7,668 research outputs found
On type II superstrings in bosonic backgrounds: the role of fermions and T-duality
We derive the actions for type II Green-Schwarz strings up to second order in
the fermions, for general bosonic backgrounds. We base our work on the
so-called normal coordinate expansion. The resulting actions are
-symmetric and, for the case of surviving background supersymmetries,
supersymmetric. We first obtain the type IIa superstring action from the 11D
supermembrane by double dimensional reduction. Then, by means of a
generalization of T-duality we derive the type IIb superstring action. The
resulting actions are surprisingly simple and elegant.Comment: 10 pages, latex. more typos correcte
Actions and Fermionic symmetries for D-branes in bosonic backgrounds
In this article we derive the full interacting effective actions for
supersymmetric D-branes in arbitrary bosonic type II supergravity backgrounds.
The actions are presented in terms of component fields up to second order in
fermions. As one expects, the actions are built from the supercovariant
derivative operator and the -symmetry projector. The results take a
compact and elegant form exhibiting -symmetry, as well as supersymmetry
in a background with Killing spinors. We give the explicit transformation rules
for these symmetries in all cases, including the M2-brane. As an example, we
analyze the N=2 super-worldvolume field theory defined by a test D4-brane in
the supergravity background produced by a large number of D0-branes. This
example displays rigid supersymmetry in a curved spacetime.Comment: 33 pages, typos correcte
The explicit form of the effective action for F1 and D-branes
In this work we consider the full interacting effective actions for
fundamental strings and D-branes in arbitrary bosonic type II supergravity
backgrounds. The explicit form of these actions is given in terms of component
fields, up to second order in the fermions. The results take a compact form
exhibiting -symmetry, as well as supersymmetry in a background with
Killing spinors. Also we give the explicit transformation rules for these
symmetries in all cases.Comment: 9 pages, based on the talk given at the RTN workshop of 2003
Copenhagen: 15/9-20/9/03. Published in the proceedings of the conferenc
Anti-Powers in Infinite Words
In combinatorics of words, a concatenation of consecutive equal blocks is
called a power of order . In this paper we take a different point of view
and define an anti-power of order as a concatenation of consecutive
pairwise distinct blocks of the same length. As a main result, we show that
every infinite word contains powers of any order or anti-powers of any order.
That is, the existence of powers or anti-powers is an unavoidable regularity.
Indeed, we prove a stronger result, which relates the density of anti-powers to
the existence of a factor that occurs with arbitrary exponent. As a
consequence, we show that in every aperiodic uniformly recurrent word,
anti-powers of every order begin at every position. We further show that every
infinite word avoiding anti-powers of order is ultimately periodic, while
there exist aperiodic words avoiding anti-powers of order . We also show
that there exist aperiodic recurrent words avoiding anti-powers of order .Comment: Revision submitted to Journal of Combinatorial Theory Series
Flag-Dipole Spinor Fields in ESK Gravities
We consider the Riemann-Cartan geometry as a basis for the
Einstein-Sciama-Kibble theory coupled to spinor fields: we focus on and
conformal gravities, regarding the flag-dipole spinor fields, type-(4) spinor
fields under the Lounesto classification. We study such theories in specific
cases given for instance by cosmological scenarios: we find that in such
background the Dirac equation admits solutions that are not Dirac spinor
fields, but in fact the aforementioned flag-dipoles ones. These solutions are
important from a theoretical perspective, as they evince that spinor fields are
not necessarily determined by their dynamics, but also a discussion on their
structural (algebraic) properties must be carried off. Furthermore, the
phenomenological point of view is shown to be also relevant, since for
isotropic Universes they circumvent the question whether spinor fields do
undergo the Cosmological Principle.Comment: 18 pages, improved versio
Differential expression and biochemical activity of the immune receptor Tim-3 in healthy and malignant human myeloid cells
The T cell immunoglobulin and mucin domain 3 (Tim-3) is a plasma membrane-associated receptor which is involved in a variety of biological responses in human immune cells. It is highly expressed in most acute myeloid leukaemia (AML) cells and therefore may serve as a possible target for AML therapy. However, its biochemical activities in primary human AML cells remain unclear. We therefore analysed the total expression and surface presence of the Tim-3 receptor in primary human AML blasts and healthy primary human leukocytes isolated from human blood. We found that Tim-3 expression was significantly higher in primary AML cells compared to primary healthy leukocytes. Tim-3 receptor molecules were distributed largely on the surface of primary AML cells, whereas in healthy leukocytes Tim-3 protein was mainly expressed intracellularly. In primary human AML blasts, both Tim-3 agonistic antibody and galectin-9 (a Tim-3 natural ligand) significantly upregulated mTOR pathway activity. This was in line with increased accumulation of hypoxia-inducible factor 1 alpha (HIF-1α) and secretion of VEGF and TNF-α. Similar results were obtained in primary human healthy leukocytes. Importantly, in both types of primary cells, Tim-3-mediated effects were compared with those induced by lipopolysaccharide (LPS) and stem cell factor (SCF). Tim-3 induced comparatively moderate responses in both AML cells and healthy leukocytes. However, Tim-3, like LPS, mediated the release of both TNF-α and VEGF, while SCF induced mostly VEGF secretion and did not upregulate TNF-α release
Robust leave-one-out cross-validation for high-dimensional Bayesian models
Leave-one-out cross-validation (LOO-CV) is a popular method for estimating
out-of-sample predictive accuracy. However, computing LOO-CV criteria can be
computationally expensive due to the need to fit the model multiple times. In
the Bayesian context, importance sampling provides a possible solution but
classical approaches can easily produce estimators whose variance is infinite,
making them potentially unreliable. Here we propose and analyze a novel mixture
estimator to compute Bayesian LOO-CV criteria. Our method retains the
simplicity and computational convenience of classical approaches, while
guaranteeing finite variance of the resulting estimators. Both theoretical and
numerical results are provided to illustrate the improved robustness and
efficiency. The computational benefits are particularly significant in
high-dimensional problems, allowing to perform Bayesian LOO-CV for a broader
range of models. The proposed methodology is easily implementable in standard
probabilistic programming software and has a computational cost roughly
equivalent to fitting the original model once.Comment: 37 pages, 8 figure
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