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@article{CFC, author = {Vaggione, Diego and S\'anchez Terraf, Pedro}, title = {Compact factor congruences imply {B}oolean factor congruences}, journal = {Algebra univers. }, abstract = {We prove that any variety $\mathcal{V}$ in which every factor congruence is compact has Boolean factor congruences, i.e., for all $A$ in $\mathcal{V}$ the set of factor congruences of A is a distributive sublattice of the congruence lattice of A.}, volume = {51}, year = {2004}, pages = {207--213}, doi = {10.1007/s00012-004-1857-1}, zbl = {1087.08001} }

@article{DFC, author = {S\'anchez Terraf, Pedro and Vaggione, Diego}, title = {Varieties with Definable Factor Congruences}, journal = {Trans. Amer. Math. Soc.}, volume = {361}, year = {2009}, pages = {5061--5088}, doi = {10.1090/S0002-9947-09-04921-6}, abstract = {We study direct product representations of algebras in varieties. We collect several conditions expressing that these representations are \emph{definable} in a first-order-logic sense, among them the concept of Definable Factor Congruences (DFC). The main results are that DFC is a Mal'cev property and that it is equivalent to all other conditions formulated; in particular we prove that $\mathcal{V}$ has DFC if and only if $\mathcal{V}$ has $\vec{0}$ \& $\vec{1}$ and \emph{Boolean Factor Congruences}. We also obtain an explicit first order definition $\Phi$ of the kernel of the canonical projections via the terms associated to the Mal'cev condition for DFC, in such a manner it is preserved by taking direct products and direct factors. The main tool is the use of \emph{central elements,} which are a generalization of both central idempotent elements in rings with identity and neutral complemented elements in a bounded lattice.}, zbl = {1223.08001}, mrnumber = {2515803} }

@article{EDFC, author = {S\'anchez Terraf, Pedro}, title = {Existentially definable factor congruences}, journal = {Acta Scientiarum Mathematicarum (Szeged)}, volume = {76}, number = {1--2}, year = {2010}, pages = {49--53}, url = {https://bit.ly/3cBZP9f}, eprint = {0906.4722}, abstract = {A variety $\mathcal{V}$ has \emph{definable factor congruences} if and only if factor congruences can be defined by a first-order formula $\Phi$ having \emph{central elements} as parameters. We prove that if $\Phi$ can be chosen to be existential, factor congruences in every algebra of $\mathcal{V}$ are compact.}, zbl = {1274.08028} }

@article{pogroupoids, author = {S\'anchez Terraf, Pedro}, title = {Directly Indecomposables in Semidegenerate Varieties of Connected po-Groupoids}, journal = {Order}, volume = {25}, number = {4}, year = {2008}, pages = {377--386}, doi = {10.1007/s11083-008-9101-9}, abstract = {We study varieties with a term-definable poset structure, \emph{po-groupoids}. It is known that connected posets have the \emph{strict refinement property} (SRP). In a previous work by Vaggione and the author it is proved that semidegenerate varieties with the SRP have definable factor congruences and if the similarity type is finite, directly indecomposables are axiomatizable by a set of first-order sentences. We obtain such a set for semidegenerate varieties of connected po-groupoids and show its quantifier complexity is bounded in general.}, zbl = {1168.08005} }

@article{Pedro20111048, title = {Unprovability of the logical characterization of bisimulation}, journal = {Information and Computation}, volume = 209, number = 7, issue = 7, pages = {1048--1056}, year = 2011, issn = {0890-5401}, doi = {10.1016/j.ic.2011.02.003}, url = {http://www.sciencedirect.com/science/article/pii/S0890540111000691}, author = {S{\'a}nchez Terraf, Pedro}, keywords = {Labelled Markov process}, keywords = {Probabilistic bisimulation}, keywords = {Modal logic}, keywords = {Nonmeasurable set}, abstract = {We quickly review \emph{labelled Markov processes (LMP)} and provide a counterexample showing that in general measurable spaces, event bisimilarity and state bisimilarity differ in LMP. This shows that the logic in the work by Desharnais does not characterize state bisimulation in non-analytic measurable spaces. Furthermore we show that, under current foundations of Mathematics, such logical characterization is unprovable for spaces that are projections of a coanalytic set. Underlying this construction there is a proof that stationary Markov processes over general measurable spaces do not have semi-pullbacks.}, zbl = {1216.68196} }

