13,038 research outputs found
Optimal Switching for Hybrid Semilinear Evolutions
We consider the optimization of a dynamical system by switching at discrete
time points between abstract evolution equations composed by nonlinearly
perturbed strongly continuous semigroups, nonlinear state reset maps at mode
transition times and Lagrange-type cost functions including switching costs. In
particular, for a fixed sequence of modes, we derive necessary optimality
conditions using an adjoint equation based representation for the gradient of
the costs with respect to the switching times. For optimization with respect to
the mode sequence, we discuss a mode-insertion gradient. The theory unifies and
generalizes similar approaches for evolutions governed by ordinary and delay
differential equations. More importantly, it also applies to systems governed
by semilinear partial differential equations including switching the principle
part. Examples from each of these system classes are discussed
A Theory of Ex Post Inefficient Renegotiation
We propose a theory of ex post inefficient renegotiation that is based on loss aversion. When two parties write a long-term contract that has to be renegotiated after the realization of the state of the world, they take the initial contract as a reference point to which they compare gains and losses of the renegotiated transaction. We show that loss aversion makes the renegotiated outcome sticky and materially inefficient. The theory has important implications for the optimal design of long-term contracts. First, it explains why parties often abstain from writing a beneficial long-term contract or why some contracts specify transactions that are never ex post efficient. Second, it shows under what conditions parties should rely on the allocation of ownership rights to protect relationship-specific investments rather than writing a specific performance contract. Third, it shows that employment contracts can be strictly optimal even if parties are free to renegotiate
Spin edge helices in a perpendicular magnetic field
We present an exact solution to the problem of the spin edge states in the
presence of equal Bychkov-Rashba and Dresselhaus spin-orbit fields in a
two-dimensional electron system, restricted by a hard-wall confining potential
and exposed to a perpendicular magnetic field. We find that the spectrum of the
spin edge states depends critically on the orientation of the sample edges with
respect to the crystallographic axes. Such a strikingly different spectral
behavior generates new modes of the persistent spin helix-spin edge helices
with novel properties, which can be tuned by the applied electric and magnetic
fields.Comment: In press in Physical Review Letters; Revised arguments in the
introductory part; 3 figure
XMM-Newton observations of GB B1428+4217: confirmation of intrinsic soft X-ray absorption
We report the results of XMM-Newton observations of the X-ray bright,
radio-loud blazar GB B1428+4217 at a redshift of z=4.72. We confirm the
presence of soft X-ray spectral flattening at energies <0.7 keV as reported in
previous ROSAT and BeppoSAX observations. At hard X-ray energies the spectrum
is consistent with a power-law although we find the spectral slope varied
between both XMM-Newton observations and is also significantly different from
that reported previously. Whilst we cannot rule-out intrinsic cold absorption
to explain the spectral depression, we favour a dust-free warm absorber. Cold
absorption requires a column density ~1.4-1.6 x 10^22 cm^-2 whilst a warm
absorber could have up to ~10^23 cm^-2 and an ionization parameter ~10^2. The
spectrum of GB B1428+4217 shows remarkable parallels with that of the z=4.4
blazar PMN J0525-3343, in which the available evidence is also most consistent
with a warm absorber model.Comment: 5 pages, 5 figures, MNRAS accepted. Minor changes to sections 3.1 and
Upper limit on the critical strength of central potentials in relativistic quantum mechanics
In the context of relativistic quantum mechanics, where the Schr\"odinger
equation is replaced by the spinless Salpeter equation, we show how to
construct a large class of upper limits on the critical value,
, of the coupling constant, , of the central potential,
. This critical value is the value of for which a first
-wave bound state appears.Comment: 8 page
Chandra observations of Abell 2199
We present results from an analysis of two Chandra observations of the rich,
nearby galaxy cluster Abell 2199. We find evidence (having corrected for
projection effects) for radial gradients in temperature and metallicity in the
X-ray emitting gas: the temperature drops from kT~4.2 keV at R=200 kpc to 1.6
keV within R=5 kpc of the centre. The metallicity rises from ~0.3 solar at
R=200 kpc to ~0.7 solar at R=30 kpc before dropping to 0.3 solar within the
central 5 kpc. We find evidence for structure in the surface brightness
distribution associated with the central radio source 3C338. No evidence is
found for the gas having a large spread in temperature at any particular
location despite the cooling time being short (<10**9yr) within the central ~15
kpc. Heating and mass cooling rates are calculated for various assumptions
about the state of the gas.Comment: 10 pages, 12 figures. Accepted by MNRAS. Minor changes following
referee's comment
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