813 research outputs found
A Framework for the Flexible Integration of a Class of Decision Procedures into Theorem Provers
The role of decision procedures is often essential in theorem proving. Decision procedures can reduce the search space of heuristic components of a prover and increase its abilities. However, in some applications only a small number of conjectures fall within the scope of the available decision procedures. Some of these conjectures could in an informal sense fall âjust outsideâ that scope. In these situations a problem arises because lemmas have to be invoked or the decision procedure has to communicate with the heuristic component of a theorem prover. This problem is also related to the general problem of how to exibly integrate decision procedures into heuristic theorem provers. In this paper we address such problems and describe a framework for the exible integration of decision procedures into other proof methods. The proposed framework can be used in different theorem provers, for different theories and for different decision procedures. New decision procedures can be simply âplugged-inâ to the system. As an illustration, we describe an instantiation of this framework within the Clam proof-planning system, to which it is well suited. We report on some results using this implementation
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Diffuse HI Disks in Isolated Galaxies
In order to investigate the contribution of diffuse components to their total
HI emission, we have obtained high precision HI line flux densities with the
100m Green Bank Telescope for a sample of 100 isolated spiral and irregular
galaxies which we have previously observed with the 43m telescope. A comparison
of the observed HI line fluxes obtained with the two different telescopes,
characterized by half-power beam widths of 9 arcmin and 21 arcmin respectively,
exploits a ``beam matching'' technique to yield a statistical determination of
the occurrence of diffuse HI components in their disks. A simple model of the
HI distribution within a galaxy well describes ~75 % of the sample and accounts
for all of the HI line flux density. The remaining galaxies are approximately
evenly divided into two categories: ones which appear to possess a
significantly more extensive HI distribution than the model predicts, and ones
for which the HI distribution is more centrally concentrated than predicted.
Examples of both extremes can be found in the literature but little attention
has been paid to the centrally concentrated HI systems. Our sample has
demonstrated that galaxies do not commonly possess extended regions of low
surface brightness HI gas which is not accounted for by our current
understanding of the structure of HI disks. Eight HI-rich companions to the
target objects are identified, and a set of extragalactic HI line flux density
calibrators is presented.Comment: 26 page
On the Design of Cryptographic Primitives
The main objective of this work is twofold. On the one hand, it gives a brief
overview of the area of two-party cryptographic protocols. On the other hand,
it proposes new schemes and guidelines for improving the practice of robust
protocol design. In order to achieve such a double goal, a tour through the
descriptions of the two main cryptographic primitives is carried out. Within
this survey, some of the most representative algorithms based on the Theory of
Finite Fields are provided and new general schemes and specific algorithms
based on Graph Theory are proposed
Gas Rich Dwarfs from the PSS-II III. HI Profiles and Dynamical Masses
We present Arecibo neutral hydrogen data on a sample of optically selected
dwarf galaxies. The sample ranges in HI mass from 10^6 M_sun to 5x10^9 M_sun,
with a mean of 7.9x10^8 M_sun. Using estimated HI radii, the HI surface
densities range from 0.6 to 20 M_sun pc^-2, all well below the critical
threshold for star formation (Kennicutt 1998). M_HI/L values of the LSB dwarfs
range from 0.3 to 12 with a mean value of 2.0. Dynamical masses, calculated
from the HI profile widths, range from 10^8 M_sun to 10^11 M_sun. There is a
strong correlation between optical luminosity and dynamical mass for LSB dwarfs
implying that the dark matter (whether baryonic or non-baryonic) follows the
detectable baryonic matter.Comment: 53 pages, AASTeX v4.0, 8 figures, to be published in ApJ Suppl,
images, tables and referee report can be found at http://zebu.uoregon.edu/~j
Starburst in the Intragroup Medium of Stephan's Quintet
Based on new ISO mid-infrared observations and ground based and
near-infrared observations, we report the detection of a bright starburst in
the intragroup medium (IGM) of the famous compact group of galaxies Stephan's
Quintet (Source A in Fig.1). We demonstrate that this starburst is caused by a
collision between a high velocity (V 1000 km/sec) intruder galaxy
(NGC7318b) and the IGM of the group. While this is the only starburst known
today that is induced by a galaxy/cold-intergalactic-medium collision, it
provides new constraints to the theory for interaction-induced starbursts, and
may hint at a new mechanism for the star formation excess seen in more distant
clusters.Comment: 17 pages, 2 PS figures. Accepted by Ap
The Tully-Fisher Relation and H_not
The use of the Tully-Fisher (TF) relation for the determination of the Hubble
Constant relies on the availability of an adequate template TF relation and of
reliable primary distances. Here we use a TF template relation with the best
available kinematical zero-point, obtained from a sample of 24 clusters of
galaxies extending to cz ~ 9,000 km/s, and the most recent set of Cepheid
distances for galaxies fit for TF use. The combination of these two ingredients
yields H_not = 69+/-5 km/(s Mpc). The approach is significantly more accurate
than the more common application with single cluster (e.g. Virgo, Coma)
samples.Comment: 10 pages, including 2 figures and 1 table; uses AAS LaTex. Submitted
to ApJ Letter
A Digital Archive of HI 21 cm Line Spectra of Optically-targeted Galaxies
We present a homogeneous compilation of HI spectral parameters extracted from
global 21 cm line spectra for some 9000 galaxies in the local universe
(heliocentric velocity -200 < V_Sun < 28,000 km/s) obtained with a variety of
large single dish radio telescopes but reanalyzed using a single set of
parameter extraction algorithms. Corrections to the observed HI line flux for
source extent and pointing offsets and to the HI line widths for instrumental
broadening and smoothing are applied according to model estimates to produce a
homogenous catalog of derived properties with quantitative error estimates.
Where the redshift is available from optical studies, we also provide flux
measurements for an additional 156 galaxies classified as marginal HI
detections and rms noise limits for 494 galaxies classified as nondetections.
Given the diverse nature of the observing programs contributing to it, the
characteristics of the combined dataset are heterogeneous, and as such, the
compilation is neither integrated HI line flux nor peak flux limited. However,
because of the large statistical base and homogenous reprocessing, the spectra
and spectral parameters of galaxies in this optically targeted sample can be
used to complement data obtained at other wavelengths to characterize the
properties of galaxies in the local universe and to explore the large scale
structures in which they reside.Comment: 13 pages, 9 figures, 3 external online tables, accepted for
publication in ApJ
That Was Close! Reward Reporting of Cybersecurity âNear Missesâ
Building, deploying, and maintaining systems with sufficient cybersecurity is challenging. Faster improvement would be valuable to society as a whole. Are we doing as much as we can to improve? We examine robust and long-standing systems for learning from near misses in aviation, and propose the creation of a Cyber Safety Reporting System (CSRS).
To support this argument, we examine the liability concerns which inhibit learning, including both civil and regulatory liability. We look to the way in which cybersecurity engineering and science is done today, and propose that a small amount of âpolicy entrepreneurshipâ could have substantial positive impact. We close by considering how a CSRS should be organized and housed
Design study of Software-Implemented Fault-Tolerance (SIFT) computer
Software-implemented fault tolerant (SIFT) computer design for commercial aviation is reported. A SIFT design concept is addressed. Alternate strategies for physical implementation are considered. Hardware and software design correctness is addressed. System modeling and effectiveness evaluation are considered from a fault-tolerant point of view
- âŠ