88 research outputs found
Bit Level Correlations in Some Pseudorandom Number Generators
We present results of extensive bit level tests on some pseudorandom number
generators which are commonly used in physics applications. The generators have
first been tested with an extended version of the -tuple test. Second, we
have developed a novel {\it cluster test} where a physical analogy of the
binary numbers with the two dimensional Ising model has been utilized. We
demonstrate that the new test is rather powerful in finding periodic
correlations on bit level. Results of both test methods are presented for each
bit of the output of the generators. Some generators exhibit clear bit level
correlations but we find no evidence of discernible correlations for
generators, which have recently produced systematic errors in Monte Carlo
simulations.Comment: University of Helsinki preprint HU-TFT-93-4
Lake zooplankton delta C-13 values are strongly correlated with the delta C-13 values of distinct phytoplankton taxa
Analyses of carbon stable isotopes are often used to estimate the contributions of allochthonous and autochthonous dietary resources to aquatic consumers. Most pelagic food web studies assume that all phytoplankton taxa have a similar delta C-13 value. We studied pelagic food web compartments (dissolved inorganic carbon [DIC], phytoplankton, bacteria, seston, cladoceran zooplankton) in 12 small (<0.1 km(2)) lakes in southern Finland. These lakes were classified as oligotrophic, mesotrophic, eutrophic, and dystrophic based on their concentrations of total phosphorus and dissolved organic carbon. Additionally, we studied phytoplankton photosynthetic carbon fractionation (epsilon(p)) in laboratory conditions. The photosynthetic fractionation in 28 phytoplankton cultures from nine different phytoplankton classes varied significantly at the class level, and fractionation correlated significantly with the DIC concentration of the growth media. In small boreal lakes, the delta C-13 values of different phytoplankton taxa, as directly measured or estimated from the delta C-13 values of biomarker fatty acids, varied greatly (-18 parts per thousand to - 44.5 parts per thousand). Phytoplankton delta C-13 values varied significantly by lake type and were most depleted in dystrophic lakes even though the delta C-13 values of the DIC was similar to mesotrophic lakes. Further within-taxa variation was found between lakes and between different depths within a lake. Vertical samples from dystrophic lakes also showed lower ep in the phytoplankton from meta-and hypolimnion, possibly as a result of reduced light intensity. Altogether, in nine of the 10 sampled lakes, the delta C-13 values of cladoceran zooplankton were between the minimum and the maximum phytoplankton delta C-13 value of each lake, and thus, phytoplankton alone could explain zooplankton delta C-13 values. We conclude that stable isotope mixing models should take into account carbon variation among different phytoplankton taxa.Peer reviewe
Physical tests for Random Numbers in Simulations
We propose three physical tests to measure correlations in random numbers
used in Monte Carlo simulations. The first test uses autocorrelation times of
certain physical quantities when the Ising model is simulated with the Wolff
algorithm. The second test is based on random walks, and the third on blocks of
n successive numbers. We apply the tests to show that recent errors in high
precision simulations using generalized feedback shift register algorithms are
due to short range correlations in random number sequences. We also determine
the length of these correlations.Comment: 16 pages, Post Script file, HU-TFT-94-
Outcome of Chair-Side Dental Fear Treatment: Long-Term Follow-Up in Public Health Setting
Aim. Purpose of this practice and data-based study was to evaluate the outcome of dental fear treatment of patients referred to the Clinic for Fearful Dental Patients (CFDP) in the primary oral health care, City of Oulu, Finland, during period 2000-2005. Methods. A psychological approach including behavioral interventions and cognitive behavioral therapy (BT/CBT) was used for all participants combined with conscious sedation or dental general anesthesia (DGA), if needed. The outcome was considered successful if later dental visits were carried out without any notifications in the patient records of behavioral problems or sedation. Data collection was made in 2006; the average length of the observation period from the last visit in the CFPD to data collection was 2y 3m (SD 1y 5m). All information was available for 163 patients (mean age 8.9y at referral). Study population was dominated by males (58.0%). Cause for referrals was mostly dental fear (81.0%) or lack of cooperation. Results. The success rate was 69.6% among females and 68.1% among males. Success seemed to be (p=0.053) higher for those treated in 12years compared with the older ones. The participants, without need for dental general anesthesia (DGA) in the CFDP, had significantly a higher success rate (81.4%) compared with those who did (54.8%, p<0.001). Use of conscious oral sedation (p=0.300) or N2O (p=0.585) was not associated with the future success. Conclusions. A chair-side approach seems successful in a primary health care setting for treating dental fear, especially in early childhood. Use of sedation seems not to improve the success rate
