14,884 research outputs found
Turing patterns in parabolic systems of conservation laws and numerically observed stability of periodic waves
Turing patterns on unbounded domains have been widely studied in systems of
reaction-diffusion equations. However, up to now, they have not been studied
for systems of conservation laws. Here, we (i) derive conditions for Turing
instability in conservation laws and (ii) use these conditions to find families
of periodic solutions bifurcating from uniform states, numerically continuing
these families into the large-amplitude regime. For the examples studied,
numerical stability analysis suggests that stable periodic waves can emerge
either from supercritical Turing bifurcations or, via secondary bifurcation as
amplitude is increased, from sub-critical Turing bifurcations. This answers in
the affirmative a question of Oh-Zumbrun whether stable periodic solutions of
conservation laws can occur. Determination of a full small-amplitude stability
diagram-- specifically, determination of rigorous Eckhaus-type stability
conditions-- remains an interesting open problem.Comment: 12 pages, 20 figure
On the Modulation Equations and Stability of Periodic GKdV Waves via Bloch Decompositions
In this paper, we complement recent results of Bronski and Johnson and of
Johnson and Zumbrun concerning the modulational stability of spatially periodic
traveling wave solutions of the generalized Korteweg-de Vries equation. In this
previous work it was shown by rigorous Evans function calculations that the
formal slow modulation approximation resulting in the Whitham system accurately
describes the spectral stability to long wavelength perturbations. Here, we
reproduce this result without reference to the Evans function by using direct
Bloch-expansion methods and spectral perturbation analysis. This approach has
the advantage of applying also in the more general multi-periodic setting where
no conveniently computable Evans function is yet devised. In particular, we
complement the picture of modulational stability described by Bronski and
Johnson by analyzing the projectors onto the total eigenspace bifurcating from
the origin in a neighborhood of the origin and zero Floquet parameter. We show
the resulting linear system is equivalent, to leading order and up to
conjugation, to the Whitham system and that, consequently, the characteristic
polynomial of this system agrees (to leading order) with the linearized
dispersion relation derived through Evans function calculation.Comment: 19 pages
Nonlinear stability of spatially-periodic traveling-wave solutions of systems of reaction diffusion equations
Using spatial domain techniques developed by the authors and Myunghyun Oh in
the context of parabolic conservation laws, we establish under a natural set of
spectral stability conditions nonlinear asymptotic stability with decay at
Gaussian rate of spatially periodic traveling-waves of systems of reaction
diffusion equations. In the case that wave-speed is identically zero for all
periodic solutions, we recover and slightly sharpen a well-known result of
Schneider obtained by renormalization/Bloch transform techniques; by the same
arguments, we are able to treat the open case of nonzero wave-speeds to which
Schneider's renormalization techniques do not appear to appl
Intervocalic consonant sequences in Korean
This paper reports the results of an instrumental phonetic study of intervocalic consonant sequences in Korean. The study explored a putative positional neutralization produced at the phonetics/phonology interface. It was designed to determine whether Korean intervocalic laryngeal consonants are phonetically distinct from geminates, plain consonants, or laryngeal consonants in consonant clusters. The results showed that the contrast between intervocalic tensed singletons and geminates was neutralized, and that both of these patterned with heterorganic consonant sequences rather than plain singletons. Moreover, we found that this neutralization persisted across (limited) variation in speaking rate, although intervocalic tense consonants were more compressible in faster speech than were post-consonantal tense consonants
Nonlinear modulational stability of periodic traveling-wave solutions of the generalized Kuramoto-Sivashinsky equation
In this paper we consider the spectral and nonlinear stability of periodic
traveling wave solutions of a generalized Kuramoto-Sivashinsky equation. In
particular, we resolve the long-standing question of nonlinear modulational
stability by demonstrating that spectrally stable waves are nonlinearly stable
when subject to small localized (integrable) perturbations. Our analysis is
based upon detailed estimates of the linearized solution operator, which are
complicated by the fact that the (necessarily essential) spectrum of the
associated linearization intersects the imaginary axis at the origin. We carry
out a numerical Evans function study of the spectral problem and find bands of
spectrally stable periodic traveling waves, in close agreement with previous
numerical studies of Frisch-She-Thual, Bar-Nepomnyashchy,
Chang-Demekhin-Kopelevich, and others carried out by other techniques. We also
compare predictions of the associated Whitham modulation equations, which
formally describe the dynamics of weak large scale perturbations of a periodic
wave train, with numerical time evolution studies, demonstrating their
effectiveness at a practical level. For the reader's convenience, we include in
an appendix the corresponding treatment of the Swift-Hohenberg equation, a
nonconservative counterpart of the generalized Kuramoto-Sivashinsky equation
for which the nonlinear stability analysis is considerably simpler, together
with numerical Evans function analyses extending spectral stability analyses of
Mielke and Schneider.Comment: 78 pages, 11 figure
Chest Pain and Costochondritis Associated with Vitamin D Deficiency: A Report of Two Cases
Vitamin D is integral for bone health, and severe deficiency can cause rickets in children and osteomalacia in adults. Although osteomalacia can cause severe generalized bone pain, there are only a few case reports of chest pain associated with vitamin D deficiency. We describe 2 patients with chest pain that were initially worked up for cardiac etiologies but were eventually diagnosed with costochondritis and vitamin D deficiency. Vitamin D deficiency is known to cause hypertrophic costochondral junctions in children (“rachitic rosaries”) and sternal pain with adults diagnosed with osteomalacia. We propose that vitamin D deficiency may be related to the chest pain associated with costochondritis. In patients diagnosed with costochondritis, physicians should consider testing and treating for vitamin D deficiency
The Shape of LITTLE THINGS Dwarf Galaxies DDO 46 and DDO 168: Understanding the stellar and gas kinematics
We present the stellar and gas kinematics of DDO 46 and DDO 168 from the
LITTLE THINGS survey and determine their respective Vmax/sigma_z,0 values. We
used the KPNO's 4-meter telescope with the Echelle spectrograph as a long-slit
spectrograph. We acquired spectra of DDO 168 along four position angles by
placing the slit over the morphological major and minor axes and two
intermediate position angles. However, due to poor weather conditions during
our observing run for DDO 46, we were able to extract only one useful data
point from the morphological major axis. We determined a central stellar
velocity dispersion perpendicular to the disk, sigma_z,0, of 13.5+/-8 km/s for
DDO 46 and of 10.7+/-2.9 km/s for DDO 168. We then derived the
maximum rotation speed in both galaxies using the LITTLE THINGS HI data. We
separated bulk motions from non-circular motions using a double Gaussian
decomposition technique and applied a tilted-ring model to the bulk velocity
field. We corrected the observed HI rotation speeds for asymmetric drift and
found a maximum velocity, Vmax, of 77.4 +/- 3.7 and 67.4 +/- 4.0 km/s for DDO
46 and DDO 168, respectively. Thus, we derived a kinematic measure,
Vmax/sigma_z,0, of 5.7 +/- 0.6 for DDO 46 and 6.3 +/- 0.3 for DDO 168.
Comparing these values to ones determined for spiral galaxies, we find that DDO
46 and DDO 168 have Vmax/sigma_z,0 values indicative of thin disks, which is in
contrast to minor-to-major axis ratio studies
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