8,140 research outputs found
Speeding up the FMMR perfect sampling algorithm: A case study revisited
In a previous paper by the second author,two Markov chain Monte Carlo perfect
sampling algorithms -- one called coupling from the past (CFTP) and the other
(FMMR) based on rejection sampling -- are compared using as a case study the
move-to-front (MTF) self-organizing list chain. Here we revisit that case study
and, in particular, exploit the dependence of FMMR on the user-chosen initial
state. We give a stochastic monotonicity result for the running time of FMMR
applied to MTF and thus identify the initial state that gives the
stochastically smallest running time; by contrast, the initial state used in
the previous study gives the stochastically largest running time. By changing
from worst choice to best choice of initial state we achieve remarkable speedup
of FMMR for MTF; for example, we reduce the running time (as measured in Markov
chain steps) from exponential in the length n of the list nearly down to n when
the items in the list are requested according to a geometric distribution. For
this same example, the running time for CFTP grows exponentially in n.Comment: 19 pages. See also http://www.mts.jhu.edu/~fill/ and
http://www.mathcs.carleton.edu/faculty/bdobrow/. Submitted for publication in
May, 200
Creating Social Media Policies in Public Libraries
James, as a librarian and as a trustee, talks about the benefits and pitfalls of creating social-media policies for a small public library in Vermont
Influence of organic films on the evaporation and condensation of water in aerosol
Uncertainties in quantifying the kinetics of evaporation and condensation of water from atmospheric aerosol are a significant contributor to the uncertainty in predicting cloud droplet number and the indirect effect of aerosols on climate. The influence of aerosol particle surface composition, particularly the impact of surface active organic films, on the condensation and evaporation coefficients remains ambiguous. Here, we report measurements of the influence of organic films on the evaporation and condensation of water from aerosol particles. Significant reductions in the evaporation coefficient are shown to result when condensed films are formed by monolayers of long-chain alcohols [C(n)H((2n+1))OH], with the value decreasing from 2.4 × 10(−3) to 1.7 × 10(−5) as n increases from 12 to 17. Temperature-dependent measurements confirm that a condensed film of long-range order must be formed to suppress the evaporation coefficient below 0.05. The condensation of water on a droplet coated in a condensed film is shown to be fast, with strong coherence of the long-chain alcohol molecules leading to islanding as the water droplet grows, opening up broad areas of uncoated surface on which water can condense rapidly. We conclude that multicomponent composition of organic films on the surface of atmospheric aerosol particles is likely to preclude the formation of condensed films and that the kinetics of water condensation during the activation of aerosol to form cloud droplets is likely to remain rapid
A proposal for a scalable universal bosonic simulator using individually trapped ions
We describe a possible architecture to implement a universal bosonic
simulator (UBS) using trapped ions. Single ions are confined in individual
traps, and their motional states represent the bosonic modes. Single-mode
linear operators, nonlinear phase-shifts, and linear beam splitters can be
realized by precisely controlling the trapping potentials. All the processes in
a bosonic simulation, except the initialization and the readout, can be
conducted beyond the Lamb-Dicke regime. Aspects of our proposal can also be
applied to split adiabatically a pair of ions in a single trap
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