Numerical Studies of the Two Dimensional XY Model with Symmetry Breaking Fields
We present results of numerical studies of the two dimensional XY model with
four and eight fold symmetry breaking fields. This model has recently been
shown to describe hydrogen induced reconstruction on the W(100) surface. Based
on mean-field and renormalization group arguments,we first show how the
interplay between the anisotropy fields can give rise to different phase
transitions in the model. When the fields are compatible with each other there
is a continuous phase transition when the fourth order field is varied from
negative to positive values. This transition becomes discontinuous at low
temperatures. These two regimes are separated by a multicritical point. In the
case of competing four and eight fold fields, the first order transition at low
temperatures opens up into two Ising transitions. We then use numerical methods
to accurately locate the position of the multicritical point, and to verify the
nature of the transitions. The different techniques used include Monte Carlo
histogram methods combined with finite size scaling analysis, the real space
Monte Carlo Renormalization Group method, and the Monte Carlo Transfer Matrix
method. Our numerical results are in good agreement with the theoretical
arguments.Comment: 29 pages, HU-TFT-94-36, to appear in Phys. Rev. B, Vol 50, November
1, 1994. A LaTeX file with no figure
A Comparative Study of Some Pseudorandom Number Generators
We present results of an extensive test program of a group of pseudorandom
number generators which are commonly used in the applications of physics, in
particular in Monte Carlo simulations. The generators include public domain
programs, manufacturer installed routines and a random number sequence produced
from physical noise. We start by traditional statistical tests, followed by
detailed bit level and visual tests. The computational speed of various
algorithms is also scrutinized. Our results allow direct comparisons between
the properties of different generators, as well as an assessment of the
efficiency of the various test methods. This information provides the best
available criterion to choose the best possible generator for a given problem.
However, in light of recent problems reported with some of these generators, we
also discuss the importance of developing more refined physical tests to find
possible correlations not revealed by the present test methods.Comment: University of Helsinki preprint HU-TFT-93-22 (minor changes in Tables
2 and 7, and in the text, correspondingly
Theory of adsorbate induced surface reconstruction on W(100)
We report results of a theoretical study on an adsorbate induced surface
reconstruction. Hydrogen adsorption on a W(100) surface causes a switching
transition in the symmetry of the displacements of the W atoms within the
ordered c(2x2) phase. This transition is modeled by an effective Hamiltonian,
where the hydrogen degrees of freedom are integrated out. Based on extensive
Monte Carlo renormalisation group calculations we show that the switching
transition is of second order at high temperatures and of first order at low
temperatures. This behavior is qualitatively explained in terms of an XY model
where there is an interplay between four and eight fold anisotropy fields. We
also compare the calculated phase diagrams with a simple mean field theory.Comment: CSC Preprint, 31 pages (plain TeX file, no figures
Functional Effects of Parasites on Food Web Properties during the Spring Diatom Bloom in Lake Pavin: A Linear Inverse Modeling Analysis
This study is the first assessment of the quantitative impact of parasitic chytrids on a planktonic food web. We used a carbon-based food web model of Lake Pavin (Massif Central, France) to investigate the effects of chytrids during the spring diatom bloom by developing models with and without chytrids. Linear inverse modelling procedures were employed to estimate undetermined flows in the lake. The Monte Carlo Markov chain linear inverse modelling procedure provided estimates of the ranges of model-derived fluxes. Model results support recent theories on the probable impact of parasites on food web function. In the lake, during spring, when ‘inedible’ algae (unexploited by planktonic herbivores) were the dominant primary producers, the epidemic growth of chytrids significantly reduced the sedimentation loss of algal carbon to the detritus pool through the production of grazer-exploitable zoospores. We also review some theories about the potential influence of parasites on ecological network properties and argue that parasitism contributes to longer carbon path lengths, higher levels of activity and specialization, and lower recycling. Considering the “structural asymmetry” hypothesis as a stabilizing pattern, chytrids should contribute to the stability of aquatic food webs
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