53,518 research outputs found
The Effect of Orbital Angular Momentum on Nondiffracting Ultrashort Optical Pulses
We introduce a new class of nondiffracting optical pulses possessing orbital
angular momentum. By generalizing the X-waves solution of the Maxwell equation,
we discover the coupling between angular momentum and the temporal degrees of
freedom of ultra-short pulses. The spatial twist of propagation invariant light
pulse turns out to be directly related to the number of optical cycles. Our
results may trigger the development on novel multi-level classical and quantum
transmission channels free of dispersion and diffraction, may also find
application in the manipulation of nano-structured objects by ultra-short
pulses, and for novel approaches to the spatio-temporal measurements in
ultrafast photonics
Automorphisms of the UHF algebra that do not extend to the Cuntz algebra
Automorphisms of the canonical core UHF-subalgebra F_n of the Cuntz algebra
O_n do not necessarily extend to automorphisms of O_n. Simple examples are
discussed within the family of infinite tensor products of (inner)
automorphisms of the matrix algebras M_n. In that case, necessary and
sufficient conditions for the extension property are presented. It is also
addressed the problem of extending to O_n the automorphisms of the diagonal
D_n, which is a regular MASA with Cantor spectrum. In particular, it is shown
the existence of product-type automorphisms of D_n that are not extensible to
(possibly proper) endomorphisms of O_n
Solitonization of the Anderson Localization
We study the affinities between the shape of the bright soliton of the
one-dimensional nonlinear Schroedinger equation and that of the disorder
induced localization in the presence of a Gaussian random potential. With
emphasis on the focusing nonlinearity, we consider the bound states of the
nonlinear Schroedinger equation with a random potential; for the state
exhibiting the highest degree of localization, we derive explicit expressions
for the nonlinear eigenvalue and for the localization length by using
perturbation theory and a variational approach following the methods of
statistical mechanics of disordered systems. We numerically investigate the
linear stability and "superlocalizations". The profile of the disorder averaged
Anderson localization is found to obey a nonlocal nonlinear Schroedinger
equationComment: 5 pages, 3 figure
Chengyu in Chinese Language Teaching: A preliminary analysis of Italian learners’ data
Chengyu, also known as Chinese four-character idioms, are a type of traditional Chinese idiom, mostly consisting of four characters. They commonly derive
from classic Chinese literary sources, including those of the three great philosophical and religious traditions that influenced the entire East Asia cultural sphere: Confucianism, Daoism and Buddhism. Chengyu, therefore, possess a wide range of cultural references, and, from Chinese, spread to the languages of the other countries of the sinosphere, such as Japan and Korea. Although many scholars have emphasized the importance of the acquisition of chengyu, not much attention has been paid to chengyu learning in Chinese Language Teaching research so far. As a preliminary attempt to address this gap, this paper reports the results of two small-scale, exploratory experiments, aimed at investigating Italian learners’ general knowledge of chengyu and their main interpretation strategies, as well as comparing the effectiveness of direct and indirect instruction in chengyu teaching. The experiments involved participants from Bachelor and Master programs of Roma Tre University. The results show a predominant effect of negative transfer from Italian, as well as a better performance of the participants who received indirect instruction
Half-flat nilmanifolds
We introduce a double complex that can be associated to certain Lie algebras,
and show that its cohomology determines an obstruction to the existence of a
half-flat SU(3)-structure. We obtain a classification of the 6-dimensional
nilmanifolds carrying an invariant half-flat structure.Comment: 14 pages; v2: corrected equation in introduction; added solvable
example; rearranged contents of Proposition 3 and Lemma 4; v3: corrected
typos, added remark about unimodular cas
- …