6,211 research outputs found
Wholeness as a Hierarchical Graph to Capture the Nature of Space
According to Christopher Alexander's theory of centers, a whole comprises
numerous, recursively defined centers for things or spaces surrounding us.
Wholeness is a type of global structure or life-giving order emerging from the
whole as a field of the centers. The wholeness is an essential part of any
complex system and exists, to some degree or other, in spaces. This paper
defines wholeness as a hierarchical graph, in which individual centers are
represented as the nodes and their relationships as the directed links. The
hierarchical graph gets its name from the inherent scaling hierarchy revealed
by the head/tail breaks, which is a classification scheme and visualization
tool for data with a heavy-tailed distribution. We suggest that (1) the degrees
of wholeness for individual centers should be measured by PageRank (PR) scores
based on the notion that high-degree-of-life centers are those to which many
high-degree-of-life centers point, and (2) that the hierarchical levels, or the
ht-index of the PR scores induced by the head/tail breaks can characterize the
degree of wholeness for the whole: the higher the ht-index, the more life or
wholeness in the whole. Three case studies applied to the Alhambra building
complex and the street networks of Manhattan and Sweden illustrate that the
defined wholeness captures fairly well human intuitions on the degree of life
for the geographic spaces. We further suggest that the mathematical model of
wholeness be an important model of geographic representation, because it is
topological oriented that enables us to see the underlying scaling structure.
The model can guide geodesign, which should be considered as the
wholeness-extending transformations that are essentially like the unfolding
processes of seeds or embryos, for creating beautiful built and natural
environments or with a high degree of wholeness.Comment: 14 pages, 7 figures, 2 table
Electromagnetism, local covariance, the Aharonov-Bohm effect and Gauss' law
We quantise the massless vector potential A of electromagnetism in the
presence of a classical electromagnetic (background) current, j, in a generally
covariant way on arbitrary globally hyperbolic spacetimes M. By carefully
following general principles and procedures we clarify a number of topological
issues. First we combine the interpretation of A as a connection on a principal
U(1)-bundle with the perspective of general covariance to deduce a physical
gauge equivalence relation, which is intimately related to the Aharonov-Bohm
effect. By Peierls' method we subsequently find a Poisson bracket on the space
of local, affine observables of the theory. This Poisson bracket is in general
degenerate, leading to a quantum theory with non-local behaviour. We show that
this non-local behaviour can be fully explained in terms of Gauss' law. Thus
our analysis establishes a relationship, via the Poisson bracket, between the
Aharonov-Bohm effect and Gauss' law (a relationship which seems to have gone
unnoticed so far). Furthermore, we find a formula for the space of electric
monopole charges in terms of the topology of the underlying spacetime. Because
it costs little extra effort, we emphasise the cohomological perspective and
derive our results for general p-form fields A (p < dim(M)), modulo exact
fields. In conclusion we note that the theory is not locally covariant, in the
sense of Brunetti-Fredenhagen-Verch. It is not possible to obtain such a theory
by dividing out the centre of the algebras, nor is it physically desirable to
do so. Instead we argue that electromagnetism forces us to weaken the axioms of
the framework of local covariance, because the failure of locality is
physically well-understood and should be accommodated.Comment: Minor corrections to Def. 4.3, acknowledgements and typos, in line
with published versio
An explicit relationship between time-domain noise correlation and spatial autocorrelation (SPAC) results
The success of recent ambient noise tomographic studies is now understood to arise due to cross-correlation properties documented in the acoustics community since the 1950s. However, despite the fact that Aki's 1957 spatial autocorrelation (SPAC) work yields identical analytical results to certain noise correlation results, the precise relationship between SPAC and time-domain cross-correlation remains not entirely transparent. Here, we present an explicit comparison of the two approaches and clarify that SPAC theory is indeed equivalent to the cross-correlation theory used for recent noise tomography studies. This equivalence allows theoretical work from each field to be applied to the other, and we illustrate a few examples of this
Level statistics across the many--body localization transition
Level statistics of systems that undergo many--body localization transition
are studied. An analysis of the gap ratio statistics from the perspective of
inter- and intra-sample randomness allows us to pin point differences between
transitions in random and quasi-random disorder, showing the effects due to
Griffiths rare events for the former case. It is argued that the transition in
the case of random disorder exhibits universal features that are identified by
constructing an appropriate model of intermediate spectral statistics which is
a generalization of the family of short-range plasma models. The considered
weighted short-range plasma model yields a very good agreement both for level
spacing distribution including its exponential tail and the number variance up
to tens of level spacings outperforming previously proposed models. In
particular, our model grasps the critical level statistics which arise at
disorder strength for which the inter-sample fluctuations are the strongest.
