1,136 research outputs found
Obtaining Formal Models through Non-Monotonic Refinement
When designing a model for formal verification, we want to\ud
be certain that what we proved about the model also holds for the system we modelled. This raises the question of whether our model represents the system, and what makes us confident about this. By performing so called, non-monotonic refinement in the modelling process, we make the steps and decisions explicit. This helps us to (1) increase the confidence that the model represents the system, (2) structure and organize the communication with domain experts and the problem owner, and (3) identify rational steps made while modelling. We focus on embedded control systems
A nonet of novel species of Monanthotaxis (Annonaceae) from around Africa
As part of an ongoing revision of the genus Monanthotaxis Baill. (Annonaceae), nine new species are described and one variety is reinstated to species rank. Two new species from West Africa (Monanthotaxis aquila P.H. Hoekstra, sp. nov. and Monanthotaxis atewensis P.H. Hoekstra, sp. nov.), four new species from Central Africa (Monanthotaxis couvreurii P.H. Hoekstra, sp. nov., Monanthotaxis latistamina P.H. Hoekstra, sp. nov., Monanthotaxis tripetala P.H. Hoekstra, sp. nov. and Monanthotaxis zenkeri P.H. Hoekstra, sp. nov.), one new species from Tanzania (Monanthotaxis filipes P.H. Hoekstra, sp. nov.), one new species from the area around Maputo (Monanthotaxis maputensis P.H. Hoekstra, sp. nov.), one new species from the Comoro Islands (Monanthotaxis komorensis P.H. Hoekstra, sp. nov.) and Monanthotaxis klainei (Engl.) Verdc. var. angustifolia (Boutique) Verdc. is raised to species level leading to the replacement name Monanthotaxis atopostema P.H. Hoekstra, nom. nov. (not Monanthotaxis angustifolia (Exell) Verdc.). Complete descriptions, comparisons with related species, ecological information and IUCN conservation assessments are given for the new species. Five species were classified as critical endangered, two species as endangered, one as vulnerable and one as least concern, warranting the need of further collecting and studying those species
A new species of Monanthotaxis from Gabon with a unique inflorescence type for Annonaceae
Monanthotaxis Baillon (1890: 878) currently consists of 56 species (Rainer & Chatrou 2006) confined to tropical Africa and Madagascar and is the second most species-rich genus of Annonaceae in Africa after Uvaria Linnaeus (1753: 536). Both genera belong to the tribe Uvarieae Hooker & Thomson (1855: 91, 92). Circumscription of this tribe has recently been modified to comply with the principle of monophyly, and it now almost exclusively consists of climbing species, all from the Old World tropics (Chatrou et al. 2012). Generic circumscription within Uvarieae has been in disarray for considerable time. Delimitation of Uvaria and related genera has recently been modified based on phylogenetic relationships (Zhou et al. 2010, Zhou et al. 2009). Monanthotaxis was monophyletic in Wang et al. (2012), based on a limited sampling of seven species. Subsequent study with increased sampling (Hoekstra, unpub.) has revealed that the African species of Friesodielsia van Steenis (1948: 458) and Exellia Boutique (1951b: 117) are nested in Monanthotaxis. Whatever the solution and taxonomic consequences, the name Monanthotaxis with the type Monanthotaxis congoensis Baillon (1890: 879) will be retained as it is the oldest valid generic name. Along with phylogenetic analysis, we are conducting a taxonomic revision. The last revision of Monanthotaxis and allied genera was published over a century ago by Engler & Diels (1901). Since then, only contributions to local floras have been published (e.g. Boutique 1951a, Le Thomas 1969, Robson 1960, Verdcourt 1971a). While studying the material of Monanthotaxis, we encountered a remarkable new species, which differs from all other species of Annonaceae in its large and lax panicle-like inflorescence. Panicle-like inflorescences are rare in Annonaceae, and those that have been recorded are either congested, as in e.g. Unonopsis and Guatteria (Erkens et al. 2008, Maas et al. 2007), or with only a few flowers, as in Monanthotaxis le-testui Pellegrin (1950: 75). This new species is probably closely related to M. congoensis since they share several characters. Verdcourt (1971b) divided the genus in three subgenera and five sections. In his classification, this new species would join M. congoensis in the typical section Monanthotaxis, which is easily distinguished by having flowers with the four to six petals in a single whorl and less than 17 stamens. Because it is so similar to M. congoensis, our new species will almost certainly be classified within Monanthotaxis, and we decided to publish it before a new generic classification has been completed
SN1998bw: The Case for a Relativistic Shock
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.Comment: A&A (in press), Rome GRB Symposium, Nov. 199
The inheritance of dynamic and deontic integrity constraints or: Does the boss have more rights?
