77 research outputs found
Active Integrity Constraints and Revision Programming
We study active integrity constraints and revision programming, two
formalisms designed to describe integrity constraints on databases and to
specify policies on preferred ways to enforce them. Unlike other more commonly
accepted approaches, these two formalisms attempt to provide a declarative
solution to the problem. However, the original semantics of founded repairs for
active integrity constraints and justified revisions for revision programs
differ. Our main goal is to establish a comprehensive framework of semantics
for active integrity constraints, to find a parallel framework for revision
programs, and to relate the two. By doing so, we demonstrate that the two
formalisms proposed independently of each other and based on different
intuitions when viewed within a broader semantic framework turn out to be
notational variants of each other. That lends support to the adequacy of the
semantics we develop for each of the formalisms as the foundation for a
declarative approach to the problem of database update and repair. In the paper
we also study computational properties of the semantics we consider and
establish results concerned with the concept of the minimality of change and
the invariance under the shifting transformation.Comment: 48 pages, 3 figure
Geospatial analysis and living urban geometry
This essay outlines how to incorporate morphological rules within the exigencies of our technological age. We propose using the current evolution of GIS (Geographical Information Systems) technologies beyond their original representational domain, towards predictive and dynamic spatial models that help in constructing the new discipline of "urban seeding". We condemn the high-rise tower block as an unsuitable typology for a living city, and propose to re-establish human-scale urban fabric that resembles the traditional city. Pedestrian presence, density, and movement all reveal that open space between modernist buildings is not urban at all, but neither is the open space found in today's sprawling suburbs. True urban space contains and encourages pedestrian interactions, and has to be designed and built according to specific rules. The opposition between traditional self-organized versus modernist planned cities challenges the very core of the urban planning discipline. Planning has to be re-framed from being a tool creating a fixed future to become a visionary adaptive tool of dynamic states in evolution
Hamilton's Turns for the Lorentz Group
Hamilton in the course of his studies on quaternions came up with an elegant
geometric picture for the group SU(2). In this picture the group elements are
represented by ``turns'', which are equivalence classes of directed great
circle arcs on the unit sphere , in such a manner that the rule for
composition of group elements takes the form of the familiar parallelogram law
for the Euclidean translation group. It is only recently that this construction
has been generalized to the simplest noncompact group , the double cover of SO(2,1). The present work develops a theory of
turns for , the double and universal cover of SO(3,1) and ,
rendering a geometric representation in the spirit of Hamilton available for
all low dimensional semisimple Lie groups of interest in physics. The geometric
construction is illustrated through application to polar decomposition, and to
the composition of Lorentz boosts and the resulting Wigner or Thomas rotation.Comment: 13 pages, Late
The Image of the City Out of the Underlying Scaling of City Artifacts or Locations
Two fundamental issues surrounding research on the image of the city
respectively focus on the city's external and internal representations. The
external representation in the context of this paper refers to the city itself,
external to human minds, while the internal representation concerns how the
city is represented in human minds internally. This paper deals with the first
issue, i.e., what trait the city has that make it imageable? We develop an
argument that the image of the city arises from the underlying scaling of city
artifacts or locations. This scaling refers to the fact that, in an imageable
city (a city that can easily be imaged in human minds), small city artifacts
are far more common than large ones; or alternatively low dense locations are
far more common than high dense locations. The sizes of city artifacts in a
rank-size plot exhibit a heavy tailed distribution consisting of the head,
which is composed of a minority of unique artifacts (vital and very important),
and the tail, which is composed of redundant other artifacts (trivial and less
important). Eventually, those extremely unique and vital artifacts in the top
head, i.e., what Lynch called city elements, make up the image of the city. We
argue that the ever-increasing amount of geographic information on cities, in
particular obtained from social media such as Flickr and Twitter, can turn
research on the image of the city, or cognitive mapping in general, into a
quantitative manner. The scaling property might be formulated as a law of
geography.
Keywords: Scaling of geographic space, face of the city, cognitive maps,
power law, and heavy tailed distributions.Comment: 13 pages, 9 figures, 2 table
Symmetries of the Dirac operators associated with covariantly constant Killing-Yano tensors
The continuous and discrete symmetries of the Dirac-type operators produced
by particular Killing-Yano tensors are studied in manifolds of arbitrary
dimensions. The Killing-Yano tensors considered are covariantly constant and
realize certain square roots of the metric tensor. Such a Killing-Yano tensor
produces simultaneously a Dirac-type operator and the generator of a
one-parameter Lie group connecting this operator with the standard Dirac one.
The Dirac operators are related among themselves through continuous or discrete
transformations. It is shown that the groups of the continuous symmetry can be
only U(1) and SU(2), specific to (hyper-)Kahler spaces, but arising even in
cases when the requirements for these special geometries are not fulfilled. The
discrete symmetries are also studied obtaining the discrete groups Z_4 and Q.
The briefly presented examples are the Euclidean Taub-NUT space and the
Minkowski spacetime.Comment: 27 pages, latex, no figures, final version to be published in Class.
Quantum Gravit
Scaling of Geographic Space as a Universal Rule for Map Generalization
Map generalization is a process of producing maps at different levels of
detail by retaining essential properties of the underlying geographic space. In
this paper, we explore how the map generalization process can be guided by the
underlying scaling of geographic space. The scaling of geographic space refers
to the fact that in a geographic space small things are far more common than
large ones. In the corresponding rank-size distribution, this scaling property
is characterized by a heavy tailed distribution such as a power law, lognormal,
or exponential function. In essence, any heavy tailed distribution consists of
the head of the distribution (with a low percentage of vital or large things)
and the tail of the distribution (with a high percentage of trivial or small
things). Importantly, the low and high percentages constitute an imbalanced
contrast, e.g., 20 versus 80. We suggest that map generalization is to retain
the objects in the head and to eliminate or aggregate those in the tail. We
applied this selection rule or principle to three generalization experiments,
and found that the scaling of geographic space indeed underlies map
generalization. We further relate the universal rule to T\"opfer's radical law
(or trained cartographers' decision making in general), and illustrate several
advantages of the universal rule.
Keywords: Head/tail division rule, head/tail breaks, heavy tailed
distributions, power law, and principles of selectionComment: 12 pages, 9 figures, 4 table
Matrix representations of a special polynomial sequence in arbitrary dimension
This paper provides an insight into different structures of a special polynomial sequence of binomial type in higher dimensions with values in a Clifford algebra. The elements of the special polynomial sequence are homogeneous hypercomplex differentiable (monogenic) functions of different degrees and their matrix representation allows to prove their recursive construction in analogy to the complex power functions. This property can somehow be considered as a compensation for the loss of multiplicativity caused by the non-commutativity of the underlying algebra.Fundação para a Ciência e a Tecnologia (FCT
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