@article{1228.08003, author = {S{\'a}nchez Terraf, Pedro}, title = {{{B}oolean factor congruences and property $(*)$}}, language = {English}, journal = {Int. J. Algebra Comput. }, volume = {21}, number = {6}, pages = {931-950}, year = {2011}, doi = {10.1142/S021819671100656X}, abstract = {{Summary: A variety ${\cal V}$ has Boolean factor congruences (BFC) if the set of factor congruences of any algebra in ${\cal V}$ is a distributive sublattice of its congruence lattice; this property holds in rings with unit and in every variety which has a semilattice operation. BFC has a prominent role in the study of uniqueness of direct product representations of algebras, since it is a strengthening of the refinement property. We provide an explicit Mal'tsev condition for BFC. With the aid of this condition, it is shown that BFC is equivalent to a variant of the definability property $(*)$, an open problem in {\it R. Willard}'s work [J. Algebra 132, No.~1, 130--153 (1990; Zbl 0737.08006)].}}, keywords = {{Boolean factor congruences; strict refinement property; definability; preservation by direct factors}}, classmath = {{*08B05 (Equational logic in varieties of algebras) }}, zbl = {1228.08003} }

@article{D'Argenio:2012:BNL:2139682.2139685, author = {D'Argenio, Pedro R. and S\'{a}nchez Terraf, Pedro and Wolovick, Nicol\'{a}s}, title = {Bisimulations for non-deterministic labelled {M}arkov processes}, journal = {Mathematical Structures in Comp. Sci.}, issue_date = {February 2012}, volume = {22}, number = {1}, issue = 1, month = feb, year = {2012}, issn = {0960-1295}, pages = {43--68}, numpages = {26}, url = {http://dx.doi.org/10.1017/S0960129511000454}, doi = {10.1017/S0960129511000454}, acmid = {2139685}, publisher = {Cambridge University Press}, address = {New York, NY, USA}, abstract = { We extend the theory of labeled Markov processes with \emph{internal} nondeterminism, a fundamental concept for the further development of a process theory with abstraction on nondeterministic continuous probabilistic systems. % We define \emph{nondeterministic labeled Markov processes (NLMP)} and provide three definition of bisimulations: a bisimulation following a traditional characterization, a \emph{state} based bisimulation tailored to our ``measurable'' non-determinism, and an \emph{event} based bisimulation. % We show the relation between them, including that the largest state bisimulation is also an event bisimulation. % We also introduce a variation of the Hennessy-Milner logic that characterizes event bisimulation and that is sound w.r.t.\ the other bisimulations for arbitrary NLMP. % This logic, however, is infinitary as it contains a denumerable $\bigvee$. % We then introduce a finitary sublogic that characterize all bisimulations for image finite NLMP whose underlying measure space is also analytic. Hence, in this setting, all notions of bisimulation we deal with turn out to be equal. % Finally, we show that all notions of bisimulations are different in the general case. The counterexamples that separate them turn to be \emph{non-probabilistic} NLMP.}, zbl = {1234.68316} }

@article{2012arXiv1211.0967S, author = {S{\'a}nchez Terraf, Pedro}, title = {Bisimilarity is not {B}orel}, abstract = {We prove that the relation of bisimilarity between countable labelled transition systems is $\Sigma_1^1$-complete (hence not Borel), by reducing the set of non-wellorders over the natural numbers continuously to it. This has an impact on the theory of probabilistic and nondeterministic processes over uncountable spaces, since logical characterizations of bisimilarity (as, for instance, those based on the unique structure theorem for analytic spaces) require a countable logic whose formulas have measurable semantics. Our reduction shows that such a logic does not exist in the case of image-infinite processes.}, ee = {http://arxiv.org/abs/1211.0967}, keywords = {Mathematics - Logic, Computer Science - Logic in Computer Science, 03B70, 03E15, 28A05, F.4.1, F.1.2}, journal = {Mathematical Structures in Computer Science}, issn = {1469-8072}, doi = {10.1017/S0960129515000535}, zbl = {1377.68150}, url = {http://journals.cambridge.org/article_S0960129515000535}, pages = {1265--1284}, number = {7}, month = {10}, year = {2017}, volume = {27} }

