34,353 research outputs found
Strong normalisation for applied lambda calculi
We consider the untyped lambda calculus with constructors and recursively
defined constants. We construct a domain-theoretic model such that any term not
denoting bottom is strongly normalising provided all its `stratified
approximations' are. From this we derive a general normalisation theorem for
applied typed lambda-calculi: If all constants have a total value, then all
typeable terms are strongly normalising. We apply this result to extensions of
G\"odel's system T and system F extended by various forms of bar recursion for
which strong normalisation was hitherto unknown.Comment: 14 pages, paper acceptet at electronic journal LMC
Practical Subtyping for System F with Sized (Co-)Induction
We present a rich type system with subtyping for an extension of System F.
Our type constructors include sum and product types, universal and existential
quantifiers, inductive and coinductive types. The latter two size annotations
allowing the preservation of size invariants. For example it is possible to
derive the termination of the quicksort by showing that partitioning a list
does not increase its size. The system deals with complex programs involving
mixed induction and coinduction, or even mixed (co-)induction and polymorphism
(as for Scott-encoded datatypes). One of the key ideas is to completely
separate the induction on sizes from the notion of recursive programs. We use
the size change principle to check that the proof is well-founded, not that the
program terminates. Termination is obtained by a strong normalization proof.
Another key idea is the use symbolic witnesses to handle quantifiers of all
sorts. To demonstrate the practicality of our system, we provide an
implementation that accepts all the examples discussed in the paper and much
more
A Theory of Explicit Substitutions with Safe and Full Composition
Many different systems with explicit substitutions have been proposed to
implement a large class of higher-order languages. Motivations and challenges
that guided the development of such calculi in functional frameworks are
surveyed in the first part of this paper. Then, very simple technology in named
variable-style notation is used to establish a theory of explicit substitutions
for the lambda-calculus which enjoys a whole set of useful properties such as
full composition, simulation of one-step beta-reduction, preservation of
beta-strong normalisation, strong normalisation of typed terms and confluence
on metaterms. Normalisation of related calculi is also discussed.Comment: 29 pages Special Issue: Selected Papers of the Conference
"International Colloquium on Automata, Languages and Programming 2008" edited
by Giuseppe Castagna and Igor Walukiewic
Multi-frequency study of a new Fe-rich supernova remnant in the Large Magellanic Cloud, MCSNR J0508-6902
We present a detailed radio, X-ray and optical study of a newly discovered
Large Magellanic Cloud (LMC) supernova remnant (SNR) which we denote MCSNR
J0508-6902. Observations from the Australian Telescope Compact Array (ATCA) and
the X-ray observatory are complemented by deep H
images and Anglo Australian Telescope AAOmega spectroscopic data to study the
SNR shell and its shock-ionisation. Archival data at other wavelengths are also
examined. The remnant follows a filled-in shell type morphology in the
radio-continuum and has a size of 74 pc 57 pc at the LMC
distance. The X-ray emission exhibits a faint soft shell morphology with
Fe-rich gas in its interior indicative of a Type Ia origin. The remnant
appears to be mostly dissipated at higher radio-continuum frequencies leaving
only the south-eastern limb fully detectable while in the optical it is the
western side of the SNR shell that is clearly detected. The best-fit
temperature to the shell X-ray emission ( keV) is
consistent with other large LMC SNRs. We determined an O/Fe ratio of and
an Fe mass of 0.5-1.8 in the interior of the remnant, both of which
are consistent with the Type Ia scenario. We find an equipartition magnetic
field for the remnant of 28 G, a value typical of older SNRs and
consistent with other analyses which also infer an older remnant
XMM-Newton observations of three poor clusters: Similarity in dark matter and entropy profiles down to low mass
(Abridged) We present an analysis of the mass and entropy profiles of three
poor clusters (A1991, A2717 and MKW9) observed with XMM-Newton. The clusters
have similar temperatures (kT=2.65, 2.53 and 2.58 keV), and similar redshifts
(0.04 < z < 0.06). We trace the surface brightness, temperature, entropy and
integrated mass profiles up to 0.5 (0.35 for MKW9) of the virial radius
(r_200). The integrated mass profiles are very similar in physical units and
are reasonably well fitted with the NFW mass model with concentration
parameters of c_200=4-6 and M_200=1.2-1.6 X 10^14 h_70^-1 \msun. The entropy
profiles are similar at large scale, but there is some scatter in the central
region (r<50 kpc). None of the clusters has an isentropic core. Including XMM
data on A1983 (kT=2.2 keV), and A1413 (kT = 6.5 keV), we discuss the structural
and scaling properties of cluster mass and entropy profiles. The scaled mass
profiles display <20% dispersion in the 0.05 - 0.5 r_200 radial range. The
c_200 parameters of these clusters, and other values from the literature, are
fully consistent with the c_200 - M_200 relation derived from simulations. The
dispersion in scaled entropy profiles is small, implying self-similarity down
to low mass (kT ~2 keV), and is reduced by 30-40% (to ~20%) if we use the
empirical relation S \propto T^0.65 instead of the standard self-similar
relation, S \propto T. The mean scaled profile is well fitted by a power law
for 0.05 < r_200 < 0.5, with a slope slightly lower than expected from pure
shock heating (\alpha = 0.94+/-0.14), and a normalisation at 0.1 r_200
consistent with previous studies. The gas history thus likely depends both on
gravitational processes and the interplay between cooling and various galaxy
feedback mechanisms.Comment: Final refereed version to appear in A&A. Minor changes. 15 pages, 12
figures (Figs 1 & 3 low res
- …