1,526 research outputs found
Experimental pressure distributions for a family of blunt bodies at Mach numbers from 2.49 to 4.63 and angles of attack from 0 deg to 15 deg
Pressure distributions for blunt body wind tunnel models at supersonic speeds and angles of attack from 0 to 15 degree
Knot Floer homology detects fibred knots
Ozsv\'ath and Szab\'o conjectured that knot Floer homology detects fibred
knots in . We will prove this conjecture for null-homologous knots in
arbitrary closed 3--manifolds. Namely, if is a knot in a closed 3--manifold
, is irreducible, and is monic, then is fibred.
The proof relies on previous works due to Gabai, Ozsv\'ath--Szab\'o, Ghiggini
and the author. A corollary is that if a knot in admits a lens space
surgery, then the knot is fibred.Comment: version 4: incorporates referee's suggestions, to appear in
Inventiones Mathematica
Assessing conceptual knowledge in three online engineering courses: theory of computation and compiler construction, operating systems, and signal and systems
In the current decade understanding conceptual knowledge should be an important area of engineering science. However, it is not as widespread in
this field as it is in the areas of education and psychology. Learning conceptual knowledge in engineering science could help instructors to adapt their
lectures in order to overcome student misconceptions, to reinforce the learning process, and to check whether students are able to identify key
features of a problem. Different methods are provided by authors to assess conceptual knowledge. One is to design and develop a concept inventory
with the objective of identifying possible student misconceptions through multiple-choice questions. Another method consists of asking each student to
answer a question by submitting a written explanation. This study provides two procedures for the assessment of conceptual knowledge based on the
latter method. The first procedure is applied to online computer science students enrolled on an Operating Systems course. The second procedure is
applied to online communication students enrolled on a Signals and Systems course. Both procedures are focused not only on assessing but also on
searching for causes of potential student misconceptions. These procedures could help other instructors to assess conceptual knowledge on other
engineering courses.Universidad a Distancia de Madrid, UDIMA2018-1
Post- and peritraumatic stress in disaster survivors: An explorative study about the influence of individual and event characteristics across different types of disasters
Background:
Examination of existing research on posttraumatic adjustment after disasters suggests that survivors’ posttraumatic stress levels might be better understood by investigating the influence of the characteristics of the event experienced on how people thought and felt, during the event as well as afterwards.
Objective:
To compare survivors’ perceived post- and peritraumatic emotional and cognitive reactions across different types of disasters. Additionally, to investigate individual and event characteristics.
Design:
In a European multi-centre study, 102 survivors of different disasters terror attack, flood, fire and collapse of a building were interviewed about their responses during the event. Survivors’ perceived posttraumatic stress levels were assessed with the Impact of Event Scale-Revised (IES-R). Peritraumatic emotional stress and risk perception were rated retrospectively. Influences of individual characteristics, such as socio-demographic data, and event characteristics, such as time and exposure factors, on post- and peritraumatic outcomes were analyzed.
Results:
Levels of reported post- and peritraumatic outcomes differed significantly between types of disasters. Type of disaster was a significant predictor of all three outcome variables but the factors gender, education, time since event, injuries and fatalities were only significant for certain outcomes.
Conclusion:
Results support the hypothesis that there are differences in perceived post- and peritraumatic emotional and cognitive reactions after experiencing different types of disasters. However, it should be noted that these findings were not only explained by the type of disaster itself but also by individual and event characteristics. As the study followed an explorative approach, further research paths are discussed to better understand the relationships between variables
Noncommutative knot theory
The classical abelian invariants of a knot are the Alexander module, which is
the first homology group of the the unique infinite cyclic covering space of
S^3-K, considered as a module over the (commutative) Laurent polynomial ring,
and the Blanchfield linking pairing defined on this module. From the
perspective of the knot group, G, these invariants reflect the structure of
G^(1)/G^(2) as a module over G/G^(1) (here G^(n) is the n-th term of the
derived series of G). Hence any phenomenon associated to G^(2) is invisible to
abelian invariants. This paper begins the systematic study of invariants
associated to solvable covering spaces of knot exteriors, in particular the
study of what we call the n-th higher-order Alexander module, G^(n+1)/G^(n+2),
considered as a Z[G/G^(n+1)$-module. We show that these modules share almost
all of the properties of the classical Alexander module. They are torsion
modules with higher-order Alexander polynomials whose degrees give lower bounds
for the knot genus. The modules have presentation matrices derived either from
a group presentation or from a Seifert surface. They admit higher-order linking
forms exhibiting self-duality. There are applications to estimating knot genus
and to detecting fibered, prime and alternating knots. There are also
surprising applications to detecting symplectic structures on 4-manifolds.
