747 research outputs found
Simulated-tempering approach to spin-glass simulations
After developing an appropriate iteration procedure for the determination of
the parameters, the method of simulated tempering has been successfully applied
to the 2D Ising spin glass. The reduction of the slowing down is comparable to
that of the multicanonical algorithm. Simulated tempering has, however, the
advantages to allow full vectorization of the programs and to provide the
canonical ensemble directly.Comment: 12 pages (LaTeX), 4 postscript figures, uufiles encoded, submitted to
Physical Review
The smallest eigenvalue of Hankel matrices
Let H_N=(s_{n+m}),n,m\le N denote the Hankel matrix of moments of a positive
measure with moments of any order. We study the large N behaviour of the
smallest eigenvalue lambda_N of H_N. It is proved that lambda_N has exponential
decay to zero for any measure with compact support. For general determinate
moment problems the decay to 0 of lambda_N can be arbitrarily slow or
arbitrarily fast. In the indeterminate case, where lambda_N is known to be
bounded below by a positive constant, we prove that the limit of the n'th
smallest eigenvalue of H_N for N tending to infinity tends rapidly to infinity
with n. The special case of the Stieltjes-Wigert polynomials is discussed
Generalizations of the Diffie-Hellman protocol : exposition and implementation
A generalisation of the Diffie-Hellman protocol is studied in this dissertation. In the generalisation polynomials are used to reduce the representation size of a public key and linear shift registers for more efficient computations. These changes are important for the implementation of the protocol in con- strained environments. The security of the Diffie-Hellman protocol and its generalisation is based on the same computations problems. Lastly three examples of the generalisation and their implementation are discussed. For two of the protocols, models are given to predict the execution time and it is determined how well these model predictions are.Dissertation (MSc (Applied Mathematics))--University of Pretoria, 2007.Mathematics and Applied MathematicsMScunrestricte
Natuurkunde-vraagstukken-oplossen : een vakdidactische studie van het leren oplossen van natuurkundevraagstukken in klas vier vwo
Determining the density of states for classical statistical models: A random walk algorithm to produce a flat histogram
We describe an efficient Monte Carlo algorithm using a random walk in energy
space to obtain a very accurate estimate of the density of states for classical
statistical models. The density of states is modified at each step when the
energy level is visited to produce a flat histogram. By carefully controlling
the modification factor, we allow the density of states to converge to the true
value very quickly, even for large systems. This algorithm is especially useful
for complex systems with a rough landscape since all possible energy levels are
visited with the same probability. In this paper, we apply our algorithm to
both 1st and 2nd order phase transitions to demonstrate its efficiency and
accuracy. We obtained direct simulational estimates for the density of states
for two-dimensional ten-state Potts models on lattices up to
and Ising models on lattices up to . Applying this approach to
a 3D spin glass model we estimate the internal energy and entropy at
zero temperature; and, using a two-dimensional random walk in energy and
order-parameter space, we obtain the (rough) canonical distribution and energy
landscape in order-parameter space. Preliminary data suggest that the glass
transition temperature is about 1.2 and that better estimates can be obtained
with more extensive application of the method.Comment: 22 pages (figures included
Separation of Time Scales in a Quantum Newton’s Cradle
We provide detailed modeling of the Bragg pulse used in quantum Newton’s-cradle-like settings or in Bragg spectroscopy experiments for strongly repulsive bosons in one dimension. We reconstruct the postpulse time evolution and study the time-dependent local density profile and momentum distribution by a combination of exact techniques. We further provide a variety of results for finite interaction strengths using a time-dependent Hartree-Fock analysis and bosonization-refermionization techniques. Our results display a clear separation of time scales between rapid and trap-insensitive relaxation immediately after the pulse, followed by slow in-trap periodic behavior
Pulmonary hypertension after transjugular intrahepatic portosystemic shunt (TIPS)
We reported the case of a patient in whom severe, and ultimately fatal,
pulmonary hypertension developed 1.5 yrs after transjugular intrahepatic
portosystemic shunt (TIPS). Pulmonary artery pressures were not affected
by 100% oxygen, prostacyclin or nifedipine. Postmortem examinations showed
pulmonary and vascular abnormalities typical of pulmonary hypertension.
Pulmonary artery pressures should be measured in each patient with
otherwise not readily explained dyspnoea following transjugular
intrahepatic portosystemic shunt
Evaluation of the distributions of hydroxylated glycerol dibiphytanyl glycerol tetraethers (GDGTs) in Holocene Baltic Sea sediments for reconstruction of sea surface temperature: The effect of changing salinity
Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT proxies for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH′ indices, were tested using a thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22 ∘C, as well as 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise in temperature, but their absolute values were not always in line with existing marine core-top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH′ index shows only a linear correlation with SST when the sediments from the Bothnian Sea and Bothnian Bay area, with a reduced salinity and increased lateral sediment influx, are omitted from the dataset. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH′ index shows a good correlation with the TEX, an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH′ (and RI-OH) index records values that are far too high, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano's water column. In the Littorina Sea, Ancylus Lake, and Yoldia Lake phases of the Baltic basin, the record of the RI-OH′ index thus most likely reflects changes in both temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH′ (and RI-OH) indices substantially, and this should be considered when applying these proxies in other settings
Electron neutrino tagging through tertiary lepton detection
We discuss an experimental technique aimed at tagging electron neutrinos in
multi-GeV artificial sources on an event-by-event basis. It exploits in a novel
manner calorimetric and tracking technologies developed in the framework of the
LHC experiments and of rare kaon decay searches. The setup is suited for
slow-extraction, moderate power beams and it is based on an instrumented decay
tunnel equipped with tagging units that intercept secondary and tertiary
leptons from the bulk of undecayed \pi^+ and protons. We show that the taggers
are able to reduce the \nue contamination originating from K_e3 decays by about
one order of magnitude. Only a limited suppression (~60%) is achieved for \nue
produced by the decay-in-flight of muons; for low beam powers, similar
performance as for K_e3 can be reached supplementing the tagging system with an
instrumented beam dump.Comment: 19 pages, 7 figures; minor changes, version to appear in EPJ
Associated syndromes and other genetic variations at a South African cleft lip and palate clinic
A retrospective study was done of data on all patients registered at one of the largest cleft lip and palate clinics in South Africa (n = 3174). The associated syndromes and other genetic variations [(abbreviation:) ASGV] found in the population of persons suffering from facial cleft deformities (FCD) were analysed. 832 (26.2%) cleft lip and/ or palate patients presented with ASGV. Fifty-seven different types of syndromes were recorded of which the Fairbaim-Robin appearance (FRA) (or Pierre Robin sequence) 169 (5.3%), the Demarque-van der Woude syndrome 40 (1.3%), and the holoprosencephaly sequence cases 32 (1.0%) were the three most common ones. The three most common genetic variations found in the non-syndromic patients, were heart involvement 53(1.7%), club foot 42 (1.3%) and various eye problems 39 (1.2%). The main facial cleft deformity, namely the cleft lip, alveolus and palate (CLAP), was found in 26.2% o f the ASGV-group. This particular cleft deformity was recorded at 39.7% in the FCD clinic. On the other hand, the hard and soft palate cleft (hPsP) group was found in 32.9% of patients who also had ASGV; in the total group o f patients registered at the clinic, it accounted for only 16.6%. This means that ASGV occur less commonly in the CLAP group of patients, than in the hPsP group of patients.http://www.curationis.org.zaam2013ay201
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