Jiří Adámek, Chase Ford, Stefan Milius, Lutz Schröder
Finitary monads on $\mathsf{Pos}$ are characterized as the precisely the free-algebra monads of varieties of algebras. These are classes of ordered algebras specified by inequations in context. Analagously, finitary enriched monads on $\mathsf{Pos}$ are characterized: here we work with varieties of coherent algebras which means that their operations are monotone.
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Strongly finitary metric monads are too strong
该研究指出强有限度量单子过于强大，通过反例证明自由代数单子并非全部强有限，并揭示了1-基本单子的特性。
Sifted Colimits, Strongly Finitary Monads and Continuous Algebras
本文刻画了 Pos、CPO 和 DCPO 范畴上强有限单子等价于保持筛选余极限的单子，并将连续代数簇等同于强有限单子的单子范畴。
We consider the equivalence of Lawvere theories and finitary monads on Set from the perspective of Endf(Set)-enriched category theory, where Endf(Set) is the category of finitary endofunctors of Set. We identify finitary monads with one-object Endf(Set)-categories, and ordinary categories admitting finite powers (i.e., n-fold products of each object with itself) with Endf(Set)-categories admitting a certain class Phi of absolute colimits; we then show that, from this perspective, the passage from a finitary monad to the associated Lawvere theory is given by completion under Phi-colimits. We al…
It is well known that classical varieties of $Σ$-algebras correspond bijectively to finitary monads on $\mathsf{Set}$. We present an analogous result for varieties of ordered $Σ$-algebras, i.e., classes presented by inequations between $Σ$-terms. We prove that they correspond bijectively to strongly finitary monads on $\mathsf{Pos}$. That is, those finitary monads which preserve reflexive coinserters. We deduce that strongly finitary monads have a coinserter presentation, analogous to the coequaliser presentation of finitary monads due to Kelly and Power. We also show that these monads are lif…
For finitary regular monads T on locally finitely presentable categories we characterize the finitely presentable objects in the category of T-algebras in the style known from general algebra: they are precisely the algebras presentable by finitely many generators and finitely many relations.
An enriched view on the extended finitary monad--Lawvere theory correspondence
本文用丰富范畴论重新阐释有限余单子与Lawvere理论间的对应，将其视为丰富范畴在绝对余极限下的自由完备化，并创新性地使用双范畴丰富化以统一处理所有局部有限可表现基。
We develop universal algebra over an enriched category $\mathcal K$ and relate it to finitary enriched monads over $\mathcal K$. Using it, we deduce recent results about ordered universal algebra where inequations are used instead of equations. Then we apply it to metric universal algebra where quantitative equations are used instead of equations. This contributes to understanding of finitary monads on the category of metric spaces.
We introduce a framework for universal algebra in categories of relational structures given by finitary relational signatures and finitary or infinitary Horn theories, with the arity $λ$ of a Horn theory understood as a strict upper bound on the number of premisses in its axioms; key examples include partial orders ($λ=ω$) or metric spaces ($λ=ω_1$). We establish a bijective correspondence between $λ$-accessible enriched monads on the given category of relational structures and a notion of $λ$-ary algebraic theories (i.e. with operations of arity $<λ$), with the syntax of algebraic theories in…
Quantitative Algebras and a Classification of Metric Monads
本文研究了定量代数与度量空间中单子的分类，证明了定量变种与强有限单子之间的双射关系，并扩展到一般富集单子与广义变种的对应。
We introduce the notion of a lax monoidal fibration and we show how it can be conveniently used to deal with various algebraic structures that play an important role in some definitions of the opetopic sets (Baez-Dolan, Hermida-Makkai-Power). We present the 'standard' such structures, the exponential fibrations of basic fibrations and three areas of applications. First area is related to the T-categories of A. Burroni. The monoids in the Burroni lax monoidal fibrations form the fibration of T-categories. The construction of the relative Burroni fibrations and free T-categories in this context,…
Strongly finitary monads and multi-sorted varieties enriched in cartesian closed concrete categories
本研究将富集范畴中代数簇与强有限单子的对偶等价结果，推广到任意具体笛卡尔闭范畴及多类情形，证明了对应范畴的对偶等价性，并给出了相关实例。
After a review of the concept of "monad with arities" we show that the category of algebras for such a monad has a canonical dense generator. This is used to extend the correspondence between finitary monads on sets and Lawvere's algebraic theories to a general correspondence between monads and theories for a given category with arities. As application we determine arities for the free groupoid monad on involutive graphs and recover the symmetric simplicial nerve characterisation of groupoids.
Birkhoff's variety theorem for relative algebraic theories
本文将集合范畴上代数理论与有限单子的等价性，通过部分霍恩逻辑推广到局部有限可表示范畴，定义$\boldsymbol{A}$-相对代数理论并证明其等价于该范畴上的有限单子，广义伯克霍夫多样性定理在此等价性证明中起关键作用。
Quantitative algebras are algebras enriched in the category $\mathsf{Met}$ of metric spaces so that all operations are nonexpanding. Mardare, Plotkin and Panangaden introduced varieties (aka $1$-basic varieties) as classes of quantitative algebras presented by quantitative equations. We prove that they bijectively correspond to strongly finitary monads $T$ on $\mathsf{Met}$. This means that $T$ is the Kan extension of its restriction to finite discrete spaces. An analogous result holds in the category $\mathsf{CMet}$ of complete metric spaces. Analogously, continuous algebras are algebras enri…
Categorical universal algebra can be developed either using Lawvere theories (single-sorted finite product theories) or using monads, and the category of Lawvere theories is equivalent to the category of finitary monads on Set. We show how this equivalence, and the basic results of universal algebra, can be generalized in three ways: replacing Set by another category, working in an enriched setting, and by working with another class of limits than finite products. An important special case involves working with sifted-colimit-preserving monads rather than filtered-colimit-preserving ones.