In [18,23], we presented a language for the specification of static, dynamic and deontic integrity constraints (IC's) for conceptual models (CM's). An important problem not discussed in that paper is how IC's are inherited in a taxonomic network of types. For example, if students are permitted to perform certain actions under certain preconditions, must we repeat these preconditions when specializing this action for the subtype of graduate students, or are they inherited, and if so, how? For static constraints, this problem is relatively trivial, but for dynamic and deontic constraints, it will turn out that it contains numerous pitfalls, caused by the fact that common sense supplies presuppositions about the structure of IC inheritance that are not warranted by logic. In this paper, we unravel some of these presuppositions and show how to avoid the pitfalls. We first formulate a number of general theorems about the inheritance of necessary and/or sufficient conditions and show that for upward inheritance, a closure assumption is needed. We apply this to dynamic and deontic IC's, where conditions arepreconditions of actions, and show that our common sense is sometimes mistaken about the logical implications of what we have specified. We also show the connection of necessary and sufficient preconditions of actions with the specification of weakest preconditions in programming logic. Finally, we argue that information analysts usually assume constraint completion in the specification of (pre)conditions analogous to predicate completion in Prolog and circumscription in non-monotonic logic. The results are illustrated with numerous examples and compared with other approaches in the literature
Clarifying the generic limits of Talbotiella and Hymenostegia (Detarieae, Caesalpinioideae, Leguminosae)
The formal description of four species of Cameroonian forest legume trees new to science has been hampered by uncertainty as to whether their correct generic placement is within Hymenostegia Harms or Talbotiella Baker f. As there has been doubt as to whether these two genera differ from one another, an investigation was undertaken so that the new species could be correctly assigned to genus. Using morphological, molecular and pollen data, our study supports the recognition of Hymenostegia and Talbotiella as distinct genera, consequently the new species are correctly placed in Talbotiella. In addition, our data reveal the extensive heterogeneous nature of Hymenostegia as currently circumscribed and the need to transfer H. breteleri to Talbotiella
Mogelijkheden voor een industriële ontwikkeling langs de oever van de Westerschelde ten oosten van Terneuzen
Constraints on Off-Axis GRB Jets in Type Ibc Supernovae From Late-Time Radio Observations
It has been suggested that the peculiar properties of the luminous Type Ic
supernova SN 1998bw and its low-energy gamma-ray burst GRB 980425 may be
understood if they originated in a standard gamma-ray burst explosion viewed
far from the axis of the relativistic jet. In this scenario, strong radio
emission is predicted from the jet on a timescale 1 to 10 years after the
explosion as it decelerates and spreads into our line of sight. To test this
hypothesis we have carried out late-time radio observations of SN 1998bw at
years, yielding upper limits which are consistent with the continued
fading of the supernova. We find these limits to be consistent with an off-axis
jet only if the progenitor mass loss rate is
M yr (for a wind velocity km s) or the
fraction of the shock energy in magnetic fields is . These values are low relative to those inferred for cosmological
GRBs. We combine the SN 1998bw measurements with existing observations for a
sample of 15 local Type Ibc supernovae to estimate that at most 6% produce
collimated, relativistic outflows.Comment: Revised version, as it appears in ApJ
Architecture-based Qualitative Risk Analysis for Availability of IT Infrastructures
An IT risk assessment must deliver the best possible quality of results in a time-effective way. Organisations are used to customise the general-purpose standard risk assessment methods in a way that can satisfy their requirements. In this paper we present the QualTD Model and method, which is meant to be employed together with standard risk assessment methods for the qualitative assessment of availability risks of IT architectures, or parts of them. The QualTD Model is based on our previous quantitative model, but geared to industrial practice since it does not require quantitative data which is often too costly to acquire. We validate the model and method in a real-world case by performing a risk assessment on the authentication and authorisation system of a large multinational company and by evaluating the results w.r.t. the goals of the stakeholders of the system. We also perform a review of the most popular standard risk assessment methods and an analysis of which one can be actually integrated with our QualTD Model
Actors, actions, and initiative in normative system specification
The logic of norms, called deontic logic, has been used to specify normative constraints for information systems. For example, one can specify in deontic logic the constraints that a book borrowed from a library should be returned within three weeks, and that if it is not returned, the library should send a reminder. Thus, the notion of obligation to perform an action arises naturally in system specification. Intuitively, deontic logic presupposes the concept of anactor who undertakes actions and is responsible for fulfilling obligations. However, the concept of an actor has not been formalized until now in deontic logic. We present a formalization in dynamic logic, which allows us to express the actor who initiates actions or choices. This is then combined with a formalization, presented earlier, of deontic logic in dynamic logic, which allows us to specify obligations, permissions, and prohibitions to perform an action. The addition of actors allows us to expresswho has the responsibility to perform an action. In addition to the application of the concept of an actor in deontic logic, we discuss two other applications of actors. First, we show how to generalize an approach taken up by De Nicola and Hennessy, who eliminate from CCS in favor of internal and external choice. We show that our generalization allows a more accurate specification of system behavior than is possible without it. Second, we show that actors can be used to resolve a long-standing paradox of deontic logic, called the paradox of free-choice permission. Towards the end of the paper, we discuss whether the concept of an actor can be combined with that of an object to formalize the concept of active objects
- …