151 research outputs found
Undecidability of a weak version of MSO+U
We prove the undecidability of MSO on ω-words extended with the second-order predicate U1(X) which says that the distance between consecutive positions in a set X⊆N is unbounded. This is achieved by showing that adding U1 to MSO gives a logic with the same expressive power as MSO+U, a logic on ω-words with undecidable satisfiability. As a corollary, we prove that MSO on ω-words becomes undecidable if allowing to quantify over sets of positions that are ultimately periodic, i.e., sets X such that for some positive integer p, ultimately either both or none of positions x and x+p belong to X
Two-Variable Logic over Countable Linear Orderings
We study the class of languages of finitely-labelled countable linear orderings definable in two-variable first-order logic. We give a number of characterisations, in particular an algebraic one in terms of circle monoids, using equations. This generalises the corresponding characterisation, namely variety DA, over finite words to the countable case. A corollary is that the membership in this class is decidable: for instance given an MSO formula it is possible to check if there is an equivalent two-variable logic formula over countable linear orderings. In addition, we prove that the satisfiability problems for two-variable logic over arbitrary, countable, and scattered linear orderings are NEXPTIME-complete
Weighted One-Deterministic-Counter Automata
We introduce weighted one-deterministic-counter automata (ODCA). These are
weighted one-counter automata (OCA) with the property of counter-determinacy,
meaning that all paths labelled by a given word starting from the initial
configuration have the same counter-effect. Weighted ODCAs are a strict
extension of weighted visibly OCAs, which are weighted OCAs where the input
alphabet determines the actions on the counter.
We present a novel problem called the co-VS (complement to a vector space)
reachability problem for weighted ODCAs over fields, which seeks to determine
if there exists a run from a given configuration of a weighted ODCA to another
configuration whose weight vector lies outside a given vector space. We
establish two significant properties of witnesses for co-VS reachability: they
satisfy a pseudo-pumping lemma, and the lexicographically minimal witness has a
special form. It follows that the co-VS reachability problem is in P.
These reachability problems help us to show that the equivalence problem of
weighted ODCAs over fields is in P by adapting the equivalence proof of
deterministic real-time OCAs by B\"ohm et al. This is a step towards resolving
the open question of the equivalence problem of weighted OCAs. Furthermore, we
demonstrate that the regularity problem, the problem of checking whether an
input weighted ODCA over a field is equivalent to some weighted automaton, is
in P. Finally, we show that the covering and coverable equivalence problems for
uninitialised weighted ODCAs are decidable in polynomial time. We also consider
boolean ODCAs and show that the equivalence problem for (non-deterministic)
boolean ODCAs is in PSPACE, whereas it is undecidable for (non-deterministic)
boolean OCAs.Comment: 36 pages, 11 figure
Wildbook: Crowdsourcing, computer vision, and data science for conservation
Photographs, taken by field scientists, tourists, automated cameras, and
incidental photographers, are the most abundant source of data on wildlife
today. Wildbook is an autonomous computational system that starts from massive
collections of images and, by detecting various species of animals and
identifying individuals, combined with sophisticated data management, turns
them into high resolution information database, enabling scientific inquiry,
conservation, and citizen science.
We have built Wildbooks for whales (flukebook.org), sharks (whaleshark.org),
two species of zebras (Grevy's and plains), and several others. In January
2016, Wildbook enabled the first ever full species (the endangered Grevy's
zebra) census using photographs taken by ordinary citizens in Kenya. The
resulting numbers are now the official species census used by IUCN Red List:
http://www.iucnredlist.org/details/7950/0. In 2016, Wildbook partnered up with
WWF to build Wildbook for Sea Turtles, Internet of Turtles (IoT), as well as
systems for seals and lynx. Most recently, we have demonstrated that we can now
use publicly available social media images to count and track wild animals.
In this paper we present and discuss both the impact and challenges that the
use of crowdsourced images can have on wildlife conservation.Comment: Presented at the Data For Good Exchange 201
Supernova search with active learning in ZTF DR3
We provide the first results from the complete SNAD adaptive learning
pipeline in the context of a broad scope of data from large-scale astronomical
surveys. The main goal of this work is to explore the potential of adaptive
learning techniques in application to big data sets. Our SNAD team used Active
Anomaly Discovery (AAD) as a tool to search for new supernova (SN) candidates
in the photometric data from the first 9.4 months of the Zwicky Transient
Facility (ZTF) survey, namely, between March 17 and December 31 2018 (58194 <
MJD < 58483). We analysed 70 ZTF fields at a high galactic latitude and
visually inspected 2100 outliers. This resulted in 104 SN-like objects being
found, 57 of which were reported to the Transient Name Server for the first
time and with 47 having previously been mentioned in other catalogues, either
as SNe with known types or as SN candidates. We visually inspected the
multi-colour light curves of the non-catalogued transients and performed
fittings with different supernova models to assign it to a probable photometric
class: Ia, Ib/c, IIP, IIL, or IIn. Moreover, we also identified unreported
slow-evolving transients that are good superluminous SN candidates, along with
a few other non-catalogued objects, such as red dwarf flares and active
galactic nuclei. Beyond confirming the effectiveness of human-machine
integration underlying the AAD strategy, our results shed light on potential
leaks in currently available pipelines. These findings can help avoid similar
losses in future large-scale astronomical surveys. Furthermore, the algorithm
enables direct searches of any type of data and based on any definition of an
anomaly set by the expert.Comment: 22 pages with appendix, 12 figures, 2 tables, accepted for
publication in Astronomy and Astrophysic
Network Geometry and Complexity
(28 pages, 11 figures)Higher order networks are able to characterize data as different as functional brain networks, protein interaction networks and social networks beyond the framework of pairwise interactions. Most notably higher order networks include simplicial complexes formed not only by nodes and links but also by triangles, tetrahedra, etc. More in general, higher-order networks can be cell-complexes formed by gluing convex polytopes along their faces. Interestingly, higher order networks have a natural geometric interpretation and therefore constitute a natural way to explore the discrete network geometry of complex networks. Here we investigate the rich interplay between emergent network geometry of higher order networks and their complexity in the framework of a non-equilibrium model called Network Geometry with Flavor. This model, originally proposed for capturing the evolution of simplicial complexes, is here extended to cell-complexes formed by subsequently gluing different copies of an arbitrary regular polytope. We reveal the interplay between complexity and geometry of the higher order networks generated by the model by studying the emergent community structure and the degree distribution as a function of the regular polytope forming its building blocks. Additionally, we discuss the underlying hyperbolic nature of the emergent geometry and we relate the spectral dimension of the higher-order network to the dimension and nature of its building blocks
Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes
As the relative performance of alkali activated slag (AAS) concretes in comparison to portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6 % Na2O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance
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