66,474 research outputs found
Combining All Pairs Shortest Paths and All Pairs Bottleneck Paths Problems
We introduce a new problem that combines the well known All Pairs Shortest
Paths (APSP) problem and the All Pairs Bottleneck Paths (APBP) problem to
compute the shortest paths for all pairs of vertices for all possible flow
amounts. We call this new problem the All Pairs Shortest Paths for All Flows
(APSP-AF) problem. We firstly solve the APSP-AF problem on directed graphs with
unit edge costs and real edge capacities in
time,
where is the number of vertices, is the number of distinct edge
capacities (flow amounts) and is the time taken
to multiply two -by- matrices over a ring. Secondly we extend the problem
to graphs with positive integer edge costs and present an algorithm with
worst case time complexity, where is
the upper bound on edge costs
Dynamics of Scalar Field in Polymer-like Representation
In recent twenty years, loop quantum gravity, a background independent
approach to unify general relativity and quantum mechanics, has been widely
investigated. We consider the quantum dynamics of a real massless scalar field
coupled to gravity in this framework. A Hamiltonian operator for the scalar
field can be well defined in the coupled diffeomorphism invariant Hilbert
space, which is both self-adjoint and positive. On the other hand, the
Hamiltonian constraint operator for the scalar field coupled to gravity can be
well defined in the coupled kinematical Hilbert space. There are 1-parameter
ambiguities due to scalar field in the construction of both operators. The
results heighten our confidence that there is no divergence within this
background independent and diffeomorphism invariant quantization approach of
matter coupled to gravity. Moreover, to avoid possible quantum anomaly, the
master constraint programme can be carried out in this coupled system by
employing a self-adjoint master constraint operator on the diffeomorphism
invariant Hilbert space.Comment: 24 pages, accepted for pubilcation in Class. Quant. Gra
Discovery limits for Techni-Omega production in Collisions
In a strongly-interacting electroweak sector with an isosinglet vector state,
such as the techni-omega, , the direct coupling
implies that an can be produced by fusion in
collisions. This is a unique feature for high energy or
colliders operating in an mode. We consider the processes and , both of which proceed via
an intermediate . We find that at a 1.5 TeV linear collider
operating in an mode with an integrated luminosity of 200 fb,
we can discover an for a broad range of masses and widths.Comment: To appear in the Proceedings of the 29th International Conference on
High Energy Physics, Vancouver, July 1998, 5 pages, Latex, 7 postscript
figure
Optimizing Stimulation and Analysis Protocols for Neonatal fMRI
The development of brain function in young infants is poorly understood. The core challenge is that infants have a limited behavioral repertoire through which brain function can be expressed. Neuroimaging with fMRI has great potential as a way of characterizing typical development, and detecting abnormal development early. But, a number of methodological challenges must first be tackled to improve the robustness and sensitivity of neonatal fMRI. A critical one of these, addressed here, is that the hemodynamic response function (HRF) in pre-term and term neonates differs from that in adults, which has a number of implications for fMRI. We created a realistic model of noise in fMRI data, using resting-state fMRI data from infants and adults, and then conducted simulations to assess the effect of HRF of the power of different stimulation protocols and analysis assumptions (HRF modeling). We found that neonatal fMRI is most powerful if block-durations are kept at the lower range of those typically used in adults (full on/off cycle duration 25-30s). Furthermore, we show that it is important to use the age-appropriate HRF during analysis, as mismatches can lead to reduced power or even inverted signal. Where the appropriate HRF is not known (for example due to potential developmental delay), a flexible basis set performs well, and allows accurate post-hoc estimation of the HRF
Induced Magnetic Ordering by Proton Irradiation in Graphite
We provide evidence that proton irradiation of energy 2.25 MeV on
highly-oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism.
