238,858 research outputs found
µ-Charts and Z: Examples and extensions
μ-Charts are a way of specifying reactive systems, i.e. systems which are in some environment to which they have to react, based on the well-established formalism Statecharts. This paper gives (very abbreviated) examples of translating μ-charts to Z, which is itself a well-established language for specifying computational systems with tried and tested methods and support tools which guide its effective use in systems development. We undertake this translation in order that investigation of the modelled system can be performed before expensive and lengthy implementation is considered. We also present an extension of the μ-charts and the related Z to deal with a simple command language, local variables and integer-valued signals
An Exercise in Invariant-based Programming with Interactive and Automatic Theorem Prover Support
Invariant-Based Programming (IBP) is a diagram-based correct-by-construction
programming methodology in which the program is structured around the
invariants, which are additionally formulated before the actual code. Socos is
a program construction and verification environment built specifically to
support IBP. The front-end to Socos is a graphical diagram editor, allowing the
programmer to construct invariant-based programs and check their correctness.
The back-end component of Socos, the program checker, computes the verification
conditions of the program and tries to prove them automatically. It uses the
theorem prover PVS and the SMT solver Yices to discharge as many of the
verification conditions as possible without user interaction. In this paper, we
first describe the Socos environment from a user and systems level perspective;
we then exemplify the IBP workflow by building a verified implementation of
heapsort in Socos. The case study highlights the role of both automatic and
interactive theorem proving in three sequential stages of the IBP workflow:
developing the background theory, formulating the program specification and
invariants, and proving the correctness of the final implementation.Comment: In Proceedings THedu'11, arXiv:1202.453
How many dark energy parameters?
For exploring the physics behind the accelerating universe a crucial question
is how much we can learn about the dynamics through next generation
cosmological experiments. For example, in defining the dark energy behavior
through an effective equation of state, how many parameters can we
realistically expect to tightly constrain? Through both general and specific
examples (including new parametrizations and principal component analysis) we
argue that the answer is 42 - no, wait, two. Cosmological parameter analyses
involving a measure of the equation of state value at some epoch (e.g. w_0) and
a measure of the change in equation of state (e.g. w') are therefore realistic
in projecting dark energy parameter constraints. More elaborate
parametrizations could have some uses (e.g. testing for bias or comparison with
model features), but do not lead to accurately measured dark energy parameters.Comment: 10 pages, 1 figur
Dark Before Light: Testing the Cosmic Expansion History through the Cosmic Microwave Background
The cosmic expansion history proceeds in broad terms from a radiation
dominated epoch to matter domination to an accelerated, dark energy dominated
epoch. We investigate whether intermittent periods of acceleration are possible
in the early universe -- between Big Bang nucleosynthesis (BBN) and
recombination and beyond. We establish that the standard picture is remarkably
robust: observations of anisotropies in the cosmic microwave background exclude
any extra period of accelerated expansion between 1 \leq z \lesssim 10^5
(corresponding to 5\times10^{-4}\ {\rm eV} \leq T \lesssim 25\ {\rm eV}).Comment: 7 pages, 5 figure
Experimental constraints on the polarizabilities of the 6s^2 1S0 and 6s6p 3P0 states of Yb
We utilize accurate experimental data available in the literature to yield
bounds on the polarizabilities of the ground and first excited states of atomic
Yb. For the 6s^2 1S0 ground state, we find the polarizability alpha to be
constrained to 134.4<alpha<144.2 in atomic units, while for the 6s6p 3P0
excited state we find 280.1<alpha<289.9. The uncertainty in each of these
values is 1.0. These constraints provide a valuable check for ab initio and
semi-empirical methods used to compute polarizabilities and other related
properties in Yb.Comment: 7 pages, 1 figur
Robust State Space Filtering under Incremental Model Perturbations Subject to a Relative Entropy Tolerance
This paper considers robust filtering for a nominal Gaussian state-space
model, when a relative entropy tolerance is applied to each time increment of a
dynamical model. The problem is formulated as a dynamic minimax game where the
maximizer adopts a myopic strategy. This game is shown to admit a saddle point
whose structure is characterized by applying and extending results presented
earlier in [1] for static least-squares estimation. The resulting minimax
filter takes the form of a risk-sensitive filter with a time varying risk
sensitivity parameter, which depends on the tolerance bound applied to the
model dynamics and observations at the corresponding time index. The
least-favorable model is constructed and used to evaluate the performance of
alternative filters. Simulations comparing the proposed risk-sensitive filter
to a standard Kalman filter show a significant performance advantage when
applied to the least-favorable model, and only a small performance loss for the
nominal model
A kinematical approach to dark energy studies
We present and employ a new kinematical approach to cosmological `dark
energy' studies. We construct models in terms of the dimensionless second and
third derivatives of the scale factor a(t) with respect to cosmic time t,
namely the present-day value of the deceleration parameter q_0 and the cosmic
jerk parameter, j(t). An elegant feature of this parameterization is that all
LCDM models have j(t)=1 (constant), which facilitates simple tests for
departures from the LCDM paradigm. Applying our model to the three best
available sets of redshift-independent distance measurements, from type Ia
supernovae and X-ray cluster gas mass fraction measurements, we obtain clear
statistical evidence for a late time transition from a decelerating to an
accelerating phase. For a flat model with constant jerk, j(t)=j, we measure
q_0=-0.81+-0.14 and j=2.16+0.81-0.75, results that are consistent with LCDM at
about the 1sigma confidence level. A standard `dynamical' analysis of the same
data, employing the Friedmann equations and modeling the dark energy as a fluid
with an equation of state parameter, w (constant), gives
Omega_m=0.306+0.042-0.040 and w=-1.15+0.14-0.18, also consistent with LCDM at
about the 1sigma level. In comparison to dynamical analyses, the kinematical
approach uses a different model set and employs a minimum of prior information,
being independent of any particular gravity theory. The results obtained with
this new approach therefore provide important additional information and we
argue that both kinematical and dynamical techniques should be employed in
future dark energy studies, where possible. Our results provide further
interesting support for the concordance LCDM paradigm.Comment: 12 pages, 5 figures, 2 tables. Accepted for publication in MNRA
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