1,402,914 research outputs found
Action minimizing solutions of the Newtonian n-body problem: from homology to symmetry
An action minimizing path between two given configurations, spatial or
planar, of the -body problem is always a true -- collision-free -- solution.
Based on a remarkable idea of Christian Marchal, this theorem implies the
existence of new "simple" symmetric periodic solutions, among which the Eight
for 3 bodies, the Hip-Hop for 4 bodies and their generalizations
Symmetry-break in Voronoi tessellations
We analyse in a common framework the properties of the Voronoi tessellations resulting from regular 2D and 3D crystals and those of tessellations generated by Poisson distributions of points, thus joining on symmetry breaking processes and the approach to uniform random distributions of seeds. We perturb crystalline structures in 2D and 3D with a spatial Gaussian noise whose adimensional strength is α and analyse the statistical properties of the cells of the resulting Voronoi tessellations using an ensemble approach. In 2D we consider triangular, square and hexagonal regular lattices, resulting into hexagonal, square and triangular tessellations, respectively. In 3D we consider the simple cubic (SC), body-centred cubic (BCC), and face-centred cubic (FCC) crystals, whose corresponding Voronoi cells are the cube, the truncated octahedron, and the rhombic dodecahedron, respectively. In 2D, for all values α>0, hexagons constitute the most common class of cells. Noise destroys the triangular and square tessellations, which are structurally unstable, as their topological properties are discontinuous in α=0. On the contrary, the honeycomb hexagonal tessellation is topologically stable and, experimentally, all Voronoi cells are hexagonal for small but finite noise with α0.5), memory of the specific initial unperturbed state is lost, because the statistical properties of the three perturbed regular tessellations are indistinguishable. When α>2, results converge to those of Poisson-Voronoi tessellations. In 2D, while the isoperimetric ratio increases with noise for the perturbed hexagonal tessellation, for the perturbed triangular and square tessellations it is optimised for specific value of noise intensity. The same applies in 3D, where noise degrades the isoperimetric ratio for perturbed FCC and BCC lattices, whereas the opposite holds for perturbed SCC lattices. This allows for formulating a weaker form of the Kelvin conjecture. By analysing jointly the statistical properties of the area and of the volume of the cells, we discover that also the cells shape heavily fluctuates when noise is introduced in the system. In 2D, the geometrical properties of n-sided cells change with α until the Poisson-Voronoi limit is reached for α>2; in this limit the Desch law for perimeters is shown to be not valid and a square root dependence on n is established, which agrees with exact asymptotic results. Anomalous scaling relations are observed between the perimeter and the area in the 2D and between the areas and the volumes of the cells in 3D: except for the hexagonal (2D) and FCC structure (3D), this applies also for infinitesimal noise. In the Poisson-Voronoi limit, the anomalous exponent is about 0.17 in both the 2D and 3D case. A positive anomaly in the scaling indicates that large cells preferentially feature large isoperimetric quotients. As the number of faces is strongly correlated with the sphericity (cells with more faces are bulkier), in 3D it is shown that the anomalous scaling is heavily reduced when we perform power law fits separately on cells with a specific number of faces
A novel nonlinear evolution equation integrable by the inverse scattering method
A Backlund transformation for an evolution equation (ut+u ux)x+u=0 transformed into new coordinates is derived. An inverse scattering problem is formulated. The inverse scattering method has a third order eigenvalue problem. A procedure for finding the exact N-soliton solution of the Vakhnenko equation via the inverse scattering method is described
Mirror Symmetry as a Gauge Symmetry
It is shown that in string theory mirror duality is a gauge symmetry (a Weyl
transformation) in the moduli space of backgrounds on group manifolds,
and we conjecture on the possible generalization to other backgrounds, such as
Calabi-Yau manifolds.Comment: 11 page
Spontaneous Symmetry Breaking and Chiral Symmetry
In this introductory lecture, some basic features of the spontaneous symmetry
breaking are discussed. More specifically, -model, non-linear
realization, and some examples of spontaneous symmetry breaking in the
non-relativistic system are discussed in details. The approach here is more
pedagogical than rigorous and the purpose is to get some simple explanation of
some useful topics in this rather wide area. .Comment: Lecture Delivered at VII Mexico Workshop on Paritcles and Fields,
Merida, Yucatan Mexico, Nov 10-17,199
Symmetry Breaking at enhanced Symmetry Points
The influence of world-sheet boundary condensates on the toroidal
compactification of bosonic string theories is considered. At the special
points in the moduli space at which the closed-string theory possesses an
enhanced unbroken symmetry (where is a semi-simple product of
simply laced groups) a scalar boundary condensate parameterizes the coset
. Fluctuations around this background define an open-string
generalization of the corresponding chiral nonlinear sigma model. Tree-level
scattering amplitudes of on-shell massless states (\lq pions') reduce to the
amplitudes of the principal chiral model for the group in the low energy
limit. Furthermore, the condition for the vanishing of the renormalization
group beta function at one loop results in the familiar equation of motion for
that model. The quantum corrections to the open-string theory generate a mixing
of open and closed strings so that the coset-space pions mix with the
closed-string gauge fields, resulting in a Higgs-like breakdown of
the symmetry to the diagonal group. The case of non-oriented strings is
also discussed.Comment: 32 pages, LaTeX, 2 figures in uuencoded fil
Symmetry, Structure and the Constitution of Objects
In this paper I focus on the impact on structuralism of the quantum treatment of objects in terms of symmetry groups and, in particular, on the question as to how we might eliminate, or better, reconceptualise such objects in structural terms. With regard to the former, both Cassirer and Eddington not only explicitly and famously tied their structuralism to the development of group theory but also drew on the quantum treatment in order to further their structuralist aims and here I sketch the relevant history with an eye on what lessons might be drawn. With regard to the latter, Ladyman has explicitly cited Castellani's work on the group-theoretical constitution of quantum objects and I indicate both how such an approach needs to be understood if it is to mesh with Ladyman's 'ontic' form of structural realism and how it might accommodate permutation symmetry through a consideration of Huggett's recent account
PT-symmetry broken by point-group symmetry
We discuss a PT-symmetric Hamiltonian with complex eigenvalues. It is based
on the dimensionless Schr\"{o}dinger equation for a particle in a square box
with the PT-symmetric potential . Perturbation theory clearly
shows that some of the eigenvalues are complex for sufficiently small values of
. Point-group symmetry proves useful to guess if some of the eigenvalues
may already be complex for all values of the coupling constant. We confirm
those conclusions by means of an accurate numerical calculation based on the
diagonalization method. On the other hand, the Schr\"odinger equation with the
potential exhibits real eigenvalues for sufficiently small
values of . Point group symmetry suggests that PT-symmetry may be broken
in the former case and unbroken in the latter one
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