Добірка наукової літератури з теми "Category FI of finite sets and injections"

AbstractWe characterize the generalized Auslander–Reiten duality on the category of finitely presented modules over some certain Hom-finite category. Examples include the category FI of finite sets with injections, and the one VI of finite-dimensional vector spaces with linear injections over a finite field.

Abstract We study representations of wreath product analogues of categories of finite sets. This includes the category of finite sets and injections (studied by Church, Ellenberg, and Farb) and the opposite of the category of finite sets and surjections (studied by the authors in previous work). We prove noetherian properties for the injective version when the group in question is polycyclic-by-finite and use it to deduce general twisted homological stability results for such wreath products and indicate some applications to representation stability. We introduce a new class of formal languages (quasi-ordered languages) and use them to deduce strong rationality properties of Hilbert series of representations for the surjective version when the group is finite.

We study the quadratic quotients of the incidence category of the Young lattice defined by the zero relations corresponding to adding two boxes to the same row, or to the same column, or both. We show that the last quotient corresponds to the Koszul dual of the original incidence category, while the first two quotients are, in a natural way, Koszul duals of each other and hence they are in particular Koszul self-dual. Both of these two quotients are known to be basic representatives in the Morita equivalence class of the category of injections between finite sets. We also present a new, rather direct, argument establishing this Morita equivalence.

AbstractA category has the amalgamation property (AP) if every pushout diagram has a cocone, and the joint embedding property (JEP) if every finite coproduct diagram has a cocone. We show that for a finitely generated category I, the following are equivalent: (i) every I-shaped diagram in a category with the AP and the JEP has a cocone; (ii) every I-shaped diagram in the category of sets and injections has a cocone; (iii) a certain canonically defined category ${\cal L}\left( {\bf{I}} \right)$ of “paths” in I has only idempotent endomorphisms. When I is a finite poset, these are further equivalent to: (iv) every upward-closed subset of I is simply-connected; (v) I can be built inductively via some simple rules. Our proof also shows that these conditions are decidable for finite I.

AbstractThe chromatic functor of a simple graph is a functorization of the chromatic polynomial. M. Yoshinaga showed that two ûnite graphs have isomorphic chromatic functors if and only if they have the same chromatic polynomial. The key ingredient in the proof is the use of stable partitions of graphs. The latter is shown to be closely related to chromatic functors. In this note, we further investigate some interesting properties of chromatic functors associated with simple graphs using stable partitions. Our ûrst result is the determination of the group of natural automorphisms of the chromatic functor, which is, in general, a larger group than the automorphism group of the graph. The second result is that the composition of the chromatic functor associated with a ûnite graph restricted to the category FI of ûnite sets and injections with the free functor into the category of complex vector spaces yields a consistent sequence of representations of symmetric groups that is representation stable in the sense of Church–Farb.

Universal Causality is a mathematical framework based on higher-order category theory, which generalizes previous approaches based on directed graphs and regular categories. We present a hierarchical framework called UCLA (Universal Causality Layered Architecture), where at the top-most level, causal interventions are modeled as a higher-order category over simplicial sets and objects. Simplicial sets are contravariant functors from the category of ordinal numbers Δ into sets, and whose morphisms are order-preserving injections and surjections over finite ordered sets. Non-random interventions on causal structures are modeled as face operators that map n-simplices into lower-level simplices. At the second layer, causal models are defined as a category, for example defining the schema of a relational causal model or a symmetric monoidal category representation of DAG models. The third layer corresponds to the data layer in causal inference, where each causal object is mapped functorially into a set of instances using the category of sets and functions between sets. The fourth homotopy layer defines ways of abstractly characterizing causal models in terms of homotopy colimits, defined in terms of the nerve of a category, a functor that converts a causal (category) model into a simplicial object. Each functor between layers is characterized by a universal arrow, which define universal elements and representations through the Yoneda Lemma, and induces a Grothendieck category of elements that enables combining formal causal models with data instances, and is related to the notion of ground graphs in relational causal models. Causal inference between layers is defined as a lifting problem, a commutative diagram whose objects are categories, and whose morphisms are functors that are characterized as different types of fibrations. We illustrate UCLA using a variety of representations, including causal relational models, symmetric monoidal categorical variants of DAG models, and non-graphical representations, such as integer-valued multisets and separoids, and measure-theoretic and topological models.

Abstract We show that Schmitt’s restriction species (such as graphs, matroids, posets, etc.) naturally induce decomposition spaces (a.k.a. unital $2$-Segal spaces), and that their associated coalgebras are an instance of the general construction of incidence coalgebras of decomposition spaces. We introduce directed restriction species that subsume Schmitt’s restriction species and also induce decomposition spaces. Whereas ordinary restriction species are presheaves on the category of finite sets and injections, directed restriction species are presheaves on the category of finite posets and convex maps. We also introduce the notion of monoidal (directed) restriction species, which induce monoidal decomposition spaces and hence bialgebras, most often Hopf algebras. Examples of this notion include rooted forests, directed graphs, posets, double posets, and many related structures. A prominent instance of a resulting incidence bialgebra is the Butcher–Connes–Kreimer Hopf algebra of rooted trees. Both ordinary and directed restriction species are shown to be examples of a construction of decomposition spaces from certain cocartesian fibrations over the category of finite ordinals that are also cartesian over convex maps. The proofs rely on some beautiful simplicial combinatorics, where the notion of convexity plays a key role. The methods developed are of independent interest as techniques for constructing decomposition spaces.

