Superforms, tropical cohomology, and Poincaré duality

We establish a canonical isomorphism between two bigraded cohomology theories for polyhedral spaces: Dolbeault cohomology of superforms and tropical cohomology. Furthermore, we prove Poincaré duality for cohomology of tropical manifolds, which are polyhedral spaces locally given by Bergman fans of matroids.

Generalized Jiang and Gottlieb groups

Given a map {f\colon X\to Y} , we extend Gottlieb’s result to the generalized Gottlieb group {G^{f}(Y,f(x_{0}))} and show that the canonical isomorphism {\pi_{1}(Y,f(x_{0}))\overset{\approx}{\to}\mathcal{D}(Y)} restricts to an isomorphism {G^{f}(Y,f(x_{0}))\overset{\approx}{\to}\mathcal{D}^{\tilde{f}_{0}}(Y)} , where {\mathcal{D}^{\tilde{f}_{0}}(Y)} is some subset of the group {\mathcal{D}(Y)} of deck transformations of Y for a fixed lifting {\tilde{f}_{0}} of f with respect to universal coverings of X and Y, respectively.

Unramified Galois Involutions

This chapter describes the fixed point building of an automorphism of a Bruhat-Tits building Ξ‎ which induces an unramified Galois involution on the building at infinity Ξ‎∞. An element of G (for example, a Galois involution of Δ‎) is unramified if the subgroup of G it generates is unramified. Before presenting the main result, the chapter presents the notation stating that Δ‎ = Ξ‎∞ is the building at infinity of Ξ‎ with respect to its complete system of apartments and G = Aut(Δ‎), followed by definitions. The central theorem shows how an unramified Galois involution of Δ‎ is obtained. Here Γ‎ := τ‎ is a descent group of both Δ‎ and Ξ‎, there is a canonical isomorphism from Δ‎Γ‎ to (Ξ‎Γ‎), where Ξ‎Γ‎ and Ξ‎Γ‎ are the fixed point buildings.

Affine Fixed Point Buildings

This chapter shows that if Ξ‎ is an affine building and Γ‎ is a finite descent group of Ξ‎, then Γ‎ is a descent group of Ξ‎∞ and (Ξ‎∞) is congruent to (Ξ‎∞). Ξ‎Γ‎ and Ξ‎ can be viewed as metric spaces. The chapter first considers the assumptions that Π‎ is an irreducible affine Coxeter diagram, Ξ‎ is a thick building of type Ξ‎, Γ‎is a finite descent group of Ξ‎, and Tits index �� = (Π‎, Θ‎, A). It then describes apartments that are endowed with reflection hyperplanes and reflection half-spaces before concluding with a theorem about a canonical isomorphism from the fixed point building Ξ‎Γ‎ to (Ξ‎Γ‎).

Coxeter Groups

This chapter develops a theory of descent for buildings by assembling various results about Coxeter groups. It begins with the notation stating that W is an arbitrary group with a distinguished set of generators S containing only elements of order 2, with MS denoting the free monoid on the set S and l: MS → ℕ denoting the length function. It then defines a Coxeter system and an automorphism of (W, S), which is an automorphism of the group W that stabilizes the set S, suggesting that there is a canonical isomorphism from Aut (W, S) to Aut(Π‎), where Π‎ is the associated Coxeter diagram with vertex set S. The chapter concludes with the proposition: Let α‎ be a root of Σ‎ and let T be the arctic region of α‎.

On planar algebraic curves and holonomic 𝒟-modules in positive characteristic

In this paper we study a correspondence between cyclic modules over the first Weyl algebra and planar algebraic curves in positive characteristic. In particular, we show that any such curve has a preimage under a morphism of certain ind-schemes. This property might pave the way to an indirect proof of existence of a canonical isomorphism between the group of algebra automorphisms of the first Weyl algebra over the field complex numbers and the group of polynomial symplectomorphisms of [Formula: see text].

Generalized Canonical Isomorphisms on Determinant

In this paper we define tensor modules (sheaves) of Schur type and of generalized Schur type associated with given modules (sheaves), using the so-called Schur functors. According to the functorial property, we give a series of tensor modules (sheaves) of Schur types in a categorical description. The main conclusion is that, by using basic ideas of algebraic geometry, there exists a canonical isomorphism of different tensor modules (sheaves) of Schur types if the original sheaf is locally free, which is in fact a generalization of results in linear algebra into locally free sheaves.

Realizing isomorphisms between first homology groups of closed 3-manifolds by borromean surgeries

We refine Matveev's result asserting that any two closed oriented 3-manifolds can be related by a sequence of borromean surgeries if and only if they have isomorphic first homology groups and linking pairings. Indeed, a borromean surgery induces a canonical isomorphism between the first homology groups of the involved 3-manifolds, which preserves the linking pairing. We prove that any such isomorphism is induced by a sequence of borromean surgeries. As an intermediate result, we prove that a given algebraic square finite presentation of the first homology group of a 3-manifold, which encodes the linking pairing, can always be obtained from a surgery presentation of the manifold.

Deligne pairing and Quillen metric

Let X → S be a smooth projective surjective morphism of relative dimension n, where X and S are integral schemes over ℂ. Let L → X be a relatively very ample line bundle. For every sufficiently large positive integer m, there is a canonical isomorphism of the Deligne pairing 〈L,…,L〉 → S with the determinant line bundle [Formula: see text] (see [D. H. Phong, J. Ross and J. Sturm, Deligne pairings and the knudsen–Mumford expansion, J. Differential Geom. 78 (2008) 475–496]). If we fix a hermitian structure on L and a relative Kähler form on X, then each of the line bundles [Formula: see text] and 〈L,…,L〉 carries a distinguished hermitian structure. We prove that the above mentioned isomorphism between 〈L,…,L〉 → S and [Formula: see text] is compatible with these hermitian structures. This holds also for the isomorphism in [Deligne pairing and determinant bundle, Electron. Res. Announc. Math. Sci. 18 (2011) 91–96] between a Deligne paring and a certain determinant line bundle.

A matroid associated with a phylogenetic tree

Special issue PRIMA 2013 International audience A (pseudo-)metric D on a finite set X is said to be a \textquotelefttree metric\textquoteright if there is a finite tree with leaf set X and non-negative edge weights so that, for all x,y ∈X, D(x,y) is the path distance in the tree between x and y. It is well known that not every metric is a tree metric. However, when some such tree exists, one can always find one whose interior edges have strictly positive edge weights and that has no vertices of degree 2, any such tree is – up to canonical isomorphism – uniquely determined by D, and one does not even need all of the distances in order to fully (re-)construct the tree\textquoterights edge weights in this case. Thus, it seems of some interest to investigate which subsets of X, 2 suffice to determine (\textquoteleftlasso\textquoteright) these edge weights. In this paper, we use the results of a previous paper to discuss the structure of a matroid that can be associated with an (unweighted) X-tree T defined by the requirement that its bases are exactly the \textquotelefttight edge-weight lassos\textquoteright for T, i.e, the minimal subsets of X, 2 that lasso the edge weights of T.