16,882 research outputs found
The A-decomposability of the Singer construction
Let denote the Singer construction on an unstable module over the
Steenrod algebra at the prime two; is canonically a subobject of
, where is the polynomial algebra on s generators of degree
one. Passage to -indecomposables gives the natural transformation , which identifies with the dual of the
composition of the Singer transfer and the Lannes-Zarati homomorphism.
The main result of the paper proves the weak generalized algebraic spherical
class conjecture, which was proposed by the first named author. Namely, this
morphism is trivial on elements of positive degree when s>2. The condition s>2
is necessary, as exhibited by the spherical classes of Hopf invariant one and
those of Kervaire invariant one.Comment: v2 15 pages. Minor revision. v3 17 pages, revision following
referee's recommendations. Accepted for publication J. Al
and decays in the perturbative QCD approach
In this work, we study six tree-dominated and
decays in the perturbative QCD(pQCD) approach, where
() is a () axial-vector meson. Based on the
perturbative calculations and phenomenological analysis, we find that: (a) the
CP-averaged branching ratio of decay in the pQCD approach
is , which agrees well with the current data and the
predictions given in the QCD factorization approach within errors; (b) the
numerical results for the decay rates of other five channels are found to be in
the order of , which could be accessed at B factories and
Large Hadron Collider(LHC) experiments; (c) other physical observables such as
polarization fractions and direct CP-violating asymmetries are also
investigated with the pQCD approach in the present work and the predictions can
be confronted with the relevant experiments in the near future; (d) the
different phenomenologies shown between and
decays are expected to be tested by the ongoing LHC and forthcoming Super-B
experiments, which could shed light on the typical QCD dynamics involved in
these decay modes, as well as in meson and meson .Comment: 1 figure, 27 pages, references added, improved version. Accepted for
publication in Phys. Rev.
Time-varying Huygens' meta-devices for parametric waves
Huygens' metasurfaces have demonstrated almost arbitrary control over the
shape of a scattered beam, however, its spatial profile is typically fixed at
fabrication time. Dynamic reconfiguration of this beam profile with tunable
elements remains challenging, due to the need to maintain the Huygens'
condition across the tuning range. In this work, we experimentally demonstrate
that a time-varying metadevice which performs frequency conversion can steer
transmitted or reflected beams in an almost arbitrary manner, with fully
dynamic control. Our time-varying Huygens' metadevice is made of both electric
and magnetic meta-atoms with independently controlled modulation, and the phase
of this modulation is imprinted on the scattered parametric waves, controlling
their shapes and directions. We develop a theory which shows how the scattering
directionality, phase and conversion efficiency of sidebands can be manipulated
almost arbitrarily. We demonstrate novel effects including all-angle beam
steering and frequency-multiplexed functionalities at microwave frequencies
around 4 GHz, using varactor diodes as tunable elements. We believe that the
concept can be extended to other frequency bands, enabling metasurfaces with
arbitrary phase pattern that can be dynamically tuned over the complete 2\pi
range
Multistability in nonlinear left-handed transmission lines
Employing a nonlinear left-handed transmission line as a model system, we
demonstrate experimentally the multi-stability phenomena predicted
theoretically for microstructured left-handed metamaterials with a nonlinear
response. We show that the bistability is associated with the period doubling
which at higher power may result in chaotic dynamics of the transmission line
Rotational tuning of interaction in metamaterials
We experimentally observe the tuning of metamaterials through the relative
rotation of the elements about their common axis. In contrast to previous
results we observe a crossing of resonances, where the symmetric and
anti-symmetric modes become degenerate. We associate this effect with an
interplay between the magnetic and electric near-field interactions and verify
this by calculations based on the interaction energy between resonators
The role of Uncertainty in Categorical Perception Utilizing Statistical Learning in Robots
At the heart of statistical learning lies the concept of uncertainty.
Similarly, embodied agents such as robots
and animals must likewise address uncertainty, as sensation
is always only a partial reflection of reality. This
thesis addresses the role that uncertainty can play in
a central building block of intelligence: categorization.
Cognitive agents are able to perform tasks like categorical perception
through physical interaction (active categorical perception; ACP),
or passively at a distance (distal categorical perception; DCP).
It is possible that the former scaffolds the learning of
the latter. However, it is unclear whether DCP indeed scaffolds
ACP in humans and animals, nor how a robot could be trained
to likewise learn DCP from ACP. Here we demonstrate a method
for doing so which involves uncertainty: robots perform
ACP when uncertain and DCP when certain.
Furthermore, we demonstrate that robots trained
in such a manner are more competent at categorizing novel
objects than robots trained to categorize in other ways.
This suggests that such a mechanism would also be
useful for humans and animals, suggesting that they
may be employing some version of this mechanism
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