3,433 research outputs found
The generalized KP hierarchy
We propose one possible generalization of the KP hierarchy, which possesses
multi bi--hamiltonian structures, and can be viewed as several KP hierarchies
coupled together.Comment: 12
The constrained modified KP hierarchy and the generalized Miura transformations
In this letter, we consider the second Hamiltonian structure of the
constrained modified KP hierarchy. After mapping the Lax operator to a pure
differential operator the second structure becomes the sum of the second and
the third Gelfand-Dickey brackets defined by this differential operator. We
simplify this Hamiltonian structure by factorizing the Lax operator into linear
terms.Comment: 8 pages, latex, no figure
HRTEM study of a new non-stoichiometric BaTiO(3-δ) structure
BaTiO3-based multilayer ceramic capacitors (MLCCs) with Ni internal electrodes are co-fired in
reducing atmospheres to avoid oxidation of the electrode. Although dielectric materials are doped by
acceptor, donor and amphoteric dopants to minimize the oxygen vacancy content, there is still a
large concentration of oxygen vacancies that are accommodated in the BaTiO3 active layers. In
general, ABO3 perovskites demonstrates a strong ability to accommodate the oxygen vacancies and
maintain a regular pseudo-cubic structure. Oxygen deficient barium titanate can be transformed to a
hexagonal polymorph (h-BT) at high temperatures1,2. In this paper, we report the new modulated and
long range ordered structures of non-stoichiometric BaTiO3-δ that are observed in the electrically
degraded Ni-BaTiO3 MLCCs at low temperature
Dark Energy as a Born-Infeld Gauge Interaction Violating the Equivalence Principle
We investigate the possibility that dark energy does not couple to
gravitation in the same way than ordinary matter, yielding a violation of the
weak and strong equivalence principles on cosmological scales. We build a
transient mechanism in which gravitation is pushed away from general relativity
by a Born-Infeld gauge interaction acting as an "Abnormally Weighting" (dark)
Energy. This mechanism accounts for the Hubble diagram of far-away supernovae
by cosmic acceleration and time variation of the gravitational constant while
accounting naturally for the present tests on general relativity.Comment: 5 pages, 3 figures, sequel of Phys. Rev. D 73 023520 (2006), to
appear in Physical Review Letter
On the Treves theorem for the AKNS equation
According to a theorem of Treves, the conserved functionals of the AKNS
equation vanish on all pairs of formal Laurent series of a specified form, both
of them with a pole of the first order. We propose a new and very simple proof
for this statement, based on the theory of B\"acklund transformations; using
the same method, we prove that the AKNS conserved functionals vanish on other
pairs of Laurent series. The spirit is the same of our previous paper on the
Treves theorem for the KdV, with some non trivial technical differences.Comment: LaTeX, 16 page
Urban Observatories in the Midst of COVID-19: Challenges & Responses
In the context of the COVID-19 pandemic, urban observatories
have demonstrated their value, but also highlighted the
challenges for boundary institutions between knowledge
generation and decision-making in a variety of different ways. We
aim here to capture some of their voices in a time of crisis.
The Connected Cities Lab, in collaboration with University
College London and UN-Habitat, and in dialogue with a variety of
urban research institutions around the planet, has been working
since 2018 to develop a review of the challenges and values of
and challenges for ‘urban observatories’. That project aims to
present evidence on the boundary-spanning roles of these
institutions, capturing the ways in which they bridge information
in and about their cities and the potential value they offer to
urban governance.
As the COVID-19 crisis took hold across cities and continents in
early 2020, it became apparent that this study could not prescind
from a closer look at how these observatories had both been
coping with, but also responding to, the pandemic. This resulted
in a series of additional interviews, document reviews and a twopart virtual workshop in August 2020 with observatories, and
urban research institutions performing observatory functions, to
give further voice to these experiences.
As a background to this ‘deep’ dive into the reality of COVID-19
for observatories, the overall study underpinning this working
paper has relied on, first, desktop research on publicly available
information to identify thirty-two cases of either explicitlynamed ‘urban observatories’ or else urban research institutions
performing ‘observatory-like’ functions. This research was then
coupled with a series of interviews with experts and senior staff
from these observatories to ground truth initial considerations
as well as to capture how the processes of boundary-spanning
worked beyond the publicly available persona of each
observatory. We then referred back to these thirty-two cases and
selected a sample of fourteen for specific analysis in relation to
COVID-specific interventions, with six of them involved directly
into two virtual workshops to capture directly their experience in
the context of the pandemic crisis. Capturing initial findings from
these engagements (which will ultimately form an integral part of
the project’s final report), this working paper offers a preliminary
snapshot of some of these lessons drawn from the study.
Essential for us has been the chance, amidst the complications of
COVID-19 lockdowns and travel bans, to better capture the voice
of observatories the world over and their tangible experiences
with spanning urban research-practice boundaries in a
turbulent historical moment. Whilst the final report for the
project will likely include more extensively analysed cases
emerging from the current crisis, we have sought to present here
much of the raw reflections emerging from our engagement with
colleagues in observatories (and ‘observatory-like’ institutions) to
both offer useful reflections to other contexts around the world as
well as to offer insights on the unique situation urban knowledge
institutions find themselves in a reality where cities and urban life
has been fundamentally recast by the pandemic.
