70,689 research outputs found
Parameterized Construction of Program Representations for Sparse Dataflow Analyses
Data-flow analyses usually associate information with control flow regions.
Informally, if these regions are too small, like a point between two
consecutive statements, we call the analysis dense. On the other hand, if these
regions include many such points, then we call it sparse. This paper presents a
systematic method to build program representations that support sparse
analyses. To pave the way to this framework we clarify the bibliography about
well-known intermediate program representations. We show that our approach, up
to parameter choice, subsumes many of these representations, such as the SSA,
SSI and e-SSA forms. In particular, our algorithms are faster, simpler and more
frugal than the previous techniques used to construct SSI - Static Single
Information - form programs. We produce intermediate representations isomorphic
to Choi et al.'s Sparse Evaluation Graphs (SEG) for the family of data-flow
problems that can be partitioned per variables. However, contrary to SEGs, we
can handle - sparsely - problems that are not in this family
From Brownian Dynamics to Markov Chain: an Ion Channel Example
A discrete rate theory for general multi-ion channels is presented, in which
the continuous dynamics of ion diffusion is reduced to transitions between
Markovian discrete states. In an open channel, the ion permeation process
involves three types of events: an ion entering the channel, an ion escaping
from the channel, or an ion hopping between different energy minima in the
channel. The continuous dynamics leads to a hierarchy of Fokker-Planck
equations, indexed by channel occupancy. From these the mean escape times and
splitting probabilities (denoting from which side an ion has escaped) can be
calculated. By equating these with the corresponding expressions from the
Markov model the Markovian transition rates can be determined. The theory is
illustrated with a two-ion one-well channel. The stationary probability of
states is compared with that from both Brownian dynamics simulation and the
hierarchical Fokker-Planck equations. The conductivity of the channel is also
studied, and the optimal geometry maximizing ion flux is computed.Comment: submitted to SIAM Journal on Applied Mathematic
A General Theory of Sample Complexity for Multi-Item Profit Maximization
The design of profit-maximizing multi-item mechanisms is a notoriously
challenging problem with tremendous real-world impact. The mechanism designer's
goal is to field a mechanism with high expected profit on the distribution over
buyers' values. Unfortunately, if the set of mechanisms he optimizes over is
complex, a mechanism may have high empirical profit over a small set of samples
but low expected profit. This raises the question, how many samples are
sufficient to ensure that the empirically optimal mechanism is nearly optimal
in expectation? We uncover structure shared by a myriad of pricing, auction,
and lottery mechanisms that allows us to prove strong sample complexity bounds:
for any set of buyers' values, profit is a piecewise linear function of the
mechanism's parameters. We prove new bounds for mechanism classes not yet
studied in the sample-based mechanism design literature and match or improve
over the best known guarantees for many classes. The profit functions we study
are significantly different from well-understood functions in machine learning,
so our analysis requires a sharp understanding of the interplay between
mechanism parameters and buyer values. We strengthen our main results with
data-dependent bounds when the distribution over buyers' values is
"well-behaved." Finally, we investigate a fundamental tradeoff in sample-based
mechanism design: complex mechanisms often have higher profit than simple
mechanisms, but more samples are required to ensure that empirical and expected
profit are close. We provide techniques for optimizing this tradeoff
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Integrating Energy Markets: Does Sequencing Matter?
This paper addresses three questions that are relevant to integrating different regional transmission areas. Market integrating normally increases the number of competitors and should therefore reduce prices but the first section shows that prices could rise when the number of generators initially increases. Regulatory effort will also be affected by market integration. If the number of generators in either market is low, then our analysis suggests that the outcome depends on whether the regulators act independently or co-ordinate. Finally, if markets are gradually combined into larger units, the choice of transmission allocation (auctions or market coupling) will affect the prospects of making further gains and hence could lead to incomplete reform.Cambridge-MIT Institut
Charm, Beauty and Top at HERA
Results on open charm and beauty production and on the search for top
production in high-energy electron-proton collisions at HERA are reviewed. This
includes a discussion of relevant theoretical aspects, a summary of the
available measurements and measurement techniques, and their impact on improved
understanding of QCD and its parameters, such as parton density functions and
charm- and beauty-quark masses. The impact of these results on measurements at
the LHC and elsewhere is also addressed.Comment: 103 pages, 60 figures, to be published in Prog. Part. Nucl. Phy
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