These modules are similar to but different from those considered by the author,
Kent Orr and Peter Teichner and are special cases of the modules considered
subsequently by Shelly Harvey for arbitrary 3-manifolds.Comment: Published by Algebraic and Geometric Topology at
http://www.maths.warwick.ac.uk/agt/AGTVol4/agt-4-19.abs.htm
Lens space surgeries on A'Campo's divide knots
It is proved that every knot in the major subfamilies of J. Berge's lens
space surgery (i.e., knots yielding a lens space by Dehn surgery) is presented
by an L-shaped (real) plane curve as a "divide knot" defined by N. A'Campo in
the context of singularity theory of complex curves. For each knot given by
Berge's parameters, the corresponding plane curve is constructed. The surgery
coefficients are also considered. Such presentations support us to study each
knot itself, and the relationship among the knots in the set of lens space
surgeries.Comment: 26 pages, 19 figures. The proofs of Theorem 1.3 and Lemma 3.5 are
written down by braid calculus. Section 4 (on the operation Adding squares)
is revised and improved the most. Section 5 is adde
Primitive Words, Free Factors and Measure Preservation
Let F_k be the free group on k generators. A word w \in F_k is called
primitive if it belongs to some basis of F_k. We investigate two criteria for
primitivity, and consider more generally, subgroups of F_k which are free
factors.
The first criterion is graph-theoretic and uses Stallings core graphs: given
subgroups of finite rank H \le J \le F_k we present a simple procedure to
determine whether H is a free factor of J. This yields, in particular, a
procedure to determine whether a given element in F_k is primitive.
Again let w \in F_k and consider the word map w:G x G x ... x G \to G (from
the direct product of k copies of G to G), where G is an arbitrary finite
group. We call w measure preserving if given uniform measure on G x G x ... x
G, w induces uniform measure on G (for every finite G). This is the second
criterion we investigate: it is not hard to see that primitivity implies
measure preservation and it was conjectured that the two properties are
equivalent. Our combinatorial approach to primitivity allows us to make
progress on this problem and in particular prove the conjecture for k=2.
It was asked whether the primitive elements of F_k form a closed set in the
profinite topology of free groups. Our results provide a positive answer for
F_2.Comment: This is a unified version of two manuscripts: "On Primitive words I:
A New Algorithm", and "On Primitive Words II: Measure Preservation". 42
pages, 14 figures. Some parts of the paper reorganized towards publication in
the Israel J. of Mat
Genome-wide mapping of the distribution of CarD, RNAP σA, and RNAP β on the Mycobacterium smegmatis chromosome using chromatin immunoprecipitation sequencing
CarD is an essential mycobacterial protein that binds the RNA polymerase (RNAP) and affects the transcriptional profile of Mycobacterium smegmatis and Mycobacterium tuberculosis [6]. We predicted that CarD was directly regulating RNAP function but our prior experiments had not determined at what stage of transcription CarD was functioning and at which genes CarD interacted with the RNAP. To begin to address these open questions, we performed chromatin immunoprecipitation sequencing (ChIP-seq) to survey the distribution of CarD throughout the M. smegmatis chromosome. The distribution of RNAP subunits β and σA were also profiled. We expected that RNAP β would be present throughout transcribed regions and RNAP σA would be predominantly enriched at promoters based on work in Escherichia coli [3], however this had yet to be determined in mycobacteria. The ChIP-seq analyses revealed that CarD was never present on the genome in the absence of RNAP, was primarily associated with promoter regions, and was highly correlated with the distribution of RNAP σA. The colocalization of σA and CarD led us to propose that in vivo, CarD associates with RNAP initiation complexes at most promoters and is therefore a global regulator of transcription initiation. Here we describe in detail the data from the ChIP-seq experiments associated with the study published by Srivastava and colleagues in the Proceedings of the National Academy of Science in 2013 [5] as well as discuss the findings from this dataset in relation to both CarD and mycobacterial transcription as a whole.
The ChIP-seq data have been deposited in the Gene Expression Omnibus (GEO) database, www.ncbi.nlm.nih.gov/geo (accession no. GSE48164)
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