Going beyond the paradigmatic examples of many-body localization in spin
systems, we show that the considered model also grasps the level statistics of
disordered Bose- and Fermi-Hubbard models. The remaining deviations for
long-range spectral correlations are discussed and attributed mainly to the
intricacies of level unfolding.Comment: 19pp. enlarged by including 1807.06983; version accepted in Phys.
Rev.
An approach to preventing spam using Access Codes with a combination of anti-spam mechanisms
Spam is becoming a more and more severe problem for individuals, networks,
organisations and businesses. The losses caused by spam are billions of dollars every
year. Research shows that spam contributes more than 80% of e-mails with an increased
in its growth rate every year. Spam is not limited to emails; it has started affecting other
technologies like VoIP, cellular and traditional telephony, and instant messaging services.
None of the approaches (including legislative, collaborative, social awareness and
technological) separately or in combination with other approaches, can prevent sufficient
of the spam to be deemed a solution to the spam problem.
The severity of the spam problem and the limitations of the state-of-the-Art solutions
create a strong need for an efficient anti-spam mechanism that can prevent significant
volumes of spam without showing any false positives. This can be achieved by an
efficient anti-spam mechanism such as the proposed anti-spam mechanism known as
"Spam Prevention using Access Codes", SPAC. SPAC targets spam from two angles i.e.
to prevent/block spam and to discourage spammers by making the infrastructure
environment very unpleasant for them.
In addition to the idea of Access Codes, SPAC combines the ideas behind some of the
key current technological anti-spam measures to increase effectiveness. The difference in
this work is that SPAC uses those ideas effectively and combines them in a unique way
which enables SPAC to acquire the good features of a number of technological anti-spam
approaches without showing any of the drawbacks of these approaches. Sybil attacks,
Dictionary attacks and address spoofing have no impact on the performance of SPAC. In
fact SPAC functions in a similar way (i.e. as for unknown persons) for these sorts of
attacks.
An application known as the "SPAC application" has been developed to test the
performance of the SPAC mechanism. The results obtained from various tests on the
SPAC application show that SPAC has a clear edge over the existing anti-spam
technological approaches
STUDY OF GATE TURN OFF THYRISTOR STRUCTURE FOR POWER CONTROL APPLICATION USING SIMULATION
The turn-on and turn-off losses in a GTO thyristor must be properly accounted for
because they can comprise upwards of 60% of the total losses. If a research can be done
to remove the defect and reduce the losses and improved the performance of the GTO
thyristor, it will be a great breakthrough to the semiconductor field. This report represents
the study on the Gate Turn- Off (GTO) Thyristor using simulation software. This project
which was done in 2 semesters (July 2006 semester and January 2007 semester) is the
study on the method to improve the performance of the GTO thyristor by reducing it
losses. During the first part of the project, familiarization with the TCAD SILVACO
ATHENA and ATLAS is done in order to continue doing a critical evaluation to the
device; furthermore this step is very important during the extraction process of the
electrical performance for the conventional device. For the second semester, the review
process of the researches done by other people began to takeplace. Each of the reference
and research found will be carefully compared. The best design or steps is then being
chosen to be adopted in the project
Large-scale low-energy excitations in 3-d spin glasses
We numerically extract large-scale excitations above the ground state in the
3-dimensional Edwards-Anderson spin glass with Gaussian couplings. We find that
associated energies are O(1), in agreement with the mean field picture. Of
further interest are the position-space properties of these excitations. First,
our study of their topological properties show that the majority of the
large-scale excitations are sponge-like. Second, when probing their geometrical
properties, we find that the excitations coarsen when the system size is
increased. We conclude that either finite size effects are very large even when
the spin overlap q is close to zero, or the mean field picture of homogeneous
excitations has to be modified.Comment: 11 pages, typos corrected, added reference
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