Recent zbMATH articles in MSC 06A https://www.zbmath.org/atom/cc/06A 2022-05-16T20:40:13.078697Z Werkzeug Catalan recursion on externally ordered bases of unit interval positroids https://www.zbmath.org/1483.05017 2022-05-16T20:40:13.078697Z "Camacho, Jan Tracy" https://www.zbmath.org/authors/?q=ai:camacho.jan-tracy "Chavez, Anastasia" https://www.zbmath.org/authors/?q=ai:chavez.anastasia Summary: The Catalan numbers form a sequence that counts over 200 combinatorial objects. A remarkable property of the Catalan numbers, which extends to these objects, is its recursive definition; that is, we can determine the $$n$$-th object from previous ones. A matroid is a combinatorial object that generalizes the notion of linear independence with connections to many fields of mathematics. A family of matroids, called unit interval positroids (UIP), are Catalan objects induced by the antiadjacency matrices of unit interval orders. Associated to each UIP is the set of externally ordered bases, which due to \textit{M. Las Vergnas} [Eur. J. Comb. 22, No. 5, 709--721 (2001; Zbl 0984.52018)], produces a lattice after adjoining a bottom element. We study the poset of externally ordered UIP bases and the implied Catalan-induced recursion. Explicitly, we describe an algorithm for constructing the lattice of a rank-$$n$$ UIP from the lattice of lower ranks. Using their inherent combinatorial structure, we define a simple formula to enumerate the bases for a given UIP. The edge-product space of phylogenetic trees is not shellable https://www.zbmath.org/1483.05024 2022-05-16T20:40:13.078697Z "Stadnyk, Grace" https://www.zbmath.org/authors/?q=ai:stadnyk.grace Summary: The edge-product space of phylogenetic trees is a regular CW complex whose maximal closed cells correspond to trivalent trees with leaves labeled by a finite set $$X$$. The face poset of this cell decomposition is isomorphic to the Tuffley poset, a poset of labeled forests, with a unique minimum adjoined. We show that the edge-product space of phylogenetic trees is gallery-connected. We then use combinatorial properties of the Tuffley poset and a related graph known as NNI-tree space to show that, although open intervals of the Tuffley poset were proven to be shellable by \textit{J. Gill} et al. [ibid. 41, No. 2, 158--176 (2008; Zbl 1149.05305)], the edge-product space is not shellable. The Scott topology on posets and continuous posets https://www.zbmath.org/1483.06001 2022-05-16T20:40:13.078697Z "Fan, Lihong" https://www.zbmath.org/authors/?q=ai:fan.lihong "He, Wei" https://www.zbmath.org/authors/?q=ai:he.wei|he.wei.3|he.wei.1|he.wei.2 (no abstract) Local directed complete sets and properties of their categories https://www.zbmath.org/1483.06002 2022-05-16T20:40:13.078697Z "Guan, Xue Chong" https://www.zbmath.org/authors/?q=ai:guan.xuechong "Wang, Ge Ping" https://www.zbmath.org/authors/?q=ai:wang.geping (no abstract) Meet continuity of posets via lim-inf-convergence https://www.zbmath.org/1483.06003 2022-05-16T20:40:13.078697Z "Li, Qingguo" https://www.zbmath.org/authors/?q=ai:li.qingguo "Li, Jibo" https://www.zbmath.org/authors/?q=ai:li.jibo (no abstract) On a characterization theorem for continuous posets https://www.zbmath.org/1483.06004 2022-05-16T20:40:13.078697Z "Wang, Xijuan" https://www.zbmath.org/authors/?q=ai:wang.xijuan "Lu, Tao" https://www.zbmath.org/authors/?q=ai:lu.tao "He, Wei" https://www.zbmath.org/authors/?q=ai:he.wei.2 (no abstract) Cartesian closeness of the categories of algebraic local complete posets and FS-local directed complete posets https://www.zbmath.org/1483.06005 2022-05-16T20:40:13.078697Z "Xu, Ai Jun" https://www.zbmath.org/authors/?q=ai:xu.ai-jun "Wang, Ge Ping" https://www.zbmath.org/authors/?q=ai:wang.geping (no abstract) $$\mathcal{Z}$$-quasidistributive and $$\mathcal{Z}$$-meet-distributive posets https://www.zbmath.org/1483.06006 2022-05-16T20:40:13.078697Z "Zhang, Wenfeng" https://www.zbmath.org/authors/?q=ai:zhang.wenfeng.1|zhang.wenfeng "Xu, Xiaoquan" https://www.zbmath.org/authors/?q=ai:xu.xiaoquan.1|xu.xiaoquan In domain theory, one fundamental result states that a poset is continuous if and only if it is quasicontinuous and meet-continuous. In this paper, the authors study two kinds of distributivity: $$Z$$-quasidistributivity and $$Z$$-meet-distributivity, which are the generalizations of quasicontinuity and meet-continuity. Here, $$Z$$ is a subset system, it becomes meaningful when $$Z$$ is replaced by adjectives such as directed'', chain'', finite'', etc. Analogous to the above fundamental result, the authors prove that, under some conditions, a poset is $$Z$$-predistributive iff it is $$Z$$-quasidistributive and $$Z$$-meet-distributive. In order theory, the Dedekind-MacNeille completion is the most well-known completion, which embeds a poset into a complete lattice. The order-theoretical properties which are invariant under the Dedekind-MacNeille completion are called completion-invariant. In this paper, one main result states that $$Z$$-quasidistributivity is a completion-invariant property whenever $$Z$$ is completion-stable. The way-below relation is a fundamental concept in domain theory. Replacing directed sets by $$Z$$-sets, one has the concept of $$Z$$-below. The last main result of this paper: if the $$Z$$-below relation on the subsets of a poset $$P$$ has the interpolation property, then $$P$$ is embeddable in a cube. Reviewer: Zhongxi Zhang (Yantai) $$\delta$$-ideals in pseudo-complemented distributive join-semilattices https://www.zbmath.org/1483.06007 2022-05-16T20:40:13.078697Z "Nimbhorkar, Shriram K." https://www.zbmath.org/authors/?q=ai:nimbhorkar.shriram-khanderao "Nehete, Jaya Y." https://www.zbmath.org/authors/?q=ai:nehete.jaya-y In this paper, the authors studied a $$\delta$$-ideal concept in a pseudo-complemented distributive join semilattice with 0. Some properties of these ideals are obtained. A characterization for an ideal to be a $$\delta$$-ideal is proved in a distributive join-semilattice. Further, from Theorem 3.1(2), it is clear that if $$I$$ is a $$\delta$$-ideal, then for any $$x \in I$$, $$x^{**} \in I$$. Note that $$\delta$$-ideals are also studied under the nomenclature Baer ideals. An ideal $$I$$ in a pseudocomplemented poset is said to be a Baer ideal, if for any $$x \in I$$, $$x^{**} \in I$$, see Remark 2.1 in [\textit{V. Joshi} and \textit{N. Mundlik}, Asian-Eur. J. Math. 9, No. 3, Article ID 1650055, 16 p. (2016; Zbl 1368.06001)]. Reviewer: Vinayak Joshi (Pune) Semilattice-ordered Clifford semigroups https://www.zbmath.org/1483.06008 2022-05-16T20:40:13.078697Z "Shao, Yong" https://www.zbmath.org/authors/?q=ai:shao.yong "Zhao, Xian Zhong" https://www.zbmath.org/authors/?q=ai:zhao.xianzhong (no abstract) Duality of graded graphs through operads https://www.zbmath.org/1483.18023 2022-05-16T20:40:13.078697Z "Giraudo, Samuele" https://www.zbmath.org/authors/?q=ai:giraudo.samuele Given a graded set $$G$$, a graded graph on this set is a graph whose set of vertices is $$G$$ and such that the grading difference between the ends of an edge is exactly one. It is possible to associate to such graphs generating series which count paths on them. For a certain notion of duality on these graphs, due to Fomin, one can recover some combinatorial identities. For instance, the Cauchy identity relating the number of standard Young tableaux and the number of permutations is given by the paths on the Young lattice and its dual graph. The author uses non-symmetric operads, an algebraic structure encoding products, to construct pairs of such dual graded graphs, with adjacency relations given by composition with a generator of the operad. These graphs are called prefix graded graphs. The author study the properties of the associated poset and describes its intervals. He also generalizes