9,783 research outputs found
Decidable Models of Recursive Asynchronous Concurrency
Asynchronously communicating pushdown systems (ACPS) that satisfy the
empty-stack constraint (a pushdown process may receive only when its stack is
empty) are a popular decidable model for recursive programs with asynchronous
atomic procedure calls. We study a relaxation of the empty-stack constraint for
ACPS that permits concurrency and communication actions at any stack height,
called the shaped stack constraint, thus enabling a larger class of concurrent
programs to be modelled. We establish a close connection between ACPS with
shaped stacks and a novel extension of Petri nets: Nets with Nested Coloured
Tokens (NNCTs). Tokens in NNCTs are of two types: simple and complex. Complex
tokens carry an arbitrary number of coloured tokens. The rules of NNCT can
synchronise complex and simple tokens, inject coloured tokens into a complex
token, and eject all tokens of a specified set of colours to predefined places.
We show that the coverability problem for NNCTs is Tower-complete. To our
knowledge, NNCT is the first extension of Petri nets, in the class of nets with
an infinite set of token types, that has primitive recursive coverability. This
result implies Tower-completeness of coverability for ACPS with shaped stacks
HoCHC: A Refutationally Complete and Semantically Invariant System of Higher-order Logic Modulo Theories
We present a simple resolution proof system for higher-order constrained Horn
clauses (HoCHC) - a system of higher-order logic modulo theories - and prove
its soundness and refutational completeness w.r.t. the standard semantics. As
corollaries, we obtain the compactness theorem and semi-decidability of HoCHC
for semi-decidable background theories, and we prove that HoCHC satisfies a
canonical model property. Moreover a variant of the well-known translation from
higher-order to 1st-order logic is shown to be sound and complete for HoCHC in
standard semantics. We illustrate how to transfer decidability results for
(fragments of) 1st-order logic modulo theories to our higher-order setting,
using as example the Bernays-Schonfinkel-Ramsey fragment of HoCHC modulo a
restricted form of Linear Integer Arithmetic
Perturbative corrections to the determination of Vub from the P+ spectrum in B->X_u l nu
We investigate the relation between the E_gamma spectrum in B->X_s gamma
decay and the P+ spectrum in semileptonic B->X_u l nu decay (P+ is the hadronic
energy minus the absolute value of the hadronic three-momentum), which provides
in principle the theoretically simplest determination of Vub from any of the
"shape function regions" of B->X_u l nu spectra. We calculate analytically the
P+ spectrum to order alpha_s^2 beta_0, and study its relation to the B->X_s
gamma photon spectrum to eliminate the leading dependence on nonperturbative
effects. We compare the result of fixed order perturbation theory to the
next-to-leading log renormalization group improved calculation, and argue that
fixed order perturbation theory is likely to be a more appropriate expansion.
Implications for the perturbative uncertainties in the determination of Vub
from the P+ spectrum are discussed.Comment: reference added, to appear in PR
Unemployment Insurance and Low-Educated Single Working Mothers Before and After Welfare Reform
Using the Survey of Income and Program Participation (SIPP), a nationally representative, longitudinal survey, this study examines changing levels of Unemployment Insurance (UI) eligibility and benefit receipt among working low-educated single mothers, 1990–2005. It also examines changing participation in cash welfare and the Food Stamp Program (FSP). Relative to single childless women, there has been no increase in UI benefit receipt among single mothers entering a spell of unemployment in the postreform period, even though single mothers have increased their relative rates of UI eligibility. Because of declining cash assistance receipt, UI became a more common income support than cash assistance for this population during the period 2001–2005. Furthermore, the probability of accessing FSP for low-educated single mothers entering a spell of unemployment increased in the years 2001–2005. As a result, the proportion of this population accessing benefits from one or more of these programs remained virtually unchanged across the study period.Welfare Reform, Unemployment Insurance, Low-educated Single Mothers
Survivor-complier effects in the presence of selection on treatment, with application to a study of prompt ICU admission
Pre-treatment selection or censoring (`selection on treatment') can occur
when two treatment levels are compared ignoring the third option of neither
treatment, in `censoring by death' settings where treatment is only defined for
those who survive long enough to receive it, or in general in studies where the
treatment is only defined for a subset of the population. Unfortunately, the
standard instrumental variable (IV) estimand is not defined in the presence of
such selection, so we consider estimating a new survivor-complier causal
effect. Although this effect is generally not identified under standard IV
assumptions, it is possible to construct sharp bounds. We derive these bounds
and give a corresponding data-driven sensitivity analysis, along with
nonparametric yet efficient estimation methods. Importantly, our approach
allows for high-dimensional confounding adjustment, and valid inference even
after employing machine learning. Incorporating covariates can tighten bounds
dramatically, especially when they are strong predictors of the selection
process. We apply the methods in a UK cohort study of critical care patients to
examine the mortality effects of prompt admission to the intensive care unit,
using ICU bed availability as an instrument
Low-coverage heats of adsorption. iii - alkali metal ions on tungsten, atom-metal interaction theory
Heats of adsorption of sodium, rubidium, and cesium ions on tungsten substrat
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