28,193 research outputs found
Under the Wing of a Creature of the Night
Magnificent in its sheer power and beauty, this owl wing has a wingspan of 18 inches and measures 10 inches from the shoulder bone to the secondary feathers. Wings such as the one displayed play a vital role in the lifestyle of owls and other hunting birds who fulfill their dietary requirements through stealthy foraging in the dark of the night. Being predatory animals, an owl depends upon its wings as a weapon, equipping it with an arsenal worthy of any hunter. Because of their composition of downy feathers, soft fringes, and comb-like primary feathers, these light appendages create less audible sound waves through air, giving an owl the advantage of nearly silent flight. [excerpt
Rocky Horror: A Study in Shadows and Flight
âRocky Horror: A Study in Shadows and Flightâ is a creative nonfiction piece that analyzes the infamous legacy left by the cult classic, The Rocky Horror Picture Show. As a first-year in college, the speaker strings together a series of vignettes from different encounters with the film in her life, from her first midnight showing to her first performance as Columbia in a live production. In a few pages, this piece examines the meaning of identity and freedom as the speaker works through repulsion, rebellion, and all things Rocky
Atomic and molecular matter fields in periodic potentials
This paper deals with the conversion between atoms and molecules in optical
lattices. We show that in the absence of collisional interaction, the atomic
and molecular components in different lattice wells combine into states with
macroscopic condensate fractions, which can be observed as a strong diffraction
signal, if the particles are abruptly released from the lattice. The condensate
population, and the diffraction signal are governed not only by the mean number
of atoms or molecules in each well, but by the precise amplitudes on state
vector components with different numbers of particles. We discuss ways to
control these amplitudes and to maximize the condensate fraction in the
molecular formation process.Comment: Invited talk at 'Quantum Challenges', Falenty, Poland, Sep. 2003.
Submitted to J. Mod. Op
Counting Components in the Lagrange Multiplier Formulation of Teleparallel Theories
We investigate the Lagrange multiplier formulation of teleparallel theories,
including f(T) gravity, in which the connection is not set to zero a priori and
compare it with the pure frame theory. We show explicitly that the two
formulations are equivalent, in the sense that the dynamical equations have the
same content. One consequence is that the manifestly local Lorentz invariant
f(T) theory cannot be expected to be free of pathologies, which were previously
found to plague f(T) gravity formulated in the usual pure frame approach.Comment: 6 pages, version accepted for publicatio
Traffic, urban growth and suburban sprawl
Cities are still getting bigger in the western world. Even though urbanpopulations are barely reproducing themselves and migration from thecountryside to the town has slowed to a trickle, the demand for more livingspace shows no sign of abating as cities continue to expand their bordersthrough suburban sprawl. The automobile, of course, makes this possiblebut we show no signs of moving to other forms of transport that mightenable our cities to become a little more compact. The problems of sprawlare pervasive. Besides congestion, time wasted, and the long term costs ofusing non-renewable energy, the lack of good social infrastructure inrapidly growing suburban areas together with the erosion of agriculturalland, often of high environmental quality, has focused the debate onwhether or not such forms of development are sustainable. In this paper,we begin by noting that suburban sprawl is an age-old phenomenon whichrepresents a fine balance between the forces that are pushing peopletogether in cities and those that are forcing them out. These lead todifferent types of sprawl in different places and at different times butwhatever the variety, there are costs to be borne. We briefly review these,noting how these affect suburban sprawl in Europe, and the efforts of theEuropean Commission to understand the problem. We conclude not with aplea that cities should be compacted and all automobile traffic removedbut that we should engage in policies for ?smart growth? such as thosebeing adopted in North America
Ranking Functions for Size-Change Termination II
Size-Change Termination is an increasingly-popular technique for verifying
program termination. These termination proofs are deduced from an abstract
representation of the program in the form of "size-change graphs".
We present algorithms that, for certain classes of size-change graphs, deduce
a global ranking function: an expression that ranks program states, and
decreases on every transition. A ranking function serves as a witness for a
termination proof, and is therefore interesting for program certification. The
particular form of the ranking expressions that represent SCT termination
proofs sheds light on the scope of the proof method. The complexity of the
expressions is also interesting, both practicaly and theoretically.
While deducing ranking functions from size-change graphs has already been
shown possible, the constructions in this paper are simpler and more
transparent than previously known. They improve the upper bound on the size of
the ranking expression from triply exponential down to singly exponential (for
certain classes of instances). We claim that this result is, in some sense,
optimal. To this end, we introduce a framework for lower bounds on the
complexity of ranking expressions and prove exponential lower bounds.Comment: 29 pages
Fourth Order Algorithms for Solving the Multivariable Langevin Equation and the Kramers Equation
We develop a fourth order simulation algorithm for solving the stochastic
Langevin equation. The method consists of identifying solvable operators in the
Fokker-Planck equation, factorizing the evolution operator for small time steps
to fourth order and implementing the factorization process numerically. A key
contribution of this work is to show how certain double commutators in the
factorization process can be simulated in practice. The method is general,
applicable to the multivariable case, and systematic, with known procedures for
doing fourth order factorizations. The fourth order convergence of the
resulting algorithm allowed very large time steps to be used. In simulating the
Brownian dynamics of 121 Yukawa particles in two dimensions, the converged
result of a first order algorithm can be obtained by using time steps 50 times
as large. To further demostrate the versatility of our method, we derive two
new classes of fourth order algorithms for solving the simpler Kramers equation
without requiring the derivative of the force. The convergence of many fourth
order algorithms for solving this equation are compared.Comment: 19 pages, 2 figure
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