The working paper is organised in a way that follows our broader
study’s key themes looking at the structure and activities of
observatories, putting our broader findings into dialogue with the
voices of observatories during the COVID-19 crisis.
Section 1 describes the proposed visions and functions performed
by observatories and puts it into dialogue with the COVID-19
crisis. Positionality of urban observatories is also discussed in
this section.
Section 2 explores outputs produced by, and themes investigated
in, observatories and how they have been shaped by and for the
crisis. In Sections 1 and 2 we endeavoured to capture vignettes
from the participating observatories through the experiences
of a set of six more specific interlocutor institutions engaged
in the project: the Gauteng City Region Observatory (GCRO)
in Johannesburg, the Karachi Urban Lab (KUL) in Karachi, the
Indian Institute for Human Settlements (IIHS) in Bangalore, the
Metropolis Observatory in Barcelona, and the World Resources
Institute Ross Centre in Washington DC.
Section 3 concludes with a commentary on the ongoing
challenges and opportunities faced by urban observatories in the
wake of COVID-19, without underestimating how the crisis might
be far from over
Quintessence Models and the Cosmological Evolution of alpha
The cosmological evolution of a quintessence-like scalar field, phi, coupled
to matter and gauge fields leads to effective modifications of the coupling
constants and particle masses over time. We analyze a class of models where the
scalar field potential V(phi) and the couplings to matter B(phi) admit common
extremum in phi, as in the Damour-Polyakov ansatz. We find that even for the
simplest choices of potentials and B(phi), the observational constraints on
delta alpha/alpha coming from quasar absorption spectra, the Oklo phenomenon
and Big Bang nucleosynthesis provide complementary constraints on the
parameters of the model. We show the evolutionary history of these models in
some detail and describe the effects of a varying mass for dark matter.Comment: 26 pages, 20 eps figure
AGN All the Way Down? AGN-like Line Ratios are Common In the Lowest-Mass Isolated Quiescent Galaxies
We investigate the lowest-mass quiescent galaxies known to exist in isolated
environments (; 1.5 Mpc from a more
massive galaxy). This population may represent the lowest stellar mass galaxies
in which internal feedback quenches galaxy-wide star formation. We present
Keck/ESI long-slit spectroscopy for 27 isolated galaxies in this regime: 20
quiescent galaxies and 7 star-forming galaxies. We measure emission line
strengths as a function of radius and place galaxies on the Baldwin Phillips
Terlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our sample
host central AGN-like line ratios. Only 5 of these quiescent galaxies were
identified as AGN-like in SDSS due to lower spatial resolution and
signal-to-noise. We find that many of the quiescent galaxies in our sample have
spatially-extended emission across the non-SF regions of BPT-space. When
considering only the central 1, we identify a tight
relationship between distance from the BPT star-forming sequence and host
galaxy stellar age as traced by , such that older stellar
ages are associated with larger distances from the star-forming locus. Our
results suggest that the presence of hard ionizing radiation (AGN-like line
ratios) is intrinsically tied to the quenching of what may be the lowest-mass
self-quenched galaxies.Comment: 16 pages, 9 figures, submitted to Ap
Mobilising urban knowledge in an infodemic: Urban observatories, sustainable development and the COVID-19 crisis
Along with disastrous health and economic implications, COVID-19 has also been an
epidemic of misinformation and rumours – an ‘infodemic’. The desire for robust, evidence-based
policymaking in this time of disruption has been at the heart of the multilateral response to the
crisis, not least in terms of supporting a continuing agenda for global sustainable development. The
role of boundary-spanning knowledge institutions in this context could be pivotal, not least in cities,
where much of the pandemic has struck. ‘Urban observatories’ have emerged as an example of such
institutions; harbouring great potential to produce and share knowledge supporting sustainable and
equitable processes of recovery. Building on four ‘live’ case studies during the crisis of institutions
based in Johannesburg, Karachi, Freetown and Bangalore, our research note aims to capture the role
of these institutions, and what it means to span knowledge boundaries in the current crisis. We do
so with an eye towards a better understanding of their knowledge mobilisation practices in
contributing towards sustainable urban development. We highlight that the crisis offers a key
window for urban observatories to play a progressive and effective role for sustainable and inclusive
development. However, we also underline continuing challenges in these boundary knowledge
dynamics: including issues of institutional trust, inequality of voices, collective memory, and the
balance between normative and advisory roles for observatories
Power-law random walks
We present some new results about the distribution of a random walk whose
independent steps follow a Gaussian distribution with exponent
. In the case we show that a stochastic
representation of the point reached after steps of the walk can be
expressed explicitly for all . In the case we show that the random
walk can be interpreted as a projection of an isotropic random walk, i.e. a
random walk with fixed length steps and uniformly distributed directions.Comment: 5 pages, 4 figure
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