We investigate the foundations of a theory of algebraic data types with variable binding inside classical universal algebra. In the first part, a category-theoretic study of monads over the nominal sets of Gabbay and Pitts leads us to introduce new notions of finitary based monads and uniform monads. In a second part we spell out these notions in the language of universal algebra, show how to recover the logics of Gabbay-Mathijssen and Clouston-Pitts, and apply classical results from universal algebra.
Poset-enriched categories and free exact completions
本文通过构造偏序集合富集范畴的精确完备化，证明每个有序代数变种都是某个子范畴的精确完备化，回答了Kurz和Velebil的问题。
Relativized universal algebra via partial Horn logic
本文基于部分Horn理论，将传统代数理论等价关系推广至任意局部可呈现范畴，提出相对代数理论概念，并推广Birkhoff定理构建相对普遍代数框架。
The category Rel(Set) of sets and relations can be described as a category of spans and as the Kleisli category for the powerset monad. A set-functor can be lifted to a functor on Rel(Set) iff it preserves weak pullbacks. We show that these results extend to the enriched setting, if we replace sets by posets or preorders. Preservation of weak pullbacks becomes preservation of exact lax squares. As an application we present Moss's coalgebraic over posets.
We describe a finitary 2-monad on a locally finitely presentable 2-category for which not every pseudoalgebra is equivalent to a strict one. This shows that having rank is not a sufficient condition on a 2-monad for every pseudoalgebra to be strictifiable. Our counterexample comes from higher category theory: the strict algebras are strict 3-categories, and the pseudoalgebras are a type of semi-strict 3-category lying in between Gray-categories and tricategories. Thus, the result follows from the fact that not every Gray-category is equivalent to a strict 3-category, connecting 2-categorical a…
For every finitary monad $T$ on sets and every endofunctor $F$ on the category of $T$-algebras we introduce the concept of an ffg-Elgot algebra for $F$, that is, an algebra admitting coherent solutions for finite systems of recursive equations with effects represented by the monad $T$. The goal is to study the existence and construction of free ffg-Elgot algebras. To this end, we investigate the locally ffg fixed point $φF$, i.e. the colimit of all $F$-coalgebras with free finitely generated carrier, which is shown to be the initial ffg-Elgot algebra. This is the technical foundation for our m…
Varieties of ordered algebras as categories
本文研究了由不等式表示的有序代数簇，刻画了等价于该簇的偏序集充实范畴，并证明其恰好是离散Lawvere理论对偶在sifted余极限下的自由完备化。
The algebraic expression $3 + 2 + 6$ can be evaluated to $11$, but it can also be partially evaluated to $5 + 6$. In categorical algebra, such partial evaluations can be defined in terms of the $1$-skeleton of the bar construction for algebras of a monad. We show that this partial evaluation relation can be seen as the relation internal to the category of algebras generated by relating a formal expression to its total evaluation. The relation is transitive for many monads which describe commonly encountered algebraic structures, and more generally for BC monads on $\mathsf{Set}$ (which are tho…
Codensity Lifting of Monads and its Dual
本文提出余密度提升方法，将纤维化基范畴上的单子提升到全范畴，并给出其对偶密度提升，同时刻画了代数运算的提升及偏序纤维化上单子提升的极限特征。
In earlier work, we had introduced the Kantorovich probability monad on complete metric spaces, extending a construction due to van Breugel. Here we extend the Kantorovich monad further to a certain class of ordered metric spaces, by endowing the spaces of probability measures with the usual stochastic order. It can be considered a metric analogue of the probabilistic powerdomain. The spaces we consider, which we call L-ordered, are spaces where the order satisfies a mild compatibility condition with the metric itself, rather than merely with the underlying topology. As we show, this is relate…
It has been known since the work of Duskin and Pelletier four decades ago that KH^op, the category opposite to compact Hausdorff spaces and continuous maps, is monadic over the category of sets. It follows that KH^op is equivalent to a possibly infinitary variety of algebras V in the sense of Slominski and Linton. Isbell showed in 1982 that the Lawvere-Linton algebraic theory of V can be generated using a finite number of finitary operations, together with a single operation of countably infinite arity. In 1983, Banaschewski and Rosicky independently proved a conjecture of Bankston, establishi…
We exhibit a monoidal structure on the category of finite sets indexed by P-trees for a finitary polynomial endofunctor P. This structure categorifies the monoid scheme (over Spec N) whose semiring of functions is (a P-version of) the Connes--Kreimer bialgebra H of rooted trees (a Hopf algebra after base change to Z and collapsing H_0). The monoidal structure is itself given by a polynomial functor, represented by three easily described set maps; we show that these maps are the same as those occurring in the polynomial representation of the free monad on P.
Terminal Coalgebras in Countably Many Steps
该研究证明了局部有限呈现范畴上有限自函子的终余代数可通过终余代数链的可数极限得到，并给出Vietoris多项式函子等多类函子终余代数的步长结果及相关证明。
For single-sorted algebras, Fujiwara defined, through the concept of family of basic mapping-formulas, a notion of morphism which generalizes the ordinary notion of homomorphism between algebras and an equivalence relation, the conjugation, on the families of basic mapping-formulas, which corresponds to the relation of inner isomorphism for algebras. In this paper we extend the theory of Fujiwara about morphisms to the many-sorted algebras, by defining the concept of polyderivor between many-sorted signatures, which assigns to basic sorts, words and to formal operations, families of derived te…
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