236 research outputs found
Osculating spaces to secant varieties
We generalize the classical Terracini's Lemma to higher order osculating
spaces to secant varieties. As an application, we address with the so-called
Horace method the case of the -Veronese embedding of the projective 3-space
Comportamento a fatica di strutture meccaniche in piena scala: risultati sperimentali e previsioni
Il lavoro si propone di presentare le principali attivitĂ di ricerca svolte, negli ultimi anni, presso il Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione (DIMNP) dellâUniversitĂ di Pisa, anche in collaborazione con lâUniversitĂ di Trento, nel campo della resistenza a fatica delle strutture meccaniche, in particolare per quanto riguarda la conduzione di âtestâ su componenti in piena scala e la loro interpretazione. Viene quindi condotta unâillustrazione di alcune recenti campagne sperimentali (Es.: giunzioni filettate in acciaio, elementi di sospensione in alluminio, ingranaggi ad elevate
prestazioni), alla quale segue una descrizione delle attivitĂ di caratterizzazione di base e di modellazione condotte al fine di costituire una adeguata base di conoscenze per la interpretazione. Infine, vengono analizzati i risultati ottenuti, evidenziando alcuni problemi aperti, sia sul piano concettuale che su quello applicativo
Lampedusa in Berlin : (Im)Mobilität innerhalb des europäischen Grenzregimes
This paper analyses the mobility practices offorced migrants within the European border regime. It investigates the relation between the control and management mechanisms of migration and the attempts of forced migrants to move freely, crisscrossing territorial and juridical borders in Europe. The paper focuses on the experiences of a group of forced migrants, who, after escaping the war in Libya, obtained humanitarian protection in Italy, but because of the current precarious socio-economic conditions in Southern Europe, decided to leave for North European countries. A group settled in Berlin, which gave rise to a protest claiming the right to stay and work against what is foreseen by European Union law. This paper draws on ethnographic work to show the tension between individual desires and practices of free mobility and the structural and juridical constraints implemented by institutions in order to control it and contain it. Focusing on this (im)mobility highlights the internal borders of Europe and how they are continuously challenged by migrant subjects. Three different kinds of mobility emerge across the European space: mobility within national territory, infra-national mobility, and \u201ccommuting-mobility\u201d. In this way, migrant subjects create new geographies and experience the whole European territory as one place: living in Berlin, renewing documents in Milan, attending education courses in Turin, and working seasonally in Sicily or Apulia. Such mobilities are supported by networks of migrants, who continuously move, and their supporters. This suggests a process of \u201cEuropeanisation from below\u201d that continuously challenges EU internal borders
Statistical mechanics of the multi-constraint continuous knapsack problem
We apply the replica analysis established by Gardner to the multi-constraint
continuous knapsack problem,which is one of the linear programming problems and
a most fundamental problem in the field of operations research (OR). For a
large problem size, we analyse the space of solution and its volume, and
estimate the optimal number of items to go into the knapsack as a function of
the number of constraints. We study the stability of the replica symmetric (RS)
solution and find that the RS calculation cannot estimate the optimal number of
items in knapsack correctly if many constraints are required.In order to obtain
a consistent solution in the RS region,we try the zero entropy approximation
for this continuous solution space and get a stable solution within the RS
ansatz.On the other hand, in replica symmetry breaking (RSB) region, the one
step RSB solution is found by Parisi's scheme. It turns out that this problem
is closely related to the problem of optimal storage capacity and of
generalization by maximum stability rule of a spherical perceptron.Comment: Latex 13 pages using IOP style file, 5 figure
Storage of correlated patterns in a perceptron
We calculate the storage capacity of a perceptron for correlated gaussian
patterns. We find that the storage capacity can be less than 2 if
similar patterns are mapped onto different outputs and vice versa. As long as
the patterns are in general position we obtain, in contrast to previous works,
that in agreement with Cover's theorem. Numerical simulations
confirm the results.Comment: 9 pages LaTeX ioplppt style, figures included using eps
Mean-field analysis of the majority-vote model broken-ergodicity steady state
We study analytically a variant of the one-dimensional majority-vote model in
which the individual retains its opinion in case there is a tie among the
neighbors' opinions. The individuals are fixed in the sites of a ring of size
and can interact with their nearest neighbors only. The interesting feature
of this model is that it exhibits an infinity of spatially heterogeneous
absorbing configurations for whose statistical properties we
probe analytically using a mean-field framework based on the decomposition of
the -site joint probability distribution into the -contiguous-site joint
distributions, the so-called -site approximation. To describe the
broken-ergodicity steady state of the model we solve analytically the
mean-field dynamic equations for arbitrary time in the cases n=3 and 4. The
asymptotic limit reveals the mapping between the statistical
properties of the random initial configurations and those of the final
absorbing configurations. For the pair approximation () we derive that
mapping using a trick that avoids solving the full dynamics. Most remarkably,
we find that the predictions of the 4-site approximation reduce to those of the
3-site in the case of expectations involving three contiguous sites. In
addition, those expectations fit the Monte Carlo data perfectly and so we
conjecture that they are in fact the exact expectations for the one-dimensional
majority-vote model
Random replicators with high-order interactions
We use tools of the equilibrium statistical mechanics of disordered systems
to study analytically the statistical properties of an ecosystem composed of N
species interacting via random, Gaussian interactions of order p >= 2, and
deterministic self-interactions u <= 0. We show that for nonzero u the effect
of increasing the order of the interactions is to make the system more
cooperative, in the sense that the fraction of extinct species is greatly
reduced. Furthermore, we find that for p > 2 there is a threshold value which
gives a lower bound to the concentration of the surviving species, preventing
then the existence of rare species and, consequently, increasing the robustness
of the ecosystem to external perturbations.Comment: 7 pages, 4 Postscript figure
Optimal static and dynamic recycling of defective binary devices
The binary Defect Combination Problem consists in finding a fully working
subset from a given ensemble of imperfect binary components. We determine the
typical properties of the model using methods of statistical mechanics, in
particular, the region in the parameter space where there is almost surely at
least one fully-working subset. Dynamic recycling of a flux of imperfect binary
components leads to zero wastage.Comment: 14 pages, 15 figure
Instance Space of the Number Partitioning Problem
Within the replica framework we study analytically the instance space of the
number partitioning problem. This classic integer programming problem consists
of partitioning a sequence of N positive real numbers \{a_1, a_2,..., a_N}
(the instance) into two sets such that the absolute value of the difference of
the sums of over the two sets is minimized. We show that there is an
upper bound to the number of perfect partitions (i.e. partitions
for which that difference is zero) and characterize the statistical properties
of the instances for which those partitions exist. In particular, in the case
that the two sets have the same cardinality (balanced partitions) we find
. Moreover, we show that the disordered model resulting from hte
instance space approach can be viewed as a model of replicators where the
random interactions are given by the Hebb rule.Comment: 7 page
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