Fomin's duality and applies this construction to several non-symmetric operads such as the associative and the diassociative operads, integer compositions, Motzkin paths and m-trees operads. Reviewer: Bérénice Delcroix-Oger (Paris) Tukey order and diversity of free abelian topological groups https://www.zbmath.org/1483.22003 2022-05-16T20:40:13.078697Z "Gartside, Paul" https://www.zbmath.org/authors/?q=ai:gartside.paul-m Summary: For a Tychonoff space $$X$$ the \textit{free abelian topological group} over $$X$$, denoted $$A(X)$$, is the free abelian group on the set $$X$$ with the coarsest topology so that for any continuous map of $$X$$ into an abelian topological group its canonical extension to a homomorphism on $$A(X)$$ is continuous. We show there is a family $$\mathcal{A}$$ of maximal size, $$2^{\mathfrak{c}}$$, consisting of separable metrizable spaces, such that if $$M$$ and $$N$$ are distinct members of $$\mathcal{A}$$ then $$A(M)$$ and $$A(N)$$ are not topologically isomorphic (moreover, $$A(M)$$ neither embeds topologically in $$A(N)$$ nor is an open image of $$A(N))$$. We show there is a chain $$\mathcal{C}=\{M_\alpha:\alpha<\mathfrak{c}^+\}$$, of maximal size, of separable metrizable spaces such that if $$\beta < \alpha$$ then $$A( M_\beta)$$ embeds as a closed subgroup of $$A( M_\alpha)$$ but no subspace of $$A( M_\beta)$$ is homeomorphic to $$A( M_\alpha)$$. We show that the character (minimal size of a local base at 0) of $$A(M)$$ is $$\mathfrak{d}$$ (minimal size of a cofinal set in $$\mathbb{N}^{\mathbb{N}})$$ for every non-discrete, analytic $$M$$, but consistently there is a co-analytic $$M$$ such that the character of $$A(M)$$ is strictly above $$\mathfrak{d}$$. The main tool used for these results is the Tukey order on the neighborhood filter at 0 in an $$A(X)$$, and a connection with the family of compact subsets of an auxiliary space. Convexity in topological betweenness structures https://www.zbmath.org/1483.54017 2022-05-16T20:40:13.078697Z "Anderson, Daron" https://www.zbmath.org/authors/?q=ai:anderson.daron "Bankston, Paul" https://www.zbmath.org/authors/?q=ai:bankston.paul "McCluskey, Aisling" https://www.zbmath.org/authors/?q=ai:mccluskey.aisling-e A betweenness structure is a pair $$\langle X,[\cdot,\cdot,\cdot] \rangle$$, where $$X$$ is a set and $$[\cdot,\cdot,\cdot]\subset X^{3}$$ is a ternary relation satisfying that \begin{itemize} \item[(B1)] Inclusivity: $$(\forall\ xy )$$ $$([x,y,y] \wedge [x,x,y])$$ \item[(B2)] Symmetry: $$(\forall\ xzy )$$ $$([x,z,y] \rightarrow [y,z,x])$$ \item[(B3)] Uniqueness: $$(\forall\ xz)$$ $$([x,z,x]\rightarrow x=z)$$ \end{itemize} Given a betweenness structure $$\langle X,[\cdot,\cdot,\cdot] \rangle$$, an interval is defined as $$[a,b]=\{c\in X:[a,c,b]\}$$, a convex subset of $$X$$ is a subset $$C$$ of $$X$$ such that $$a,b\in C$$, implies $$[a,b]\subset C$$. The span of a subset $$A$$ of $$X$$ is defined as $$[A]=\bigcup\{[a,b]:a,b\in A\}$$. The convex hull of $$A$$ is defined as $$[A]^{\omega}=\bigcup\{[A]^{n}:n\in\omega\}$$, where $$[A]^{0}=A$$ and $$[A]^{n+1}=[[A]^{n}]$$. With a detailed analysis of examples, in this paper the authors show how the notion of betweenness is related to several important concepts in mathematics. In particular if besides a betweenness structure, $$X$$ has a topology, it is possible to define interesting relations between the two structures, starting by asking that intervals are closed. In this sense, the authors define local convexity, upper (and lower) semi-continuity of betweenness, and a type of internal continuity of the betweenness. They obtain results connecting the convexity and the topology in compact connected Hausdorff spaces which are aposyndetic or hereditary unicoherent. In particular, they study how the span and the convex hull interact with the topological closure and interior operators. Reviewer: Alejandro Illanes (Ciudad de México)