75 research outputs found
Nonreciprocal Cahn-Hilliard model emerges as a universal amplitude equation
Oscillatory behavior is ubiquitous in out-of-equilibrium systems showing
spatio-temporal pattern formation. Starting from a linear large-scale
oscillatory instability -- a conserved-Hopf instability -- that naturally
occurs in many active systems with two conservation laws, we derive the
corresponding amplitude equation. This completes the set of such universal
equations for the eight types of instabilities in homogeneous isotropic systems
resulting from the combination of three features: large-scale vs. small-scale
instability, stationary vs. oscillatory instability, and instability without
and with conservation law(s). The derived universal equation generalizes a
phenomenological model of considerable recent interest, namely, the
nonreciprocal Cahn-Hilliard equation, and plays a similar role in the
systematics of pattern formation as the complex Ginzburg-Landau equation
Stabilization of localized structures by inhomogeneous injection in Kerr resonators
We consider the formation of temporal localized structures or Kerr comb
generation in a microresonator with inhomogeneities. We show that the
introduction of even a small inhomogeneity in the injected beam widens the
stability region of localized solutions. The homoclinic snaking bifurcation
associated with the formation of localized structures and clusters of them with
decaying oscillatory tails is constructed. Furthermore, the inhomogeneity
allows not only to control the position of localized solutions, but strongly
affects their stability domains. In particular, a new stability domain of a
single peak localized structure appears outside of the region of multistability
between multiple peaks of localized states. We identify a regime of larger
detuning, where localized structures do not exhibit a snaking behavior. In this
regime, the effect of inhomogeneities on localized solutions is far more
complex: they can act either attracting or repelling. We identify the pitchfork
bifurcation responsible for this transition. Finally, we use a potential well
approach to determine the force exerted by the inhomogeneity and summarize with
a full analysis of the parameter regime where localized structures and
therefore Kerr comb generation exist and analyze how this regime changes in the
presence of an inhomogeneity
Towards a New Paradigm of Non-Captive Research on Cetacean Cognition
Contemporary knowledge of impressive neurophysiology and behavior in cetaceans, combined with increasing opportunities for studying free-ranging cetaceans who initiate sociable interaction with humans, are converging to highlight serious ethical considerations and emerging opportunities for a new era of progressive and less-invasive cetacean research. Most research on cetacean cognition has taken place in controlled captive settings, e.g., research labs, marine parks. While these environments afford a certain amount of experimental rigor and logistical control they are fraught with limitations in external validity, impose tremendous stress on the part of the captive animals, and place burdens on populations from which they are often captured. Alternatively, over the past three decades, some researchers have sought to focus their attention on the presence of free-ranging cetacean individuals and groups who have initiated, or chosen to participate in, sociable interactions with humans in the wild. This new approach, defined as Interspecies Collaborative Research between cetacean and human, involves developing novel ways to address research questions under natural conditions and respecting the individual cetacean's autonomy. It also offers a range of potential direct benefits to the cetaceans studied, as well as allowing for unprecedented cognitive and psychological research on sociable mysticetes. Yet stringent precautions are warranted so as to not increase their vulnerability to human activities or pathogens. When conducted in its best and most responsible form, collaborative research with free-ranging cetaceans can deliver methodological innovation and invaluable new insights while not necessitating the ethical and scientific compromises that characterize research in captivity. Further, it is representative of a new epoch in science in which research is designed so that the participating cetaceans are the direct recipients of the benefits
Nonreciprocity induces resonances in two-field Cahn-Hilliard model
We consider a non-reciprocically coupled two-field Cahn-Hilliard system that
has been shown to allow for oscillatory behaviour, a suppression of coarsening
as well as the existence of localised states. Here, after introducing the model
we first briefly review the linear stability of homogeneous states and show
that all instability thresholds are identical to the ones for a corresponding
Turing system (i.e., a two-species reaction-diffusion system). Next, we discuss
possible interactions of linear modes and analyse the specific case of a
``Hopf-Turing'' resonance by discussing corresponding amplitude equations in a
weakly nonlinear approach. The thereby obtained states are finally compared
with fully nonlinear simulations for a specific conserved amended