@article{fact_slat, author = {S{\'a}nchez Terraf, Pedro}, title = {Factor Congruences in Semilattices}, journal = {Revista de la {U}ni\'on {M}atem\'atica {A}rgentina}, volume = {52}, number = {1}, year = {2011}, ee = {http://arxiv.org/abs/0809.3822v2}, eprint = {0809.3822}, keywords = {semilattice, direct factor, factor congruence, generalized direct sum, generalized ideal}, pages = {1--10}, url = {http://inmabb.criba.edu.ar/revuma/pdf/v52n1/v52n1a03.pdf}, abstract = {We characterize factor congruences in semilattices by using generalized notions of order ideal and of direct sum of ideals. When the semilattice has a minimum (maximum) element, these generalized ideals turn into ordinary (dual) ideals.}, zbl = {1242.06006} }

@article{2014arXiv1405.7141D, author = {Doberkat, Ernst-Erich and S{\'a}nchez Terraf, Pedro}, title = {Stochastic Nondeterminism and Effectivity Functions}, journal = {Journal of Logic and Computation}, archiveprefix = {arXiv}, eprint = {1405.7141}, primaryclass = {cs.LO}, keywords = {stochastic effectivity function, non-deterministic labelled Markov process, state bisimilarity, coalgebra}, year = 2015, doi = {10.1093/logcom/exv049}, abstract = {This paper investigates stochastic nondeterminism on continuous state spaces by relating nondeterministic kernels and stochastic effectivity functions to each other. Nondeterministic kernels are functions assigning each state a set o subprobability measures, and effectivity functions assign to each state an upper-closed set of subsets of measures. Both concepts are generalizations of Markov kernels used for defining two different models: Nondeterministic labelled Markov processes and stochastic game models, respectively. We show that an effectivity function that maps into principal filters is given by an image-countable nondeterministic kernel, and that image-finite kernels give rise to effectivity functions. We define state bisimilarity for the latter, considering its connection to morphisms. We provide a logical characterization of bisimilarity in the finitary case. A generalization of congruences (event bisimulations) to effectivity functions and its relation to the categorical presentation of bisimulation are also studied.} }

@article{2015arXiv150401789A, author = {Areces, Carlos and Campercholi, Miguel and Penazzi, Daniel and S{\'a}nchez Terraf, Pedro}, title = {{The Lattice of Congruences of a Finite Linear Frame}}, journal = {Journal of Logic and Computation}, archiveprefix = {arXiv}, eprint = {1504.01789}, primaryclass = {math.LO}, keywords = {Mathematics - Logic, Computer Science - Logic in Computer Science, 03B45 (Primary), 06B10, 06E25, 03B70 (Secondary), F.4.1, F.1.2}, doi = {10.1093/logcom/exx026}, zbl = {06981765}, pages = {2653--2688}, volume = 27, issue = 8, abstract = {Let $\mathbf{F}=\left\langle F,R\right\rangle $ be a finite Kripke frame. A congruence of $\mathbf{F}$ is a bisimulation of $\mathbf{F}$ that is also an equivalence relation on F. The set of all congruences of $\mathbf{F}$ is a lattice under the inclusion ordering. In this article we investigate this lattice in the case that $\mathbf{F}$ is a finite linear frame. We give concrete descriptions of the join and meet of two congruences with a nontrivial upper bound. Through these descriptions we show that for every nontrivial congruence $\rho$, the interval $[\mathrm{Id_{F},\rho]}$ embeds into the lattice of divisors of a suitable positive integer. We also prove that any two congruences with a nontrivial upper bound permute.}, year = 2017, month = apr, adsurl = {http://adsabs.harvard.edu/abs/2015arXiv150401789A}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

