A Characterization of Twin Buildings by Twin Apartments

A twin building is a pair of buildings along with a certain codistance' between chambers of one building, and chambers of the other. All spherical buildings can be regarded as twin buildings (such a building is twinned with itself in a natural way), and the aim of this paper is to give a definition of twin buildings along the lines that Tits originally gave for spherical buildings. This alternative approach via apartments has been used by the first author in studying some group actions on twin buildings.     

Loop Groups and Twin Buildings

In these notes we describe some buildings related to complex Kac–Moody groups. First we describe the spherical building of SL_n() (i.e. the projective geometry PG(ⁿ)) and its Veronese representation. Next we recall the construction of the affine building associated to a discrete valuation on the rational function field (z). Then we describe the same building in terms of complex Laurent polynomials, and introduce the Veronese representation, which is an equivariant embedding of the building into an affine Kac–Moody algebra. Next, we introduce topological twin buildings. These buildings can be used for a proof which is a variant of the proof by Quillen and Mitchell, of Bott periodicity which uses only topological geometry. At the end we indicate very briefly that the whole process works also for affine real almost split Kac–Moody groups.Supported by a Heisenberg fellowship by the Deutsche Forschungsgemeinschaft.     

Symplectic polarities of buildings of type E 6

A symplectic polarity of a building Δ of type E ₆ is a polarity whose fixed point structure is a building of type F ₄ containing residues isomorphic to symplectic polar spaces. In this paper, we present two characterizations of such polarities among all dualities. Firstly, we prove that, if a duality θ of Δ never maps a point to a neighbouring symp, and maps some element to a non-opposite element, then θ is a symplectic duality. Secondly, we show that, if a duality θ never maps a chamber to an opposite chamber, then it is a symplectic polarity. The latter completes the programme for dualities of buildings of type E ₆ of determining all domestic automorphisms of spherical buildings, and it also shows that symplectic polarities are the only polarities in buildings of type E ₆ for which the Phan geometry is empty.     

Superrigidity of Hyperbolic Buildings

In this paper, we study the behavior of harmonic maps into complexes with branching differentiable manifold structure. The main examples of such target spaces are Euclidean and hyperbolic buildings. We show that a harmonic map from an irreducible symmetric space of noncompact type other than real or complex hyperbolic into these complexes are non-branching. As an application, we prove rank-one and higher-rank superrigidity for the isometry groups of a class of complexes which includes hyperbolic buildings as a special case.     

Affine Twin Buildings

It is well known from work of Bruhat and Tits that an affinebuilding has a spherical building at infinity. The paper studiesthe structure at infinity for an affine twin building. It isshown that the twinning restricts the structure at infinityin a natural way, producing two smaller spherical buildingsthat are canonically isomorphic to one another. In the processit is shown that to each wall and panel of these buildings atinfinity is attached a twin tree.     

Lagrangian Matroids and Cohomology

We prove that \bigtriangleup \bigtriangleup -matroids associated with maps on compact closed surfaces are representable, with the space of representation provided by cohomology of the surface with punctured points.     

Vertex opposition in spherical buildings

We study to which extent all pairs of opposite vertices of self-opposite type determine a given building. We provide complete answers in the case of buildings related to projective spaces, to polar spaces and the exceptional buildings, but for the latter we restrict to the vertices whose Grassmannian defines a parapolar space of point diameter 3. Some results about non-self opposite types for buildings of types ${\mathsf{A}_n}$ , ${\mathsf{D}_m}$ (m odd), and ${\mathsf{E}_6}$ are also provided.     

Embeddings of Twin Trees

In this article we provide sufficient conditions for when a pair of trees having a semi-codistance function can be embedded in a twin tree with codistance function extending . We use these conditions to show that given a twin tree T of bidegree (d1,d2) there exists a twin tree of bidegree (e1,e2) containing T as a substructure as long as di ei.     

Simplicity and superrigidity of twin building lattices

Kac–Moody groups over finite fields are finitely generated groups. Most of them can naturally be viewed as irreducible lattices in products of two closed automorphism groups of non-positively curved twinned buildings: those are the most important (but not the only) examples of twin building lattices. We prove that these lattices are simple if the corresponding buildings are irreducible and not of affine type (i.e. they are not Bruhat–Tits buildings). Many of them are finitely presented and enjoy property (T). Our arguments explain geometrically why simplicity fails to hold only for affine Kac–Moody groups. Moreover we prove that a nontrivial continuous homomorphism from a completed Kac–Moody group is always proper. We also show that Kac–Moody lattices fulfill conditions implying strong superrigidity properties for isometric actions on non-positively curved metric spaces. Most results apply to the general class of twin building lattices. Dedicated to Jacques Tits with our admiration     

The geometry of the chamber system of a semimodular lattice

In this paper geometric properties of the following metric space C are studied. Its elements are called chambers and are the maximal chains of a semimodular lattice X of finite height and its metric d is the gallery distance. We show that X has many properties in common with buildings. More specifically, Tits [17] has recently described buildings in terms of Weyl-group valued distance functions. We consider the Jordan-Hölder permutation (C, D) corresponding to a pair C, D of chambers and show that it has most properties of such a distance with values in the symmetric group.     

