9,254 research outputs found
Non-local fractional derivatives. Discrete and continuous
We prove maximum and comparison principles for fractional discrete
derivatives in the integers. Regularity results when the space is a mesh of
length , and approximation theorems to the continuous fractional derivatives
are shown. When the functions are good enough, these approximation procedures
give a measure of the order of approximation. These results also allows us to
prove the coincidence, for good enough functions, of the Marchaud and
Gr\"unwald-Letnikov derivatives in every point and the speed of convergence to
the Gr\"unwald-Letnikov derivative. The fractional discrete derivative will be
also described as a Neumann-Dirichlet operator defined by a semi-discrete
extension problem. Some operators related to the Harmonic Analysis associated
to the discrete derivative will be also considered, in particular their
behavior in the Lebesgue spaces $\ell^p(\mathbb{Z}).
Thermal boundary resistance at Si/Ge interfaces determined by approach-to-equilibrium molecular dynamics simulations
The thermal boundary resistance of Si/Ge interfaces as been determined using
approach-to-equilibrium molecular dynamics simulations. Assuming a reciprocal
linear dependence of the thermal boundary resistance, a length-independent bulk
thermal boundary resistance could be extracted from the calculation resulting
in a value of 3.76x10 m K/W for a sharp Si/Ge interface and thermal
transport from Si to Ge. Introducing an interface with finite thickness of 0.5
nm consisting of a SiGe alloy, the bulk thermal resistance slightly decreases
compared to the sharp Si/Ge interface. Further growth of the boundary leads to
an increase in the bulk thermal boundary resistance. When the heat flow is
inverted (Ge to Si), the thermal boundary resistance is found to be higher.
From the differences in the thermal boundary resistance for different heat flow
direction, the rectification factor of the Si/Ge has been determined and is
found to significantly decrease when the sharp interface is moderated by
introduction of a SiGe alloy in the boundary layer.Comment: 7 pages, 6 figure
Challenging the paradigm of singularity excision in gravitational collapse
A paradigm deeply rooted in modern numerical relativity calculations
prescribes the removal of those regions of the computational domain where a
physical singularity may develop. We here challenge this paradigm by performing
three-dimensional simulations of the collapse of uniformly rotating stars to
black holes without excision. We show that this choice, combined with suitable
gauge conditions and the use of minute numerical dissipation, improves
dramatically the long-term stability of the evolutions. In turn, this allows
for the calculation of the waveforms well beyond what previously possible,
providing information on the black-hole ringing and setting a new mark on the
present knowledge of the gravitational-wave emission from the stellar collapse
to a rotating black hole.Comment: 4 pages, 4 figures, accepted for publication on Phys. Rev. Let
Redox control of multidrug resistance and Its possible modulation by antioxidants
Clinical efficacy of anticancer chemotherapies is dramatically hampered by multidrug resistance (MDR) dependent on inherited traits, acquired defence against toxins, and adaptive mechanisms mounting in tumours. There is overwhelming evidence that molecular events leading to MDR are regulated by redox mechanisms. For example, chemotherapeutics which overrun the first obstacle of redox-regulated cellular uptake channels (MDR1, MDR2, and MDR3) induce a concerted action of phase I/II metabolic enzymes with a temporal redox-regulated axis. This results in rapid metabolic transformation and elimination of a toxin. This metabolic axis is tightly interconnected with the inducible Nrf2-linked pathway, a key switch-on mechanism for upregulation of endogenous antioxidant enzymes and detoxifying systems. As a result, chemotherapeutics and cytotoxic by-products of their metabolism (ROS, hydroperoxides, and aldehydes) are inactivated and MDR occurs. On the other hand, tumour cells are capable of mounting an adaptive antioxidant response against ROS produced by chemotherapeutics and host immune cells. The multiple redox-dependent mechanisms involved in MDR prompted suggesting redox-active drugs (antioxidants and prooxidants) or inhibitors of inducible antioxidant defence as a novel approach to diminish MDR. Pitfalls and progress in this direction are discussed
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