466 research outputs found
Invariants and separating morphisms for algebraic group actions
The first part of this paper is a refinement of Winkelmannâs work on invariant rings and quotients of algebraic group actions on affine varieties, where we take a more geometric point of view. We show that the (algebraic) quotient X//G given by the possibly not finitely generated ring of invariants is âalmostâ an algebraic variety, and that the quotient morphism Ï: X â X//G has a number of nice properties. One of the main difficulties comes from the fact that the quotient morphism is not necessarily surjective. These general results are then refined for actions of the additive group Ga, where we can say much more. We get a rather explicit description of the so-called plinth variety and of the separating variety, which measures how much orbits are separated by invariants. The most complete results are obtained for representations. We also give a complete and detailed analysis of Robertsâ famous example of a an action of Ga on 7-dimensional affine space with a non-finitely generated ring of invariants
On the ideals of equivariant tree models
We introduce equivariant tree models in algebraic statistics, which unify and
generalise existing tree models such as the general Markov model, the strand
symmetric model, and group based models. We focus on the ideals of such models.
We show how the ideals for general trees can be determined from the ideals for
stars. The main novelty is our proof that this procedure yields the entire
ideal, not just an ideal defining the model set-theoretically. A corollary of
theoretical importance is that the ideal for a general tree is generated by the
ideals of its flattenings at vertices.Comment: 23 pages. Greatly improved exposition, in part following suggestions
by a referee--thanks! Also added exampl
Drought alters the spatial distribution, grazing patterns, and radula morphology of a fungal-farming salt marsh snail
Contains fulltext :
208086.pdf (publisher's version ) (Open Access
Colorectal cancer care and patientsâ perceptions before and during COVID-19: implications for subsequent SARS-CoV-2 infection waves
BACKGROUND: Changes in colorectal cancer (CRC) care planning due to the coronavirus disease 2019 (COVID-19) pandemic and associated health-related quality of life (HRQoL) and well-being of patients with CRC are unknown. We report changes in CRC care and patient-reported outcomes (PROs) including HRQoL, distress, and loneliness during the first wave of SARS-CoV-2. METHODS: In April 2020, 4,984 patients included in the nationwide Prospective Dutch Colorectal Cancer cohort were invited to complete a COVID-19-specific questionnaire, together with the validated EORTC QLQ-C30, De Jong Gierveld, and HADS. Clinical data were obtained from the Netherlands Cancer Registry. Scores were compared with the year prior to COVID-19, and with an age- and sex-matched control population during COVID-19. RESULTS: In total, 3,247 (65.1%) patients responded between April and June 2020. Seventeen percent of patients had cancelled/postponed/changed hospital visits into a telephone- or video consult while 5.3% had adjusted/postponed/cancelled treatment. Compared to controls, patients reported worse HRQoL, but comparable distress and less social loneliness (patients = 21.2%; controls = 32.9%). Compared to pre-COVID-19, clinically meaningful deterioration of HRQoL was more prevalent in patients with changes in cancer care planning than in patients without changes. Prior to undergoing or currently undergoing treatment, and infection worries were associated with lower HRQoL. CONCLUSIONS: CRC patients reported equal anxiety and depression, but worse HRQoL than the control population. Changes in care planning were associated with deterioration of HRQoL and increased anxiety. In case of one or more risk factors, healthcare specialists should discuss (mental) health status and possible support during future SARS-CoV-2 infection waves or comparable pandemics
A functorial construction of moduli of sheaves
We show how natural functors from the category of coherent sheaves on a
projective scheme to categories of Kronecker modules can be used to construct
moduli spaces of semistable sheaves. This construction simplifies or clarifies
technical aspects of existing constructions and yields new simpler definitions
of theta functions, about which more complete results can be proved.Comment: 52 pp. Dedicated to the memory of Joseph Le Potier. To appear in
Inventiones Mathematicae. Slight change in the definition of the Kronecker
algebra in Secs 1 (p3) and 2.2 (p6), with corresponding small alterations
elsewhere, to make the constructions work for non-reduced schemes. Section
6.5 rewritten. Remark 2.6 and new references adde
On Arnold's 14 `exceptional' N=2 superconformal gauge theories
We study the four-dimensional superconformal N=2 gauge theories engineered by
the Type IIB superstring on Arnold's 14 exceptional unimodal singularities
(a.k.a. Arnold's strange duality list), thus extending the methods of 1006.3435
to singularities which are not the direct sum of minimal ones. In particular,
we compute their BPS spectra in several `strongly coupled' chambers.
From the TBA side, we construct ten new periodic Y-systems, providing
additional evidence for the existence of a periodic Y-system for each isolated
quasi-homogeneous singularity with (more generally, for each N=2
superconformal theory with a finite BPS chamber whose chiral primaries have
dimensions of the form N/l).Comment: 73 pages, 7 figure
Quiver Structure of Heterotic Moduli
We analyse the vector bundle moduli arising from generic heterotic
compactifications from the point of view of quiver representations. Phenomena
such as stability walls, crossing between chambers of supersymmetry, splitting
of non-Abelian bundles and dynamic generation of D-terms are succinctly encoded
into finite quivers. By studying the Poincar\'e polynomial of the quiver moduli
space using the Reineke formula, we can learn about such useful concepts as
Donaldson-Thomas invariants, instanton transitions and supersymmetry breaking.Comment: 38 pages, 5 figures, 1 tabl
Radio Astronomy
Contains table of contents for Section 4 and reports on five research projects.National Science Foundation Grant AST 92-24191MIT Lincoln Laboratory Agreement BX-4975National Aeronautics and Space Administration/Goddard Space Flight Center Grant NAS 5-31376National Aeronautics and Space Administration/Goddard Space Flight Center Grant NAG5-10MIT Leaders for Manufacturing Progra
Predicting mental imagery based BCI performance from personality, cognitive profile and neurophysiological patterns
Mental-Imagery based Brain-Computer Interfaces (MI-BCIs) allow their users to send commands
to a computer using their brain-activity alone (typically measured by ElectroEncephaloGraphyâ
EEG), which is processed while they perform specific mental tasks. While very
promising, MI-BCIs remain barely used outside laboratories because of the difficulty
encountered by users to control them. Indeed, although some users obtain good control
performances after training, a substantial proportion remains unable to reliably control an
MI-BCI. This huge variability in user-performance led the community to look for predictors of
MI-BCI control ability. However, these predictors were only explored for motor-imagery
based BCIs, and mostly for a single training session per subject. In this study, 18 participants
were instructed to learn to control an EEG-based MI-BCI by performing 3 MI-tasks, 2
of which were non-motor tasks, across 6 training sessions, on 6 different days. Relationships
between the participantsâ BCI control performances and their personality, cognitive
profile and neurophysiological markers were explored. While no relevant relationships with
neurophysiological markers were found, strong correlations between MI-BCI performances
and mental-rotation scores (reflecting spatial abilities) were revealed. Also, a predictive
model of MI-BCI performance based on psychometric questionnaire scores was proposed.
A leave-one-subject-out cross validation process revealed the stability and reliability of this
model: it enabled to predict participantsâ performance with a mean error of less than 3
points. This study determined how usersâ profiles impact their MI-BCI control ability and
thus clears the way for designing novel MI-BCI training protocols, adapted to the profile of
each user
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