119 research outputs found
Weighted Birkhoff Averages and the Parameterization Method
This work provides a systematic recipe for computing accurate high order
Fourier expansions of quasiperiodic invariant circles in area preserving maps.
The recipe requires only a finite data set sampled from the quasiperiodic
circle. Our approach, being based on the parameterization method, uses a Newton
scheme to iteratively solve a conjugacy equation describing the invariant
circle. A critical step in properly formulating the conjugacy equation is to
determine the rotation number of the quasiperiodic subsystem. For this we
exploit a the weighted Birkhoff averaging method. This approach facilities
accurate computation of the rotation number given nothing but the already
mentioned orbit data.
The weighted Birkhoff averages also facilitate the computation of other
integral observables like Fourier coefficients of the parameterization of the
invariant circle. Since the parameterization method is based on a Newton
scheme, we only need to approximate a small number of Fourier coefficients with
low accuracy to find a good enough initial approximation so that Newton
converges. Moreover, the Fourier coefficients may be computed independently, so
we can sample the higher modes to guess the decay rate of the Fourier
coefficients. This allows us to choose, a-priori, an appropriate number of
modes in the truncation. We illustrate the utility of the approach for explicit
example systems including the area preserving Henon map and the standard map.
We present example computations for invariant circles with period as low as 1
and up to more than 100. We also employ a numerical continuation scheme to
compute large numbers of quasiperiodic circles in these systems. During the
continuation we monitor the Sobolev norm of the Parameterization to
automatically detect the breakdown of the family.Comment: 38 pages, 15 figure
Design of a Mechanical NaK Pump for Fission Space Power Systems
Alkali liquid metal cooled fission reactor concepts are under development for mid-range spaceflight power requirements. One such concept utilizes a sodium-potassium eutectic (NaK) as the primary loop working fluid. Traditionally, linear induction pumps have been used to provide the required flow and head conditions for liquid metal systems but can be limited in performance. This paper details the design, build, and check-out test of a mechanical NaK pump. The pump was designed to meet reactor cooling requirements using commercially available components modified for high temperature NaK service
Cheon\u27s algorithm, pairing inversion and the discrete logarithm problem
We relate the fixed argument pairing inversion problems (FAPI) and the discrete logarithm problem on an elliptic curve. This is done using the reduction from the DLP to the Diffie-Hellman problem developed by Boneh, Lipton, Maurer and Wolf. This approach fails when only one of the FAPI problems can be solved. In this case we use Cheon\u27s algorithm to get a reduction
Moir\'e band structures of twisted phosphorene bilayers
We report on the theoretical electronic spectra of twisted phosphorene
bilayers exhibiting moir\'e patterns, as computed by means of a continuous
approximation to the moir\'e superlattice Hamiltonian. Our model is constructed
by interpolating between effective -point conduction- and valence-band
Hamiltonians for the different stacking configurations approximately realized
across the moir\'e supercell, formulated on symmetry grounds. We predict the
realization of three distinct regimes for -point electrons and holes at
different twist angle ranges: a Hubbard regime for small twist angles , where the electronic states form arrays of quantum-dot-like states,
one per moir\'e supercell. A Tomonaga-Luttinger regime at intermediate twist
angles , characterized by the appearance of
arrays of quasi-1D states. Finally, a ballistic regime at large twist angles
, where the band-edge states are delocalized, with
dispersion anisotropies modulated by the twist angle. Our method correctly
reproduces recent results based on large-scale ab initio calculations at a much
lower computational cost, and with fewer restrictions on the twist angles
considered.Comment: 20 pages, including 10 figures and 5 appendice
Pairings on hyperelliptic curves with a real model
We analyse the efficiency of pairing computations on hyperelliptic curves given by a real model using a balanced divisor at infinity. Several optimisations are proposed and analysed. Genus two curves given by a real model arise when considering pairing friendly groups of order dividing . We compare the performance of pairings on such groups in both elliptic and hyperelliptic versions. We conclude that pairings can be efficiently computable in real models of hyperelliptic curves
Securing the legacy of TESS through the care and maintenance of TESS planet ephemerides
Much of the science from the exoplanets detected by the TESS mission relies
on precisely predicted transit times that are needed for many follow-up
characterization studies. We investigate ephemeris deterioration for simulated
TESS planets and find that the ephemerides of 81% of those will have expired
(i.e. 1 mid-transit time uncertainties greater than 30 minutes) one
year after their TESS observations. We verify these results using a sample of
TESS planet candidates as well. In particular, of the simulated planets that
would be recommended as JWST targets by Kempton et al. (2018), 80% will
have mid-transit time uncertainties 30 minutes by the earliest time JWST
would observe them. This rapid deterioration is driven primarily by the
relatively short time baseline of TESS observations. We describe strategies for
maintaining TESS ephemerides fresh through follow-up transit observations. We
find that the longer the baseline between the TESS and the follow-up
observations, the longer the ephemerides stay fresh, and that 51% of simulated
primary mission TESS planets will require space-based observations. The
recently-approved extension to the TESS mission will rescue the ephemerides of
most (though not all) primary mission planets, but the benefits of these new
observations can only be reaped two years after the primary mission
observations. Moreover, the ephemerides of most primary mission TESS planets
(as well as those newly discovered during the extended mission) will again have
expired by the time future facilities such as the ELTs, Ariel and the possible
LUVOIR/OST missions come online, unless maintenance follow-up observations are
obtained.Comment: 16 pages, 10 figures, accepted to AJ; main changes are cross-checking
results against the sample of real TOIs, and addressing the impact of the
TESS extended missio
- …