifold there exist always two unstable
modes in the restricted state space. Furthermore the analysis of the remaining
types, guarantees that tilted perfect fluid models of types III, IV, V and
VII cannot admit a proper HVF strongly suggesting that these models either
may not be asymptotically self-similar (type V) or may be extreme tilted at
late times. Finally for each Bianchi type, we give the extreme tilted
equilibrium points of their state space.Comment: Latex, 15 pages, no figures; to appear in Classical Quantum Gravity
(uses iopart style/class files); (v2) minor corrections to match published
versio
THE INCIDENCE OF THE INFRAPATELLAR PLICAE IN THE ELDERLY WELSH POPULATION
There are several studies reporting the incidence of suprapatellar, medial and lateral plicae, but there is very limited information regarding the incidence of the infrapatellar plica. The purpose of our study was to record the incidence of the infrapatelar plicae in the elderly Welsh population suffering of knee osteoarthritis. A prospective study was performed and 90 knees with severe osteoarthritis of the knee joint (Kellgren-Lawrence type III and IV) were investigated during total knee arthroplasty surgery. Documentation was performed at every total knee replacement surgery for the length of the study. Knee replacement was performed by one senior surgeon. Infrapatellar plica was investigated by a medial parapatellar approach and was classified into five types according to Kimâs classification. The overall incidence of the infrapatellar plicae was 37.7%. The most common type of plicae was the separate type (23.3 %). There was no significant difference found between male and female patients. The fenestra type was the least common (2.22%). The incidence of infrapatellar plicae in the elderly Welsh population suffering of knee osteoarthritis was significantly lower when compared to a study that recorded the incidence of infrapatellar plica in young patients. Possibly, the degenerative changes of the knee joint can cause the reabsorption of the infrapatellar plica decreasing by this way its incidence in the elderly population.Key words: infrapatellar plicae, incidence, knee osteoarthritis, elderl
Self-similar Bianchi models: I. Class A models
We present a study of Bianchi class A tilted cosmological models admitting a
proper homothetic vector field together with the restrictions, both at the
geometrical and dynamical level, imposed by the existence of the simply
transitive similarity group. The general solution of the symmetry equations and
the form of the homothetic vector field are given in terms of a set of
arbitrary integration constants. We apply the geometrical results for tilted
perfect fluids sources and give the general Bianchi II self-similar solution
and the form of the similarity vector field. In addition we show that
self-similar perfect fluid Bianchi VII models and irrotational Bianchi
VI models do not exist.Comment: 14 pages, Latex; to appear in Classical and Quantum Gravit
A geometric description of the intermediate behaviour for spatially homogeneous models
A new approach is suggested for the study of geometric symmetries in general
relativity, leading to an invariant characterization of the evolutionary
behaviour for a class of Spatially Homogeneous (SH) vacuum and orthogonal
law perfect fluid models. Exploiting the 1+3 orthonormal frame
formalism, we express the kinematical quantities of a generic symmetry using
expansion-normalized variables. In this way, a specific symmetry assumption
lead to geometric constraints that are combined with the associated
integrability conditions, coming from the existence of the symmetry and the
induced expansion-normalized form of the Einstein's Field Equations (EFE), to
give a close set of compatibility equations. By specializing to the case of a
\emph{Kinematic Conformal Symmetry} (KCS), which is regarded as the direct
generalization of the concept of self-similarity, we give the complete set of
consistency equations for the whole SH dynamical state space. An interesting
aspect of the analysis of the consistency equations is that, \emph{at least}
for class A models which are Locally Rotationally Symmetric or lying within the
invariant subset satisfying , a proper KCS \emph{always
exists} and reduces to a self-similarity of the first or second kind at the
asymptotic regimes, providing a way for the ``geometrization'' of the
intermediate epoch of SH models.Comment: Latex, 15 pages, no figures (uses iopart style/class files); added
one reference and minor corrections; (v3) improved and extended discussion;
minor corrections and several new references are added; to appear in Class.
