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At least 73 records · Page 4Linked to original sources
PubMed2003

Survival probability of a diffusing particle in the presence of Poisson-distributed mobile traps.

The problem of a diffusing particle moving among diffusing traps is analyzed in general space dimension d. We consider the case where the traps are initially randomly distributed in space, with uniform density rho, and derive upper and lower bounds for the probability Q(t) (averaged over all particle and trap trajectories) that the particle survives up to time t. We show that, for 1< or =d< or =2, the bounds converge asymptotically to give Q(t) approximately exp(-lambda(d)t(d/2)) for 1< or =d<2, where lambda(d)=(2/pid)sin(pid/2)(4piD)(d/2)rho and D is the diffusion constant of the traps, and that Q(t) approximately exp(-4pirhoDt/ln t) for d=2. For d>2 bounds can still be derived, but they no longer converge for large t. For 1< or =d< or =2, these asymptotic forms are independent of the diffusion constant of the particle. The results are compared with simulation results obtained using a new algorithm [V. Mehra and P. Grassberger, Phys. Rev. E 65, 050101 (2002)] which is described in detail. Deviations from the predicted asymptotic forms are found to be large even for very small values of Q(t), indicating slowly decaying corrections whose form is consistent with the bounds. We also present results in d=1 for the case where the trap densities on either side of the particle are different. For this case we can still obtain exact bounds but they no longer converge.

Journal Article↗
PubMed1982

A method of fitting the gravity model based on the Poisson distribution.

"In this paper, [the authors] suggest an alternative method for fitting the gravity model. In this method, the interaction variable is treated as the outcome of a discrete probability process, whose mean is a function of the size and distance variables. This treatment seems appropriate when the dependent variable represents a count of the number of items (people, vehicles, shipments) moving from one place to another. It would seem to have special advantages where there are some pairs of places between which few items move. The argument will be illustrated with reference to data on the numbers of migrants moving in 1970-1971 between pairs of the 126 labor market areas defined for Great Britain...."

Demography↗
PubMed1976

[Application of a truncated Poisson distribution for the counting of virus plaques by micromethod. Variation of its relative inaccuracy].

The microculture plaque titration of virus shows an important disadvantage : the reduction of the areas observation brings out plaque convergence and prevents from their exact enumeration. Nevertheless, a method, proposed by Moore, enables the estimation of virus title. The influence of the dilution and repetition factors on the title precision has been investigated. Two examples are treated.

Methods↗
PubMed1984

The effects of sample size on reticulocyte counting and stool examination. The binomial and poisson distributions in laboratory medicine.

In tests that involve counting or searching for discrete objects, clinical laboratory specimens that are too small produce insensitive and imprecise test results. We found that ten of 57 surveyed New England hospital laboratories examine less than a 50-mg stool sample for parasite eggs; they are, therefore, unlikely to detect infections characterized by low concentrations of eggs in stool. Conversely, all 57 laboratories use the traditional, appropriate sample size of at least 1,000 RBCs for reticulocyte counting. In this report, we discuss theoretical and practical considerations in small sample testing.

Blood Cell Count↗
PubMed2006

Cellular mitochondrial heterogeneity in cultured astrocytes as demonstrated by immunogold labeling of alpha-ketoglutarate dehydrogenase.

In brain cells, various metabolites and metabolic pathways, largely of mitochondrial origin, have been shown to be compartmentalized. Attention has therefore been focused on the possible existence of mitochondrial heterogeneity in the brain at the cellular level. To determine whether mitochondria in cultured cortical and cerebellar astrocytes are heterogeneous at the single cell level, immunogold electron microscopy and an antibody against the alpha-ketoglutarate dehydrogenase component of the alpha-ketoglutarate dehydrogenase complex, a marker enzyme for the tricarboxylic acid (TCA) cycle, were employed. The number of gold particles was counted in the mitochondria of 36 and 42 cells from cultured cerebellar and cortical astrocytes, respectively. A test for random distribution (Poisson distribution) of mitochondria according to the number of gold particles was subsequently performed for every one of the 36 and 42 cells as the ratio variance/mean (= index of dispersion). This should be approximately distributed as chi2/degrees of freedom (df) = n - 1, n = number of mitochondria), if the observations obeyed a Poisson distribution. For 26 of the 36 (cerebellar astrocytes) distributions and for 28 of the 42 (cortical astrocytes) distributions a random distribution had to be rejected. These findings therefore strongly indicate that alpha-ketoglutarate dehydrogenase is heterogeneously distributed in mitochondria within individual astrocytes originating either from cerebellum or cerebral cortex. In conclusion, this study underlines the probability that mitochondrial heterogeneity at the single cell level might be extended to involve other metabolic pathways and metabolites.

Algorithms↗