21 research outputs found
Heegaard Floer homology and concordance bounds on the Thurston norm
We prove that twisted correction terms in Heegaard Floer homology provide
lower bounds on the Thurston norm of certain cohomology classes determined by
the strong concordance class of a 2-component link in . We then
specialise this procedure to knots in , and obtain a lower bound
on their geometric winding number. Furthermore we produce an obstruction for a
knot in to have untwisting number 1. We then provide an infinite family
of null-homologous knots with increasing geometric winding number, on which the
bound is sharp.Comment: With an appendix with Adam Simon Levine; 24 pages, 8 figures;
comments welcome! V2: Fixed a few typos, wrong citations and figures, removed
a proposition. This version to appear in Transactions of the AM
Cuspidal curves and Heegaard Floer homology
We give bounds on the gap functions of the singularities of a cuspidal plane
curve of arbitrary genus, generalising recent work of Borodzik and Livingston.
We apply these inequalities to unicuspidal curves whose singularity has one
Puiseux pair: we prove two identities tying the parameters of the singularity,
the genus, and the degree of the curve; we improve on some degree-multiplicity
asymptotic inequalities; finally, we prove some finiteness results, we
construct infinite families of examples, and in some cases we give an almost
complete classification.Comment: 39 pages, 4 figures. Exposition improved. This preprint version
differs from the final version which is to appear in the Proceedings of the
London Mathematical Societ
Porting of DSMC to multi-GPUs using OpenACC
The Direct Simulation Monte Carlo has become the method of choice for studying gas flows characterized by variable rarefaction and non-equilibrium effects, rising interest in industry for simulating flows in micro-, and nano-electromechanical systems.
However, rarefied gas dynamics represents an open research challenge from the computer science perspective, due to its computational expense compared to continuum computational fluid dynamics methods.
Fortunately, over the last decade, high-performance computing has seen an exponential growth of performance. Actually, with the breakthrough of General-Purpose GPU computing, heterogeneous systems have become widely used for scientific computing, especially in large-scale clusters and supercomputers.
Nonetheless, developing efficient, maintainable and portable applications for hybrid systems is, in general, a non-trivial task.
Among the possible approaches, directive-based programming models, such as OpenACC, are considered the most promising for porting scientific codes to hybrid CPU/GPU systems, both for their simplicity and portability.
This work is an attempt to port a simplified version of the fm dsmc code developed at FLOW Matters Consultancy B.V., a start-up company supporting this project, on a multi-GPU distributed hybrid system, such as Marconi100 hosted at CINECA, using OpenACC.
Finally, we perform a detailed performance analysis of our DSMC application on Volta (NVIDIA V100 GPU) architecture based computing platform as well as a comparison with previous results obtained with x64 86 (Intel Xeon CPU) and ppc64le (IBM Power9 CPU) architectures
Primordial Non-Gaussianity in Supersolid Inflation
We study primordial non-gaussianity in supersolid inflation. The dynamics of
supersolid is formulated in terms of an effective field theory based on four
scalar fields with a shift symmetric action minimally coupled with gravity. In
the scalar sector, there are two phonon-like excitations with a kinetic mixing
stemming from the completely spontaneous breaking of diffeomorphism. In a
squeezed configuration, of scalar perturbations is angle
dependent and not proportional to slow-roll parameters showing a blunt
violation of the Maldacena consistency relation. Contrary to solid inflation,
the violation persists even after an angular average and generically the amount
of non-gaussianity is significant. During inflation, non-gaussianity in the TSS
and TTS sector is enhanced in the same region of the parameters space where the
secondary production of gravitational waves is sizeable enough to enter in the
sensitivity region of LISA, while the scalar is still within
the current experimental limits.Comment: 54 pages, 13 figures. Latex file. A number of typos corrected with
the JHEP versio
Beyond Perturbation Theory in Inflation
Inflationary perturbations are approximately Gaussian and deviations from
Gaussianity are usually calculated using in-in perturbation theory. This
method, however, fails for unlikely events on the tail of the probability
distribution: in this regime non-Gaussianities are important and perturbation
theory breaks down for .
In this paper we show that this regime is amenable to a semiclassical
treatment, . In this limit the wavefunction of the Universe can be
calculated in saddle-point, corresponding to a resummation of all the
tree-level Witten diagrams. The saddle can be found by solving numerically the
classical (Euclidean) non-linear equations of motion, with prescribed boundary
conditions. We apply these ideas to a model with an inflaton self-interaction
. Numerical and analytical methods show that the
tail of the probability distribution of goes as
, with a clear non-perturbative dependence on
the coupling. Our results are relevant for the calculation of the abundance of
primordial black holes.Comment: 37 pages, 14 figures. Matches JCAP versio