@article{2017arXiv170602801P, author = {Pachl, Jan and S{\'a}nchez Terraf, Pedro}, title = {Semipullbacks of labelled {M}arkov processes}, journal = {Logical Methods in Computer Science}, volume = 17, number = 2, year = 2021, month = apr, archive = {arXiv}, eprint = {1706.02801}, primaryclass = {math.PR}, keywords = {Mathematics - Probability, Computer Science - Logic in Computer Science, 28A35, 28A60, 68Q85, F.4.1, F.1.2}, adsurl = {http://adsabs.harvard.edu/abs/2017arXiv170602801P}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, doi = {10.23638/LMCS-17(2:3)2021}, url = {https://lmcs.episciences.org/7361}, abstract = {A \emph{labelled Markov process (LMP)} consists of a measurable space $S$ together with an indexed family of Markov kernels from $S$ to itself. This structure has been used to model probabilistic computations in Computer Science, and one of the main problems in the area is to define and decide whether two LMP $S$ and $S'$ ``behave the same''. There are two natural categorical definitions of sameness of behavior: $S$ and $S'$ are \emph{bisimilar} if there exist an LMP $ T$ and measure preserving maps forming a diagram of the shape $ S\leftarrow T \rightarrow{S'}$; and they are \emph{behaviorally equivalent} if there exist some $ U$ and maps forming a dual diagram $ S\rightarrow U \leftarrow{S'}$. These two notions differ for general measurable spaces but Edalat proved that they coincide for analytic Borel spaces, showing that from every diagram $ S\rightarrow U \leftarrow{S'}$ one can obtain a bisimilarity diagram as above. Moreover, the resulting square of measure preserving maps is commutative (a \emph{semipullback}). In this paper, we extend Edalat's result to measurable spaces $S$ isomorphic to a universally measurable subset of a Polish space with the trace of the Borel $\sigma$-algebra, using a version of Strassen's theorem on common extensions of finitely additive measures.} }

@article{minimal-dual-quasi, author = {Caicedo, Xavier and Campercholi, Miguel and Kearnes, Keith A. and S{\'a}nchez Terraf, Pedro and Szendrei, {\'A}gnes and Vaggione, Diego}, year = 2021, title = {Every minimal dual discriminator variety is minimal as a quasivariety}, journal = {Algebra universalis}, month = {Apr}, day = 29, volume = 82, number = 2, pages = 36, abstract = {Let $\dagger$ denote the following property of a variety $\mathcal{V}$: \emph{Every subquasivariety of $\mathcal{V}$ is a variety}. In this paper, we prove that every idempotent dual discriminator variety has property $\dagger$ . Property $\dagger$ need not hold for nonidempotent dual discriminator varieties, but $\dagger$ does hold for \emph{minimal} nonidempotent dual discriminator varieties. Combining the results for the idempotent and nonidempotent cases, we obtain that every minimal dual discriminator variety is minimal as a quasivariety}, issn = {1420-8911}, doi = {10.1007/s00012-021-00715-8}, url = {https://doi.org/10.1007/s00012-021-00715-8} }

@article{moroni2020zhou, title = {The {Z}hou Ordinal of Labelled {M}arkov Processes over Separable Spaces}, author = {Moroni, MartÃn Santiago and S\'anchez Terraf, Pedro}, journal = {arXiv e-prints}, month = may, year = 2020, eprint = {2005.03630}, archive = {arXiv}, primaryclass = {cs.LO}, note = {Accepted for publication at the Review of Symbolic Logic}, abstract = {There exist two notions of equivalence of behavior between states of a Labelled Markov Process (LMP): state bisimilarity and event bisimilarity. The first one can be considered as an appropriate generalization to continuous spaces of Larsen and Skou's probabilistic bisimilarity, while the second one is characterized by a natural logic. C. Zhou expressed state bisimilarity as the greatest fixed point of an operator $\mathcal{O}$, and thus introduced an ordinal measure of the discrepancy between it and event bisimilarity. We call this ordinal the "Zhou ordinal" of $\mathbb{S}$, $\mathfrak{Z}(\mathbb{S})$. When $\mathfrak{Z}(\mathbb{S})=0$, $\mathbb{S}$ satisfies the Hennessy-Milner property. The second author proved the existence of an LMP $\mathbb{S}$ with $\mathfrak{Z}(\mathbb{S}) \geq 1$ and Zhou showed that there are LMPs having an infinite Zhou ordinal. In this paper we show that there are LMPs $\mathbb{S}$ over separable metrizable spaces having arbitrary large countable $\mathfrak{Z}(\mathbb{S})$ and that it is consistent with the axioms of $\mathit{ZFC}$ that there is such a process with an uncountable Zhou ordinal.} }

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