1,806 research outputs found
Counting irregular multigraphs
AbstractGagliardi et al. (1996, unpublished manuscript) defined an irregular multigraph to be a loopless multigraph with degree sequence n, n − 1,…, 1, and they posed the problem of determining the number of different irregular multigraphs fn on n vertices. In Gagliardi et al. (1996) they showed that if n ≡ 0 or 3 (mod 4) then fn > n − 1. In this note our aim is to show that there are constants 1 < c1 < c2 and n0 > 0 such that if n ⩾ n0 and n ≡ 0 or 3 (mod 4) then (c1)n2 < fn < (c2)n2. Indeed, we show that c1 = 1.19 and c2 = 1.65 can be chosen
Seismic modeling using the frozen Gaussian approximation
We adopt the frozen Gaussian approximation (FGA) for modeling seismic waves.
The method belongs to the category of ray-based beam methods. It decomposes
seismic wavefield into a set of Gaussian functions and propagates these
Gaussian functions along appropriate ray paths. As opposed to the classic
Gaussian-beam method, FGA keeps the Gaussians frozen (at a fixed width) during
the propagation process and adjusts their amplitudes to produce an accurate
approximation after summation. We perform the initial decomposition of seismic
data using a fast version of the Fourier-Bros-Iagolnitzer (FBI) transform and
propagate the frozen Gaussian beams numerically using ray tracing. A test using
a smoothed Marmousi model confirms the validity of FGA for accurate modeling of
seismic wavefields.Comment: 5 pages, 8 figure
High-Dispersion Optical Spectra of Nearby Stars Younger Than The Sun
We present high-dispersion (R~16,000) optical (3900-8700 A) spectra of 390
stars obtained with the Palomar 60 inch telescope. The majority of stars
observed are part of the Spitzer Legacy Science Program "The Formation and
Evolution of Planetary Systems." Through detailed analysis we determine stellar
properties for this sample, including radial and rotational velocities, Li I
6708 and Ha equivalent widths, the chromospheric activity index R'_HK, and
temperature- and gravity-sensitive line ratios. Several spectroscopic binaries
are also identified. From our tabulations, we illustrate basic age- and
rotation-related correlations among measured indices. One novel result is that
Ca II chromospheric emission appears to saturate at vsini values above ~30
km/s, similar to the well established saturation of X-rays that originate in
the spatially separate coronal regions.Comment: 1 electronic table; published in the Astronomical Journa
Atom-optics hologram in the time domain
The temporal evolution of an atomic wave packet interacting with object and
reference electromagnetic waves is investigated beyond the weak perturbation of
the initial state. It is shown that the diffraction of an ultracold atomic beam
by the inhomogeneous laser field can be interpreted as if the beam passes
through a three-dimensional hologram, whose thickness is proportional to the
interaction time. It is found that the diffraction efficiency of such a
hologram may reach 100% and is determined by the duration of laser pulses. On
this basis a method for reconstruction of the object image with matter waves is
offered.Comment: RevTeX, 13 pages, 8 figures; minor grammatical change
In-plane Magnetoconductivity of Si-MOSFET's: A Quantitative Comparison between Theory and Experiment
For densities above cm in the strongly
interacting system of electrons in two-dimensional silicon inversion layers,
excellent agreement between experiment and the theory of Zala, Narozhny and
Aleiner is obtained for the response of the conductivity to a magnetic field
applied parallel to the plane of the electrons. However, the Fermi liquid
parameter and the valley splitting obtained from
fits to the magnetoconductivity, although providing qualitatively correct
behavior (including sign), do not yield quantitative agreement with the
temperature dependence of the conductivity in zero magnetic field. Our results
suggest the existence of additional scattering processes not included in the
theory in its present form
Electron-electron interaction at decreasing
The contribution of the electron-electron interaction to conductivity is
analyzed step by step in gated GaAs/InGaAs/GaAs heterostructures with different
starting disorder. We demonstrate that the diffusion theory works down to , where is the Fermi quasimomentum, is the mean free
paths. It is shown that the e-e interaction gives smaller contribution to the
conductivity than the interference independent of the starting disorder and its
role rapidly decreases with decrease.Comment: 5 pages, 6 figure
Multiperiodicity, modulations and flip-flops in variable star light curves I. Carrier fit method
The light curves of variable stars are commonly described using simple
trigonometric models, that make use of the assumption that the model parameters
are constant in time. This assumption, however, is often violated, and
consequently, time series models with components that vary slowly in time are
of great interest. In this paper we introduce a class of data analysis and
visualization methods which can be applied in many different contexts of
variable star research, for example spotted stars, variables showing the
Blazhko effect, and the spin-down of rapid rotators. The methods proposed are
of explorative type, and can be of significant aid when performing a more
thorough data analysis and interpretation with a more conventional method.Our
methods are based on a straightforward decomposition of the input time series
into a fast "clocking" periodicity and smooth modulating curves. The fast
frequency, referred to as the carrier frequency, can be obtained from earlier
observations (for instance in the case of photometric data the period can be
obtained from independently measured radial velocities), postulated using some
simple physical principles (Keplerian rotation laws in accretion disks), or
estimated from the data as a certain mean frequency. The smooth modulating
curves are described by trigonometric polynomials or splines. The data
approximation procedures are based on standard computational packages
implementing simple or constrained least-squares fit-type algorithms.Comment: 14 pages, 23 figures, submitted to Astronomy and Astrophysic
A Fully Tunable Single-Walled Carbon Nanotube Diode
We demonstrate a fully tunable diode structure utilizing a fully suspended
single-walled carbon nanotube (SWNT). The diode's turn-on voltage under forward
bias can be continuously tuned up to 4.3 V by controlling gate voltages, which
is ~6 times the nanotube bandgap energy. Furthermore, the same device design
can be configured into a backward diode by tuning the band-to-band tunneling
current with gate voltages. A nanotube backward diode is demonstrated for the
first time with nonlinearity exceeding the ideal diode. These results suggest
that a tunable nanotube diode can be a unique building block for developing
next generation programmable nanoelectronic logic and integrated circuits.Comment: 14 pages, 4 figure
Asymptotically stable phase synchronization revealed by autoregressive circle maps
A new type of nonlinear time series analysis is introduced, based on phases,
which are defined as polar angles in spaces spanned by a finite number of
delayed coordinates. A canonical choice of the polar axis and a related
implicit estimation scheme for the potentially underlying auto-regressive
circle map (next phase map) guarantee the invertibility of reconstructed phase
space trajectories to the original coordinates. The resulting Fourier
approximated, Invertibility enforcing Phase Space map (FIPS map) is well suited
to detect conditional asymptotic stability of coupled phases. This rather
general synchronization criterion unites two existing generalisations of the
old concept and can successfully be applied e.g. to phases obtained from ECG
and airflow recordings characterizing cardio-respiratory interaction.Comment: PDF file, 232 KB, 24 pages, 3 figures; cheduled for Phys. Rev. E
(Nov) 200
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