Cubes and Hypercubes of Opposition,with Ethical Ruminations on Inviolability

We show that we in ways related to the classical Square of Opposition may define a Cube of Opposition for some useful statements, and we as a by-product isolate a distinct directive of being inviolable which deserves attention; a second central purpose is to show that we may extend our construction to isolate hypercubes of opposition of any finite cardinality when given enough independent modalities. The cube of opposition for obligations was first introduced publically in a lecture for the Square of Opposition Conference in the Vatican in May 2014.     

Gauss maps of the Ricci-mean curvature flow

In this paper, we investigate the Gauss maps of a Ricci-mean curvature flow. A Ricci-mean curvature flow is a coupled equation of a mean curvature flow and a Ricci flow on the ambient manifold. Ruh and Vilms (Trans Am Math Soc 149: 569–573, 1970) proved that the Gauss map of a minimal submanifold in a Euclidean space is a harmonic map, and Wang (Math Res Lett 10(2–3):287–299, 2003) extended this result to a mean curvature flow in a Euclidean space by proving its Gauss maps satisfy the harmonic map heat flow equation. In this paper, we deduce the evolution equation for the Gauss maps of a Ricci-mean curvature flow, and as a direct corollary we prove that the Gauss maps of a Ricci-mean curvature flow satisfy the vertically harmonic map heat flow equation when the codimension of submanifolds is 1.     

An Edge-Isoperimetric Problem for Powers of the Petersen Graph

In this paper we introduce a new order on the set of n-dimensional tuples and prove that this order preserves nestedness in the edge isoperimetric problem for the graph Pⁿ, defined as the nth cartesian power of the well-known Petersen graph. The cutwidth and wirelength of Pⁿ are also derived. These results are then generalized for the cartesian product of Pⁿ and the m-dimensional binary hypercube.     

The Classification of Certain Four-Weight Spin Models

In this paper we show that if one of the matrices {Wi, 1 h i h 4} of a four-weight spin model (X, W1, W2, W3, W4; D) is equivalent to the matrix of a Potts model or a cyclic model as type II matrix and |X| S 5, then the spin model is gauge equivalent to a Potts model or a cyclic model up to simultaneous permutations on rows and columns. Using this fact and Nomura's result [12] we show that every four-weight spin model of size |X| = 5 is gauge equivalent to either a Potts model or a cyclic model up to simultaneous permutations on rows and columns.     

Enumeration of r-regular maps on the torus. Part I: Rooted maps on the torus,the projective plane and the Klein bottle. Sensed maps on the torus

The work that consists of two parts is devoted to the problem of enumerating unrooted $r$-regular maps on the torus up to all its symmetries. We begin with enumerating near-$r$-regular rooted maps on the torus, the projective plane and the Klein bottle, as well as some special kinds of maps on the sphere: near-$r$-regular maps, maps with multiple leaves and maps with multiple root darts. For $r=3$ and $r=4$ we obtain exact analytical formulas. For larger $r$ we derive recurrence relations. Then we enumerate $r$-regular maps on the torus up to homeomorphisms that preserve its orientation — so-called sensed maps. Using the concept of a quotient map on an orbifold we reduce this problem to enumeration of certain above-mentioned classes of rooted maps. For $r=3$ and $r=4$ we obtain closed-form expressions for the numbers of $r$-regular sensed maps by edges. All these results will be used in the second part of the work to enumerate $r$-regular maps on the torus up to all homeomorphisms — so-called unsensed maps.     

Partially isometric immersions and free maps

In this paper we investigate the existence of “partially” isometric immersions. These are maps \({f:M\rightarrow \mathbb{R}^q}\) which, for a given Riemannian manifold M, are isometries on some sub-bundle \({\mathcal{H}\subset TM}\). The concept of free maps, which is essential in the Nash–Gromov theory of isometric immersions, is replaced here by that of \({\mathcal{H}}\) –free maps, i.e. maps whose restriction to \({\mathcal{H}}\) is free. We prove, under suitable conditions on the dimension q of the Euclidean space, that \({\mathcal{H}}\) –free maps are generic and we provide, for the smallest possible value of q, explicit expressions for \({\mathcal{H}}\) –free maps in the following three settings: 1–dimensional distributions in \({\mathbb{R}^2}\), Lagrangian distributions of completely integrable systems, Hamiltonian distributions of a particular kind of Poisson Bracket.     

Combinatorics of Geometrically Distributed Random Variables: Inversions and a Parameter of Knuth

. For words of length n, generated by independent geometric random variables, we consider the mean and variance of the number of inversions and of a parameter of Knuth from permutation in situ. In this way, q-analogues for these parameters from the usual permutation model are obtained.