244 research outputs found
Two isomorphism criteria for directed colimits
Using the general notions of finitely presentable and finitely generated
object introduced by Gabriel and Ulmer in 1971, we prove that, in any (locally
small) category, two sequences of finitely presentable objects and morphisms
(or two sequences of finitely generated objects and monomorphisms) have
isomorphic colimits (=direct limits) if, and only if, they are confluent. The
latter means that the two given sequences can be connected by a back-and-forth
chain of morphisms that is cofinal on each side, and commutes with the
sequences at each finite stage. In several concrete situations, analogous
isomorphism criteria are typically obtained by ad hoc arguments. The abstract
results given here can play the useful r\^ole of discerning the general from
the specific in situations of actual interest. We illustrate by applying them
to varieties of algebras, on the one hand, and to dimension groups---the
ordered of approximately finite-dimensional C*-algebras---on the other.
The first application encompasses such classical examples as Kurosh's
isomorphism criterion for countable torsion-free Abelian groups of finite rank.
The second application yields the Bratteli-Elliott Isomorphism Criterion for
dimension groups. Finally, we discuss Bratteli's original isomorphism criterion
for approximately finite-dimensional C*-algebras, and show that his result does
not follow from ours.Comment: 10 page
Representation of Perfect and Local MV-algebras
We describe representation theorems for local and perfect MV-algebras in
terms of ultraproducts involving the unit interval [0,1]. Furthermore, we give
a representation of local Abelian lattice-ordered groups with strong unit as
quasi-constant functions on an ultraproduct of the reals. All the above
theorems are proved to have a uniform version, depending only on the
cardinality of the algebra to be embedded, as well as a definable construction
in ZFC. The paper contains both known and new results and provides a complete
overview of representation theorems for such classes
Canonical formulas for k-potent commutative, integral, residuated lattices
Canonical formulas are a powerful tool for studying intuitionistic and modal
logics. Actually, they provide a uniform and semantic way to axiomatise all
extensions of intuitionistic logic and all modal logics above K4. Although the
method originally hinged on the relational semantics of those logics, recently
it has been completely recast in algebraic terms. In this new perspective
canonical formulas are built from a finite subdirectly irreducible algebra by
describing completely the behaviour of some operations and only partially the
behaviour of some others. In this paper we export the machinery of canonical
formulas to substructural logics by introducing canonical formulas for
-potent, commutative, integral, residuated lattices (-).
We show that any subvariety of - is axiomatised by canonical
formulas. The paper ends with some applications and examples.Comment: Some typo corrected and additional comments adde
Advances in the theory of μŁΠ algebras
Recently an expansion of ŁΠ1/2 logic with fixed points has been considered [23]. In the present work we study the algebraic semantics of this logic, namely μŁΠ algebras, from algebraic, model theoretic and computational standpoints. We provide a characterisation of free μŁΠ algebras as a family of particular functions from [0,1]n to [0,1]. We show that the first-order theory of linearly ordered μŁΠ algebras enjoys quantifier elimination, being, more precisely, the model completion of the theory of linearly ordered ŁΠ1/2 algebras. Furthermore, we give a functional representation of any ŁΠ1/2 algebra in the style of Di Nola Theorem for MV-algebras and finally we prove that the equational theory of μŁΠ algebras is in PSPACE. © The Author 2010. Published by Oxford University Press. All rights reserved.Marchioni acknowledges partial support of the Spanish projects MULOG2 (TIN2007-68005-C04), Agreement Technologies (CONSOLIDER CSD2007-0022, INGENIO 2010), the Generalitat de Catalunya grant 2009-SGR-1434, and Juan de la Cierva Program of the Spanish MICINN, as well as the ESF Eurocores-LogICCC/MICINN project (FFI2008-03126-E/FILO). Spada acknowledges partially supported of the FWF project P 19872-N18.Peer Reviewe
Stone-Gelfand duality for metrically complete lattice-ordered groups
We extend Yosida's 1941 version of Stone-Gelfand duality to metrically
complete unital lattice-ordered groups that are no longer required to be real
vector spaces. This calls for a generalised notion of compact Hausdorff space
whose points carry an arithmetic character to be preserved by continuous maps.
The arithmetic character of a point is (the complete isomorphism invariant of)
a metrically complete additive subgroup of the real numbers containing ,
namely, either for an integer , or the
whole of . The main result needed to establish the extended duality
theorem is a substantial generalisation of Urysohn's Lemma to such "arithmetic"
compact Hausdorff spaces. The original duality is obtained by considering the
full subcategory of spaces whose each point is assigned the entire group of
real numbers. In the introduction we indicate motivations from and connections
with the theory of dimension groups.Comment: 24 pages, 2 figure
Social anxiety and Internet gaming disorder: The role of motives and metacognitions
AbstractBackground and aimsIn recent years, Internet Gaming Disorder (IGD) has been recognized as a mental health problem. Although research has found that social anxiety, motives, the preference for online social interactions (POSI), and metacognitions about online gaming are independent predictors of IGD, less is known about their relative contribution to IGD. The aim of the current study was to model the relationship between social anxiety, motives, POSI, metacognitions about online gaming, and IGD.MethodsFive hundred and forty three Italian gamers who play more than 7 h a week (mean age = 23.9 years; SD = 6.15 years; 82.5% males) were included in the study. The pattern of relationships specified by the theoretical model was examined through path analysis.ResultsResults showed that social anxiety was directly associated with four motives (escape, coping, fantasy, and recreation), POSI, and positive and negative metacognitions about online gaming, and IGD. The Sobel test showed that negative metacognitions about online gaming played the strongest mediating role in the relationship between social anxiety and IGD followed by escape, POSI, and positive metacognitions. The model accounted for 54% of the variance for IGD.Discussion and conclusionsOverall, our findings show that, along with motives and POSI, metacognitions about online gaming may play an important role in the association between social anxiety and IGD. The clinical and preventive implications of these findings are discussed
A systematic and critical review of life cycle approaches to assess circular economy pathways in the agri-food sector
This study provides a literature review of life cycle approaches used to assess circular economy (CE) pathways in the agri-food sector. The scope of this review is to understand how and how much the LC-based analysis is useful to evaluate if CE strategies are more sustainable than linear/traditional economic models in agri-food production systems. To carry out the systematic and critical literature review the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) protocol was employed. The literature search was performed employing scientific databases (Scopus and Web of Science). The results highlight that 52 case studies out of 84 (62% of the total) use stand-alone life cycle assessment (LCA) to evaluate the benefits/impacts of circular economy strategies. Only eight studies (9.5%) deal with the life cycle costing (LCC) approach combined with other analyses, while no paper deals with the social life cycle assessment (S-LCA) methodology. We argue that experts in life cycle methodologies must strive to adopt some key elements to ensure that the results obtained fit perfectly with the measurements of circularity and that these can even be largely based on a common basis
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