2,953 research outputs found
Linear resolutions of powers and products
The goal of this paper is to present examples of families of homogeneous
ideals in the polynomial ring over a field that satisfy the following
condition: every product of ideals of the family has a linear free resolution.
As we will see, this condition is strongly correlated to good primary
decompositions of the products and good homological and arithmetical properties
of the associated multi-Rees algebras. The following families will be discussed
in detail: polymatroidal ideals, ideals generated by linear forms and Borel
fixed ideals of maximal minors. The main tools are Gr\"obner bases and Sagbi
deformation
High Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?
Observations with the Green Bank Telescope of 21cm HI emission from the
high-velocity cloud Complex H suggest that it is interacting with the Milky
Way. A model in which the cloud is a satellite of the Galaxy in an inclined,
retrograde circular orbit reproduces both the cloud's average velocity and its
velocity gradient with latitude. The model places Complex H at approximately 33
kpc from the Galactic Center on a retrograde orbit inclined about 45 degrees to
the Galactic plane. At this location it has an HI mass > 6 10^6 Msun and
dimensions of at least 10 by 5 kpc. Some of the diffuse HI associated with the
cloud has apparently been decelerated by interaction with Galactic gas. Complex
H has similarities to the dwarf irregular galaxy Leo A and to some compact
high-velocity clouds, and has an internal structure nearly identical to parts
of the Magellanic Stream, with a pressure P/k about 100 cm^{-3} K.Comment: 12 pages includes 4 figures. To be published in Astrophysical Journal
Letters, 1 July 200
Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO
LaTiO is known as Mott-insulator which orders antiferromagnetically at
K. We report on results of thermal expansion and temperature
dependent x-ray diffraction together with measurements of the heat capacity,
electrical transport measurements, and optical spectroscopy in untwinned single
crystals. At significant structural changes appear, which are
volume conserving. Concomitant anomalies are also observed in the
dc-resistivity, in bulk modulus, and optical reflectivity spectra. We interpret
these experimental observations as evidence of orbital order.Comment: 4 pages, 4 figures; published in Phys. Rev. Lett. 91, 066403 (2003
Few smooth d-polytopes with n lattice points
We prove that, for fixed n there exist only finitely many embeddings of
Q-factorial toric varieties X into P^n that are induced by a complete linear
system. The proof is based on a combinatorial result that for fixed nonnegative
integers d and n, there are only finitely many smooth d-polytopes with n
lattice points. We also enumerate all smooth 3-polytopes with at most 12
lattice points. In fact, it is sufficient to bound the singularities and the
number of lattice points on edges to prove finiteness.Comment: 20+2 pages; major revision: new author, new structure, new result
Political hashtag publics and counter-visuality: a case study of #fertilityday in Italy
In 2016 the Italian health ministry launched the âFertility Dayâ campaign, aimed at tackling Italyâs low birth rate. Under the accusation of delivering sexist and racist messages, the campaign became a trending topic on Twitter, and a protest was launched to be held during Fertility Day. By applying a combination of digital methods and visual content analysis to the #fertilityday Twitter stream, this paper contributes to existing research on the deliberative strength of political hashtag publics, with a particular focus on their power structures, communication patterns and visual content use. Findings on gatekeeping dynamics downsize optimistic views on the democratising potential of Twitterâs socio-technical infrastructure as they point to the emergence of online satirical media and âtweetstarsâ â along with mainstream news mediaâ as main producers of spreadable content, with ordinary users only surfacing when traditional media elites and new satirical actors lack or lose interest in the debate. Results confirm that political hashtag publics follow acute event communication patterns, with users highly engaged in retweeting and referencing external material and visual content playing a key role in these gatewatching practices. The transient counter-visuality â or critical stance â of tweets with user-manipulated images, however, also suggests that the deliberative potential of these publics is not easily sustainable over time
Automated Certification of Authorisation Policy Resistance
Attribute-based Access Control (ABAC) extends traditional Access Control by
considering an access request as a set of pairs attribute name-value, making it
particularly useful in the context of open and distributed systems, where
security relevant information can be collected from different sources. However,
ABAC enables attribute hiding attacks, allowing an attacker to gain some access
by withholding information. In this paper, we first introduce the notion of
policy resistance to attribute hiding attacks. We then propose the tool ATRAP
(Automatic Term Rewriting for Authorisation Policies), based on the recent
formal ABAC language PTaCL, which first automatically searches for resistance
counter-examples using Maude, and then automatically searches for an Isabelle
proof of resistance. We illustrate our approach with two simple examples of
policies and propose an evaluation of ATRAP performances.Comment: 20 pages, 4 figures, version including proofs of the paper that will
be presented at ESORICS 201
The Hilbert basis method for D-flat directions and the superpotential
We discuss, using the Hilbert basis method, how to efficiently construct a
complete basis for D-flat directions in supersymmetric Abelian and non-Abelian
gauge theories. We extend the method to discrete (R and non-R) symmetries. This
facilitates the construction of a basis of all superpotential terms in a theory
with given symmetries.Comment: 11 pages; a related mathematica code can be found at
http://einrichtungen.ph.tum.de/T30e/codes/NonAbelianHilbert
Relative blocking in posets
Poset-theoretic generalizations of set-theoretic committee constructions are
presented. The structure of the corresponding subposets is described. Sequences
of irreducible fractions associated to the principal order ideals of finite
bounded posets are considered and those related to the Boolean lattices are
explored; it is shown that such sequences inherit all the familiar properties
of the Farey sequences.Comment: 29 pages. Corrected version of original publication which is
available at http://www.springerlink.com, see Corrigendu
Banks' risk assessment of Swedish SMEs
Building on the literatures on asymmetric information and risk taking, this paper applies conjoint experiments to investigate lending officers' probabilities of supporting credit to established or existing SMEs. Using a sample of 114 Swedish lending officers, we test hypotheses concerning how information on the borrower's ability to repay the loan; alignment of risk preferences; and risk sharing affect their willingness to grant credit. Results suggest that features that reduce the risk to the bank and shift the risk to the borrower have the largest impact. The paper highlights the interaction between factors that influence the credit decision. Implications for SMEs, banks and research are discussed
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