AbstractThe commutative differential graded algebra $A_{\mathrm {PL}}(X)$ of polynomial forms on a simplicial set $X$ is a crucial tool in rational homotopy theory. In this note, we construct an integral version $A^{\mathcal {I}}(X)$ of $A_{\mathrm {PL}}(X)$. Our approach uses diagrams of chain complexes indexed by the category of finite sets and injections $\mathcal {I}$ to model $E_{\infty }$ differential graded algebras (dga) by strictly commutative objects, called commutative $\mathcal {I}$-dgas. We define a functor $A^{\mathcal {I}}$ from simplicial sets to commutative $\mathcal {I}$-dgas and show that it is a commutative lift of the usual cochain algebra functor. In particular, it gives rise to a new construction of the $E_{\infty }$ dga of cochains. The functor $A^{\mathcal {I}}$ shares many properties of $A_{\mathrm {PL}}$, and can be viewed as a generalization of $A_{\mathrm {PL}}$ that works over arbitrary commutative ground rings. Working over the integers, a theorem by Mandell implies that $A^{\mathcal {I}}(X)$ determines the homotopy type of $X$ when $X$ is a nilpotent space of finite type.

Abstract We show that if X n {X_{n}} is a variety of c × n {c\times n} -matrices that is stable under the group Sym ⁡ ( [ n ] ) {\operatorname{Sym}([n])} of column permutations and if forgetting the last column maps X n {X_{n}} into X n - 1 {X_{n-1}} , then the number of Sym ⁡ ( [ n ] ) {\operatorname{Sym}([n])} -orbits on irreducible components of X n {X_{n}} is a quasipolynomial in n for all sufficiently large n. To this end, we introduce the category of affine 𝐅𝐈 𝐨𝐩 {\mathbf{FI^{op}}} -schemes of width one, review existing literature on such schemes, and establish several new structural results about them. In particular, we show that under a shift and a localisation, any width-one 𝐅𝐈 𝐨𝐩 {\mathbf{FI^{op}}} -scheme becomes of product form, where X n = Y n {X_{n}=Y^{n}} for some scheme Y in affine c-space. Furthermore, to any 𝐅𝐈 𝐨𝐩 {\mathbf{FI^{op}}} -scheme of width one we associate a component functor from the category 𝐅𝐈 {\mathbf{FI}} of finite sets with injections to the category 𝐏𝐅 {\mathbf{PF}} of finite sets with partially defined maps. We present a combinatorial model for these functors and use this model to prove that Sym ⁡ ( [ n ] ) {\operatorname{Sym}([n])} -orbits of components of X n {X_{n}} , for all n, correspond bijectively to orbits of a groupoid acting on the integral points in certain rational polyhedral cones. Using the orbit-counting lemma for groupoids and theorems on quasipolynomiality of lattice point counts, this yields our Main Theorem. We present applications of our methods to counting fixed-rank matrices with entries in a prescribed set and to counting linear codes over finite fields up to isomorphism.

Abstract The purpose of this paper is to show that various convolution products are fully homotopical, meaning that they preserve weak equivalences in both variables without any cofibrancy hypothesis. We establish this property for diagrams of simplicial sets indexed by the category of finite sets and injections and for tame $M$-simplicial sets, with $M$ the monoid of injective self-maps of the positive natural numbers. We also show that a certain convolution product studied by Nikolaus and the 1st author is fully homotopical. This implies that every presentably symmetric monoidal $\infty $-category can be represented by a symmetric monoidal model category with a fully homotopical monoidal product.

Dans cette thèse on introduit différentes notions (forte et faibles) de foncteurs polynomiaux sur les catégories FId et on étudie leur comportement. On adapte aussi la définition classique de foncteurs polynomiaux (basée sur les effets croisés) au cadre de FId, et on montre que les deux définitions obtenues coïncident. Les foncteurs polynomiaux sur FId s'avèrent plus dificiles à étudier que sur FI. Par exemple, les projectifs standards sont fortement polynomiaux sur FI et on montre que ce n'est plus le cas sur FId pour d > 1. On étudie alors diférents quotients polynomiaux de ces foncteurs. On amorce également l'étude de la polynomialité des foncteurs considérés par Ramos en calculant explicitement les foncteurs associés aux graphes linéaires. Cependant, la notion forte de foncteurs polynomiaux manque de propriétés essentielles concernant les phénomènes stables. On introduit alors les foncteurs faiblement polynomiaux en considérant le quotient par une sous-catégorie afin de supprimer les foncteurs problématiques. Alors que les foncteurs faiblement polynomiaux de degré 0 sur FI sont les foncteurs constants, on donne une description de ceux sur FId qui forment une catégorie plus complexe. On en déduit que l'adaptation directe des méthodes utilisées par Djament et Vespa pour FI ne fonctionne pas
In this thesis we introduce different notions (strong and weak) of polynomial functors over the categories FId and we study their behaviour. We also adapt the classical definition of polynomial functors (based on cross effects) to the framework of FId, and we show that the two definitions obtained coincide. The polynomial functors over FId turn out to be harder to study than over FI. For example, the standard projectives are strong polynomial over FI and we show that this is no longer the case over FId for d > 1. We then study different polynomial quotients of these functors. We also initiate the study of the polynomiality of the functors considered by Ramos by explicitly calculating the functors associated with linear graphs. However, the strong notion of polynomial functors lacks essential properties concerning stable phenomena. We then introduce the weak polynomial functors by considering the quotient by a subcategory in order to eliminate the problematic functors. While the weak polynomial functors of degree 0 over FI are the constant functors, we give a description of those over FId which form a more complex category. We deduce that a direct adaptation of the methods used by Djament and Vespa for FI does not work