Simplicial Endomorphisms

The monoids of simplicial endomorphisms, i.e., the monoids of endomorphisms in the simplicial category, are submonoids of monoids one finds in Temperley–Lieb algebras, and as the monoids of Temperley–Lieb algebras are linked to situations where an endofunctor is adjoint to itself, so the monoids of simplicial endomorphisms are linked to arbitrary adjoint situations. This link is established through diagrams of the kind found in Temperley–Lieb algebras. Results about these matters, which were previously prefigured up to a point, are here surveyed and reworked. A presentation of monoids of simplicial endomorphisms by generators and relations has been given a long time ago. Here a closely related presentation is given, with completeness proved in a new and self-contained manner.     

Simplicial cross-polytopic d-arrangements

Branko Grünbaum has observed that the projective d-arrangements formed by the facet hyperplanes of a regular polytope together with some of its hyperplanes of mirror symmetry and possibly the hyperiplane at infinity are sometimes simplicial.We investigate the projective d-arrangements associated in this manner with a cross-polytope. Fourteen such simplicial arrangements are known: three 1-arrangements, four 2-arrangements, six 3-arrangements, and one 4-arrangement. In this paper we prove that no other arrangement so associated with a cross-polytope is simplicial.     

Simplicial nonpositive curvature

We introduce a family of conditions on a simplicial complex that we call local k-largeness (k≥6 is an integer). They are simply stated, combinatorial and easily checkable. One of our themes is that local 6-largeness is a good analogue of the non-positive curvature: locally 6-large spaces have many properties similar to non-positively curved ones. However, local 6-largeness neither implies nor is implied by non-positive curvature of the standard metric. One can think of these results as a higher dimensional version of small cancellation theory. On the other hand, we show that k-largeness implies non-positive curvature if k is sufficiently large. We also show that locally k-large spaces exist in every dimension. We use this to answer questions raised by D. Burago, M. Gromov and I. Leary.     

Colourful Simplicial Depth

Inspired by Barany’s Colourful Caratheodory Theorem, we introduce a colourful generalization of Liu's simplicial depth. We prove a parity property and conjecture that the minimum colourful simplicial depth of any core point in any d-dimensional configuration is d² + 1 and that the maximum is d^d+1 + 1. We exhibit configurations attaining each of these depths, and apply our results to the problem of bounding monochrome (non-colourful) simplicial depth.     

Cores of Simplicial Complexes

We introduce a method to reduce the study of the topology of a simplicial complex to that of a simpler one. Applying this method to complexes arising from graphs, we give topological meaning to classical graph invariants. As a consequence, we answer some questions raised in (Ehrenborg and Hetyei in Eur. J. Comb. 27(6):906–923, 2006) on the independence complex and the dominance complex of a forest and obtain improved algorithms to compute their homotopy types.     

Fibrations of Simplicial Sets

There are infinitely many variants of the notion of Kan fibration that, together with suitable choices of cofibrations and the usual notion of weak equivalence of simplicial sets, satisfy Quillen’s axioms for a homotopy model category. The combinatorics underlying these fibrations is purely finitary and seems interesting both for its own sake and for its interaction with homotopy types. To show that these notions of fibration are indeed distinct, one needs to understand how iterates of Kan’s Ex functor act on graphs and on nerves of small categories.     

Simplicial orders and chordality

Chordal clutters in the sense of Bigdeli et al. (J Comb Theory Ser A 145:129–149, 2017) and Morales et al. (Ann Fac Sci Toulouse Sér 6 23(4):877–891, 2014) are defined via simplicial orders. Their circuit ideal has a linear resolution, independent of the characteristic of the base field. We show that any Betti sequence of an ideal with linear resolution appears as the Betti sequence of the circuit ideal of such a chordal clutter. Associated with any simplicial order is a sequence of integers which we call the \(\lambda \)-sequence of the chordal clutter. All possible \(\lambda \)-sequences are characterized. They are intimately related to the Hilbert function of a suitable standard graded K-algebra attached to the chordal clutter. By the \(\lambda \)-sequence of a chordal clutter, we determine other numerical invariants of the circuit ideal, such as the \(\mathbf h \)-vector and the Betti numbers.     

A Braided Simplicial Group

By studying the braid group action on Milnor's constructionof the 1-sphere, we show that the general higher homotopy groupof the 3-sphere is the fixed set of the pure braid group actionon certain combinatorially described groups. This establishesa relation between the braid groups and the homotopy groupsof the sphere. 2000 Mathematical Subject Classification: 20F36, 55P35, 55Q05,55Q40, 55U10.     

Simplicial ramified covering maps

In this paper we define the concept of a ramified covering map in the category of simplicial sets and we show that it has properties analogous to those of the topological ramified covering maps. We show that the geometric realization of a simplicial ramified covering map is a topological ramified covering map, and we also consider the relation with ramified covering maps in the category of simplicial complexes.     

Simplicial ramified covering maps

In this paper we define the concept of a ramified covering map in the category of simplicial sets and we show that it has properties analogous to those of the topological ramified covering maps. We show that the geometric realization of a simplicial ramified covering map is a topological ramified covering map, and we also consider the relation with ramified covering maps in the category of simplicial complexes.     

Simplicial embeddings between pants graphs

We prove that, except in some low-complexity cases, every locally injective simplicial map between pants graphs is induced by a π ₁-injective embedding between the corresponding surfaces.     

Simplicial Homotopical Algebra and Satellites

We study here a notion of simplicial satellites, as a first step towards a characterisation of simplicial derived functors, a problem unsolved since the latter were introduced.The problem comes from the fact that, in contrast with the abelian case, simplicial derived functors do not produce by themselves an exact sequence. Our solution consists in extending them to commutative k-cubes, for all k, forming thus an exact system of functors universal within the connected ones; or, in other words, a system of simplicial satellites. The tool we develop here for this extension is the homotopy kernel of a commutative k-dimensional cubic diagram, generalising the homotopy kernel of a map; its 2-dimensional version has already been proved essential in other homotopical topics.     

Simplicial gauge theory on spacetime

We define a discrete gauge-invariant Yang–Mills–Higgs action on spacetime cylindrical meshes with simplicial spatial base. The formulation is a generalization of classical lattice gauge theory, and we prove consistency of the action in the finite element sense. In addition, we perform numerical tests of convergence towards exact continuum results for several choices of gauge fields in pure gauge theory.     

Simplicial complexes with rigid depth

We extend a result of Minh and Trung (Adv. Math. 226:1285–1306, 2011) to get criteria for depth ${I = \rm {depth}\sqrt{I}}$ , where I is an unmixed monomial ideal of the polynomial ring S?=?K[x ₁, . . . , x _n ]. As an application we characterize all the pure simplicial complexes Δ which have rigid depth, that is, which satisfy the condition that for every unmixed monomial ideal ${I\subset S}$ with ${\sqrt{I}=I_\Delta}$ one has depth(I)?=?depth(I _Δ).