10,982 research outputs found
Combining constraint satisfaction and local improvement algorithms to construct anaesthetists' rotas
A system is described which was built to compile weekly rotas for the anaesthetists in a large hospital. The rota compilation problem is an optimization problem (the number of tasks which cannot be assigned to an anaesthetist must be minimized) and was formulated as a constraint satisfaction problem (CSP). The forward checking algorithm is used to find a feasible rota, but because of the size of the problem, it cannot find an optimal (or even a good enough) solution in an acceptable time. Instead, an algorithm was devised which makes local improvements to a feasible solution. The algorithm makes use of the constraints as expressed in the CSP to ensure that feasibility is maintained, and produces very good rotas which are being used by the hospital involved in the project. It is argued that formulation as a constraint satisfaction problem may be a good approach to solving discrete optimization problems, even if the resulting CSP is too large to be solved exactly in an acceptable time. A CSP algorithm may be able to produce a feasible solution which can then be improved, giving a good, if not provably optimal, solution
Dynamic response of two composite prop-fan models on a nacelle/wing/fuselage half model
Results are presented for blade response wind tunnel tests of two 62.2 cm diameter Prop-Fan (advanced turboprop) models with swept and unswept graphite/epoxy composite blades. Measurements of dynamic response were made with the rotors mounted on a simulated nacelle/wing/fuselage model, with varying tilt, at flow speeds up to 0.85 Mach number. The presence of the wing, downstream of the rotor, induced 1-P responses that were about twice those previously measured for an isolated nacelle installation. The swept blade had less 1-P response than the unswept (straight) blade. The 2-P response was significant for both blades, and was closely correlated to wing lift. Higher order response was not important for the straight blade, but possibly important for the swept blade near critical speeds, due to the proximity of the blade tips to the wing leading edge. Measurements are compared with theoretically based prediction. Correlations between calculated and measured 1-P response were good for the straight blade, and fair for the swept blade. Improvements to the calculation method were identified and implemented
Dissipation signatures of the normal and superfluid phases in torsion pendulum experiments with 3He in aerogel
We present data for energy dissipation factor (Q^{-1}) over a broad
temperature range at various pressures of a torsion pendulum setup used to
study 3He confined in a 98% open silica aerogel. Values for Q^{-1} above T_c
are temperature independent and have a weak pressure dependence. Below T_c, a
deliberate axial compression of the aerogel by 10% widens the range of
metastability for a superfluid Equal Spin Pairing (ESP) state; we observe this
ESP phase on cooling and the B phase on warming over an extended temperature
region. While the dissipation for the B phase tends to zero as T goes to 0,
Q^{-1} exhibits a peak value greater than that at T_c at intermediate
temperatures. Values for Q^{-1} in the ESP phase are consistently higher than
in the B phase and are proportional to \rho_s/\rho until the ESP to B phase
transition is attained. We apply a viscoelastic collision-drag model, which
couples the motion of the helium and the aerogel through a frictional
relaxation time \tau_f. Our dissipation data is not sensitive to the damping
due to the presumed small but non-zero value of \tau_f. The result is that an
additional mechanism to dissipate energy not captured in the collision-drag
model and related to the emergence of the superfluid order must exist. The
extra dissipation below T_c is possibly associated with mutual friction between
the superfluid phases and the clamped normal fluid. The pressure dependence of
the measured dissipation in both superfluid phases is likely related to the
pressure dependence of the gap structure of the "dirty" superfluid. The large
dissipation in the ESP state is consistent with the phase being the A or the
Polar with the order parameter nodes oriented in the plane of the cell and
perpendicular to the aerogel anisotropy axis.Comment: 12 pages, 7 figure
Semantic categories underlying the meaning of ‘place’
This paper analyses the semantics of natural language expressions that are associated with the intuitive notion of ‘place’. We note that the nature of such terms is highly contested, and suggest that this arises from two main considerations: 1) there are a number of logically