Measurements performed with a superconducting quantum interferometer device
(SQUID) and magnetic force microscopy (MFM) reveal that the magnetic ordering
is stable at room temperature.Comment: 3 Figure
The Circumstellar Extinction of Planetary Nebulae
We analyze the dependence of circumstellar extinction on core mass for the
brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show
that in all three galaxies, a statistically significant correlation exists
between the two quantities, such that high core mass objects have greater
extinction. We model this behavior, and show that the relation is a simple
consequence of the greater mass loss and faster evolution times of high mass
stars. The relation is important because it provides a natural explanation for
the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF)
with population age: bright Population I PNe are extinguished below the cutoff
of the PNLF. It also explains the counter-intuitive observation that
intrinsically luminous Population I PNe often appear fainter than PNe from
older, low-mass progenitors.Comment: 12 pages, 2 figures, accepted for ApJ, April 10, 199
The triple degenerate star WD1704+481
WD1704+481 is a visual binary in which both components are white dwarfs. We
present spectra of the H-alpha line of both stars which show that one component
(WD1704+481.2 = Sanduleak B = GR 577) is a close binary with two white dwarf
components. Thus, WD1704+481 is the first known triple degenerate star. From
radial velocity measurements of the close binary we find an orbital period of
0.1448d, a mass ratio, q=Mbright/Mfaint of q=0.70+-0.03 and a difference in the
gravitational redshifts of 11.5+-2.3km/s. The masses of the close pair of white
dwarfs predicted by the mass ratio and gravitational redshift difference
combined with theoretical cooling curves are 0.39+-0.05 solar mass and
0.56+-0.07 solar masses. WD1704+481 is therefore also likely to be the first
example of a double degenerate in which the less massive white dwarf is
composed of helium and the other white dwarf is composed of carbon and oxygen.Comment: 5 pages, 4 figure
Population synthesis of accreting white dwarfs: II. X-ray and UV emission
Accreting white dwarfs (WDs) with non-degenerate companions are expected to
emit in soft X-rays and the UV, if accreted H-rich material burns stably. They
are an important component of the unresolved emission of elliptical galaxies,
and their combined ionizing luminosity may significantly influence the optical
line emission from warm ISM. In an earlier paper we modeled populations of
accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red
giant companions with the population synthesis code \textsc{BSE}, and then
following their evolution with a grid of evolutionary tracks computed with
\textsc{MESA}. Now we use these results to estimate the soft X-ray
(0.3-0.7keV), H- and He II-ionizing luminosities of nuclear burning WDs and the
number of super-soft X-ray sources for galaxies with different star formation
histories. For the starburst case, these quantities peak at Gyr and
decline by orders of magnitude by the age of 10 Gyr. For stellar
ages of ~10 Gyr, predictions of our model are consistent with soft X-ray
luminosities observed by Chandra in nearby elliptical galaxies and He II
4686 line ratio measured in stacked SDSS spectra of retired
galaxies, the latter characterising the strength and hardness of the UV
radiation field. However, the soft X-ray luminosity and
He~II~4686 ratio are significantly overpredicted for stellar
ages of Gyr. We discuss various possibilities to resolve this
discrepancy and tentatively conclude that it may be resolved by a modification
of the typically used criteria of dynamically unstable mass loss for giant
stars.Comment: 13 pages, 12 figures, MNRAS accepte
Next generation population synthesis of accreting white dwarfs: I. Hybrid calculations using BSE + MESA
Accreting, nuclear-burning white dwarfs have been deemed to be candidate
progenitors of type Ia supernovae, and to account for supersoft X-ray sources,
novae, etc. depending on their accretion rates. We have carried out a binary
population synthesis study of their populations using two algorithms. In the
first, we use the binary population synthesis code \textsf{BSE} as a baseline
for the "rapid" approach commonly used in such studies. In the second, we
employ a "hybrid" approach, in which we use \textsf{BSE} to generate a
population of white dwarfs (WD) with non-degenerate companions on the verge of
filling their Roche lobes. We then follow their mass transfer phase using the
detailed stellar evolution code \textsf{MESA}. We investigate the evolution of
the number of rapidly accreting white dwarfs (RAWDs) and stably nuclear-burning
white dwarfs (SNBWDs), and estimate the type Ia supernovae (SNe Ia) rate
produced by "single-degenerate" systems (SD). We find significant differences
between the two algorithms in the predicted numbers of SNBWDs at early times,
and also in the delay time distribution (DTD) of SD SNe Ia. Such differences in
the treatment of mass transfer may partially account for differences in the SNe
Ia rate and DTD found by different groups. Adopting 100\% efficiency for helium
burning, the rate of SNe Ia produced by the SD-channel in a Milky-way-like
galaxy in our calculations is , more than an
order of magnitude below the observationally inferred value. In agreement with
previous studies, our calculated SD DTD is inconsistent with observations.Comment: 13 pages,11 figures, accepted by MNRA
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