FitzHugh-Nagumo system. We conclude by a discussion of the limitations of the
weakly nonlinear approach
Stationary localized structures and the effect of the delayed feedback in the Brusselator model
The Brusselator reaction-diffusion model is a paradigm for the understanding
of dissipative structures in systems out of equilibrium. In the first part of
this paper, we investigate the formation of stationary localized structures in
the Brusselator model. By using numerical continuation methods in two spatial
dimensions, we establish a bifurcation diagram showing the emergence of
localized spots. We characterize the transition from a single spot to an
extended pattern in the form of squares. In the second part, we incorporate
delayed feedback control and show that delayed feedback can induce a
spontaneous motion of both localized and periodic dissipative structures. We
characterize this motion by estimating the threshold and the velocity of the
moving dissipative structures.Comment: 18 pages, 11 figure
Swimming with captive dolphins: current debates and post-experience dissonance
Dolphins have widespread contemporary appeal and anthropomorphic social representations of dolphins have fuelled a growing desire in tourist populations to seek interaction with them. This paper is concerned with the staged performance of swim-with-dolphin interaction programmes in aquaria. Qualitative interviews with tourists who have swum with captive dolphins identified their immediate recollections and stressed the grace, size and power of dolphins, but also a belief that the experience was too staged, too short and too expensive. Post-purchase dissonance focused on concerns with the size of enclosures and about captivity, too many tricks, limited interpretation and unfulfilled expectations of a quality interaction
Suppression of coarsening and emergence of oscillatory behavior in a Cahn-Hilliard model with nonvariational coupling
We investigate a generic two-field Cahn-Hilliard model with variational and
nonvariational coupling. It describes, for instance, passive and active ternary
mixtures, respectively. Already a linear stability analysis of the homogeneous
mixed state shows that activity not only allows for the usual large-scale
stationary (Cahn-Hilliard) instability of the well known passive case but also
for small-scale stationary (Turing) and large-scale oscillatory (Hopf)
instabilities. In consequence of the Turing instability, activity may
completely suppress the usual coarsening dynamics. In a fully nonlinear
analysis we first briefly discuss the passive case before focusing on the
active case. Bifurcation diagrams and selected direct time simulations are
presented that allow us to establish that nonvariational coupling (i) can
partially or completely suppress coarsening and (ii) may lead to the emergence
of drifting and oscillatory states. Throughout, we emphasize the relevance of
conservation laws and related symmetries for the encountered intricate
bifurcation behavior
Antiretroviral Therapy Outcomes in HIV-Infected Children after Adjusting Protease Inhibitor Dosing during Tuberculosis Treatment
Modification of ritonavir-boosted lopinavir (LPV/r)-based antiretroviral therapy is required for HIV-infected children co-treated for tuberculosis (TB). We aimed to determine virologic and toxicity outcomes among TB/HIV co-treated children with the following modifications to their antiretroviral therapy (ART): (1) super-boosted LPV/r, (2) double-dose LPV/r or (3) ritonavir.A medical record review was conducted at two clinical sites in Johannesburg, South Africa. The records of children 6-24 months of age initiating LPV/r-based therapy were reviewed. Children co-treated for TB were categorized based on the modifications made to their ART regimen and were compared to children of the same age at each site not treated for TB. Included are 526 children, 294 (56%) co-treated for TB. All co-treated children had more severe HIV disease, including lower CD4 percents and worse growth indicators, than comparisons. Children in the super-boosted group (n = 156) were as likely to be virally suppressed (<400 copies/ml) at 6 months as comparisons (69.2% vs. 74.8%, p = 0.36). Children in the double-dose (n = 47) and ritonavir groups (n = 91) were significantly less likely to be virally suppressed at 6 months (53.1% and 49.3%) than comparisons (74.8% and 82.1%; p = 0.02 and p<0.0001, respectively). At 12 months only children in the ritonavir group still had lower rates of virological suppression relative to comparisons (63.9% vs 83.3% p<0.05). Grade 1 or greater ALT elevations were more common in the super-boosted (75%) than double-dose (54.6%) or ritonavir (33.9%) groups (p = 0.09 and p<0.0001) but grade 3/4 elevations were observed in 3 (13.6%) of the super-boosted, 7 (15.9%) of the double-dose and 5 (8.9%) of the ritonavir group (p = 0.81 and p = 0.29).Good short-term virologic outcomes were achieved in children co-treated for TB and HIV who received super-boosted LPV/r. Treatment limiting toxicity was rare. Strategies for increased dosing of LPV/r with TB treatment warrant further investigation
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