distinct categories of place expression, which are not always clearly distinguished in discourse about ‘place’; 2) the many non-substantive place count nouns (such as ‘place’, ‘region’, ‘area’, etc.) employed in natural
language are highly ambiguous. With respect to consideration 1), we propose that place-related expressions
should be classified into the following distinct logical types: a) ‘place-like’ count nouns (further subdivided into abstract, spatial and substantive varieties), b) proper names of ‘place-like’ objects, c) locative property phrases, and d) definite descriptions of ‘place-like’ objects. We outline possible formal representations for each of these. To address consideration 2), we examine meanings, connotations and ambiguities of the English vocabulary of abstract and generic place count nouns, and identify underlying elements of meaning, which explain both
similarities and differences in the sense and usage of the various terms
Cosmological Perturbations from Cosmic Strings
Some aspects of the theory of cosmological perturbations from cosmic strings
and other topological defects are outlined, with particular reference to a
simple example: a spatially flat CDM-dominated universe. The conserved
energy-momentum pseudo-tensor is introduced, and the equation for the density
perturbation derived from it. It is shown how the scaling hypothesis for defect
evolution results in a Harrison-Zel'dovich spectrum for wavelengths well inside
the horizon.Comment: LaTeX, 6pp. From Proceedings of `Trends in Astroparticle Physics',
Stockholm, Sweden 22-25 September 1994, edited by L. Bergstr\"om, P. Carlson,
P.O. Hulth and H. Snellman (to be published in Nucl.~Phys~B, Proceedings
Supplements Section
Noisy Preprocessing and the Distillation of Private States
We provide a simple security proof for prepare & measure quantum key
distribution protocols employing noisy processing and one-way postprocessing of
the key. This is achieved by showing that the security of such a protocol is
equivalent to that of an associated key distribution protocol in which, instead
of the usual maximally-entangled states, a more general {\em private state} is
distilled. Besides a more general target state, the usual entanglement
distillation tools are employed (in particular, Calderbank-Shor-Steane
(CSS)-like codes), with the crucial difference that noisy processing allows
some phase errors to be left uncorrected without compromising the privacy of
the key.Comment: 4 pages, to appear in Physical Review Letters. Extensively rewritten,
with a more detailed discussion of coherent --> iid reductio
The quantum dynamic capacity formula of a quantum channel
The dynamic capacity theorem characterizes the reliable communication rates
of a quantum channel when combined with the noiseless resources of classical
communication, quantum communication, and entanglement. In prior work, we
proved the converse part of this theorem by making contact with many previous
results in the quantum Shannon theory literature. In this work, we prove the
theorem with an "ab initio" approach, using only the most basic tools in the
quantum information theorist's toolkit: the Alicki-Fannes' inequality, the
chain rule for quantum mutual information, elementary properties of quantum
entropy, and the quantum data processing inequality. The result is a simplified
proof of the theorem that should be more accessible to those unfamiliar with
the quantum Shannon theory literature. We also demonstrate that the "quantum
dynamic capacity formula" characterizes the Pareto optimal trade-off surface
for the full dynamic capacity region. Additivity of this formula simplifies the
computation of the trade-off surface, and we prove that its additivity holds
for the quantum Hadamard channels and the quantum erasure channel. We then
determine exact expressions for and plot the dynamic capacity region of the
quantum dephasing channel, an example from the Hadamard class, and the quantum
erasure channel.Comment: 24 pages, 3 figures; v2 has improved structure and minor corrections;
v3 has correction regarding the optimizatio
When should we stop mammography screening for breast cancer in elderly women?
There is insufficient evidence to recommend for or against routine screening mammography beyond the age of 69 years. The best candidates to stop screening are elderly women who have significant comorbidities, poor functional status, low bone mineral density (BMD), little interest in preventive care, or an unwillingness to accept the potential harm of screening. (Grade of Recommendation: C, based on retrospective cohort studies.
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