Quantum Gra
Self-similar Bianchi type VIII and IX models
It is shown that in transitively self-similar spatially homogeneous tilted
perfect fluid models the symmetry vector is not normal to the surfaces of
spatial homogeneity. A direct consequence of this result is that there are no
self-similar Bianchi VIII and IX tilted perfect fluid models. Furthermore the
most general Bianchi VIII and IX spacetime which admit a four dimensional group
of homotheties is given.Comment: 5 pages, Latex; One reference and minor clarifications added. To
appear in General Relativity and Gravitatio
The influence of carbon on the resistivity recovery of proton irradiated Feâ11at.% Cr alloys
AbstractThe effect of carbon on the point defect migration properties in FeâCr alloys with a concentration of 11 at.% Cr is studied by means of resistivity recovery measurements after low temperature proton irradiation. The presence of carbon mainly affects features of the resistivity recovery spectra in the temperature ranges of (a) 150â200K, which are linked to self-interstitial defects, and (b) 400â500K, which are probably due to vacancy and vacancy-carbon complexes. The experimental results are discussed in terms of the possible interactions of carbon with radiation defects and its influence on solute atom re-ordering
Symmetries of Bianchi I space-times
All diagonal proper Bianchi I space-times are determined which admit certain
important symmetries. It is shown that for Homotheties, Conformal motions and
Kinematic Self-Similarities the resulting space-times are defined explicitly in
terms of a set of parameters whereas Affine Collineations, Ricci Collineations
and Curvature Collineations, if they are admitted, they determine the metric
modulo certain algebraic conditions. In all cases the symmetry vectors are
explicitly computed. The physical and the geometrical consequences of the
results are discussed and a new anisitropic fluid, physically valid solution
which admits a proper conformal Killing vector, is given.Comment: 19 pages, LaTex, Accepted for publication in Journal of Mathematical
Physic
Structural Comparison of Allogeneic and Syngeneic T Cell ReceptorâPeptide-Major Histocompatibility Complex Complexes: A Buried Alloreactive Mutation Subtly Alters Peptide Presentation Substantially Increasing VÎČ Interactions
The crystal structures of the 2C/H-2Kbm3âdEV8 allogeneic complex at 2.4 Ă
and H-2Kbm3âdEV8 at 2.15 Ă
, when compared with their syngeneic counterparts, elucidate structural changes that induce an alloresponse. The Asp77Ser mutation that imbues H-2Kbm3âdEV8 with its alloreactive properties is located beneath the peptide and does not directly contact the T cell receptor (TCR). However, the buried mutation induces local rearrangement of the peptide itself to preserve hydrogen bonding interactions between the peptide and the α1 77 residue. The COOH terminus of the peptide main chain is tugged toward the α1-helix such that its presentation to the TCR is altered. These changes increase the stability of the allogeneic peptide-major histocompatibility complex (pMHC) complex and increase complementarity in the TCRâpMHC interface, placing greater emphasis on recognition of the pMHC by the TCR ÎČ-chain, evinced by an increase in shape complementarity, buried surface area, and number of TCRâpMHC contacting residues. A nearly fourfold increase in the number of ÎČ-chainâpMHC contacts is accompanied by a concomitant 64% increase in ÎČ-chainâpMHC shape complementarity. Thus, the allogeneic mutation causes the same peptide to be presented differently, temporally and spatially, by the allogeneic and syngeneic MHCs
Multi-level optical signal generation using a segmented-electrode InP IQ-MZM with integrated CMOS binary drivers
We present a segmented-electrode InP IQ-MZM, capable of multi-level optical signal generation (5-bit per I/Q arm) by employing direct digital drive from integrated, low-power (1W) CMOS binary drivers. Programmable, multi-level operation is demonstrated experimentally on one MZM of the device
Cyclization of PLP 139-151 peptide reduces its encephalitogenic potential in experimental autoimmune encephalomyelitis
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