298 research outputs found
On the properties of level spacings for decomposable systems
In this paper we show that the quantum theory of chaos, based on the
statistical theory of energy spectra, presents inconsistencies difficult to
overcome. In classical mechanics a system described by an hamiltonian (decomposable) cannot be ergodic, because there are always two dependent
integrals of motion besides the constant of energy. In quantum mechanics we
prove the existence of decomposable systems \linebreak
whose spacing distribution agrees with the Wigner law and we show that in
general the spacing distribution of is not the Poisson law, even if it
has often the same qualitative behaviour. We have found that the spacings of
are among the solutions of a well defined class of homogeneous linear
systems. We have obtained an explicit formula for the bases of the kernels of
these systems, and a chain of inequalities which the coefficients of a generic
linear combination of the basis vectors must satisfy so that the elements of a
particular solution will be all positive, i.e. can be considered a set of
spacings.Comment: LateX, 13 page
Non-linear Quantization of Integrable Classical Systems
It is demonstrated that the so-called "unavoidable quantum anomalies" can be
avoided in the farmework of a special non-linear quantization scheme. A simple
example is discussed in detail.Comment: LaTeX, 14 p
E7 groups from octonionic magic square
In this paper we continue our program, started in [2], of building up
explicit generalized Euler angle parameterizations for all exceptional compact
Lie groups. Here we solve the problem for E7, by first providing explicit
matrix realizations of the Tits construction of a Magic Square product between
the exceptional octonionic algebra J and the quaternionic algebra H, both in
the adjoint and the 56 dimensional representations. Then, we provide the Euler
parametrization of E7 starting from its maximal subgroup U=(E6 x U(1))/Z3.
Next, we give the constructions for all the other maximal compact subgroups.Comment: 23 pages, added sections with new construction
Mapping the geometry of the F4 group
In this paper we present a construction of the compact form of the
exceptional Lie group F4 by exponentiating the corresponding Lie algebra f4. We
realize F4 as the automorphisms group of the exceptional Jordan algebra, whose
elements are 3 x 3 hermitian matrices with octonionic entries. We use a
parametrization which generalizes the Euler angles for SU(2) and is based on
the fibration of F4 via a Spin(9) subgroup as a fiber. This technique allows us
to determine an explicit expression for the Haar invariant measure on the F4
group manifold. Apart from shedding light on the structure of F4 and its coset
manifold OP2=F4/Spin(9), the octonionic projective plane, these results are a
prerequisite for the study of E6, of which F4 is a (maximal) subgroup.Comment: 50 pages; some typos correcte
Mapping the geometry of the E6 group
In this paper we present a construction for the compact form of the
exceptional Lie group E6 by exponentiating the corresponding Lie algebra e6,
which we realize as the the sum of f4, the derivations of the exceptional
Jordan algebra J3 of dimension 3 with octonionic entries, and the right
multiplication by the elements of J3 with vanishing trace. Our parametrization
is a generalization of the Euler angles for SU(2) and it is based on the
fibration of E6 via a F4 subgroup as the fiber. It makes use of a similar
construction we have performed in a previous article for F4. An interesting
first application of these results lies in the fact that we are able to
determine an explicit expression for the Haar invariant measure on the E6 group
manifold.Comment: 30 page
Flow network indices signal a directional change in ecosystems: Evidence from a small mountain lake (Lake Santo, northern Italy)
Empirical evidence of the theoretically expected trends of ecosystem development is scarce so far. In this
research, we used long-term empirical data about the plankton community of a small mountain lake (Lake Santo,
northern Apennines, Italy) to reconstruct its developmental trajectory during a period comprised between early
1970 s and 2010 s. We exploited these data to build yearly ecological networks and from their configuration of
energy flows we computed network information indices. The trends of these indices enlighten about the
developmental trajectory of this ecosystem during the period covered by the data set. In particular, they indicate
that Lake Santo evolved in the direction of increasing stability at the expense of efficiency in energy transfer. We
compared these results with current hypotheses about the directionality of ecosystem development, which are
rooted in ecosystem theory, and discussed the possibility that, counter to some theoretical models of ecosystem
development, Lake Santo followed an unimpeded direction of development rather than a trajectory typical of an
ecosystem under stress. Finally, the long-term trends of flow network indices provided insights about the health
status of the ecosystem
Understanding Social–Ecological Systems using Loop Analysis
The sustainable management of social–ecological systems (SESs) requires that we understand the complex structure of relationships and feedbacks among ecosystem components and socioeconomic entities. Therefore, the construction and analysis of models integrating ecological and human actors is crucial for describing the functioning of SESs, and qualitative modeling represents an ideal tool since it allows studying dependencies among variables of diverse types. In particular, the qualitative technique of loop analysis yields predictions about how a system’s variables respond to stress factors. Different interaction types, scarce information about functional relationships among variables, and uncertainties in the values of the parameters are the rule rather than exceptions when studying SESs. Accordingly, loop analysis seems to be perfectly suitable to investigate them. Here, we introduce the key aspects of loop analysis, discuss its applications to SESs, and suggest it enables making the first steps toward the integration of the three dimensions of sustainability
Euler angles for G2
We provide a simple parametrization for the group G2, which is analogous to
the Euler parametrization for SU(2). We show how to obtain the general element
of the group in a form emphasizing the structure of the fibration of G2 with
fiber SO(4) and base H, the variety of quaternionic subalgebras of octonions.
In particular this allows us to obtain a simple expression for the Haar measure
on G2. Moreover, as a by-product it yields a concrete realization and an
Einstein metric for H.Comment: 21 pages, 2 figures, some misprints correcte
Comparison between high-field 3 Tesla MRI and computed tomography with and without arthrography for visualization of canine carpal ligaments: a cadaveric study
OBJECTIVE:To compare the quality of visualization of canine carpal ligaments by using computed tomography (CT), MRI, CT arthrography (CTA), and magnetic resonance arthrography (MRA).
STUDY DESIGN: Prospective descriptive study.
STUDY POPULATION: Cadavers from dogs weighing more than 20 kg.
METHODS: A 16-slice CT scanner and a 3 Tesla MRI were used for the investigation. A dilute contrast medium was injected into the middle carpal and radiocarpal joints under fluoroscopic control, and CTA and MRA images were acquired. To evaluate the difference between imaging modalities, 3 observers graded carpal ligaments of clinical interest using a scale from 0 to 4 for their quality of visualization. Data were analyzed by using a random-effect ordinal logistic regression with Bonferroni adjustment. The interobserver agreement was calculated by using the weighted Cohen's Îş.
RESULTS: Normal carpal joints (n = 9) were investigated. Magnetic resonance arthrography improved visualization of the majority of carpal ligaments compared with MRI (P < .05) and offered the best visualization overall. Magnetic resonance imaging and MRA offered better visualization compared with both CT and CTA (P < .05). There was no difference between CT and CTA. Interobserver agreement was discrete (0.2 < κ ≤ 0.4) for all observers.
CONCLUSION: Arthrography improved the capabilities of MRI but not of CT for visualization of the canine carpal ligaments. Magnetic resonance arthrography was particularly useful for evaluation of the stabilizers of the antebrachiocarpal joint.
CLINICAL SIGNIFICANCE: 3 Tesla MRA and MRI allow excellent visualization of the ligamentous morphology and may be helpful in the diagnostic process of carpal sprains in dogs
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