Morse quasiflats I

This is the first in a series of papers concerned with Morse quasiflats, which are a generalization of Morse quasigeodesics to arbitrary dimension. In this paper we introduce a number of alternative definitions, and under appropriate assumptions on the ambient space we show that they are equivalent and quasi-isometry invariant; we also give a variety of examples. The second paper proves that Morse quasiflats are asymptotically conical and have canonically defined Tits boundaries; it also gives some first applications.

Nontrivial Isometric Embeddings for Flat Spaces

Nontrivial isometric embeddings for flat metrics (i.e., those which are not just planes in the ambient space) can serve as useful tools in the description of gravity in the embedding gravity approach. Such embeddings can additionally be required to have the same symmetry as the metric. On the other hand, it is possible to require the embedding to be unfolded so that the surface in the ambient space would occupy the subspace of the maximum possible dimension. In the weak gravitational field limit, such a requirement together with a large enough dimension of the ambient space makes embedding gravity equivalent to general relativity, while at lower dimensions it guarantees the linearizability of the equations of motion. We discuss symmetric embeddings for the metrics of flat Euclidean three-dimensional space and Minkowski space. We propose the method of sequential surface deformations for the construction of unfolded embeddings. We use it to construct such embeddings of flat Euclidean three-dimensional space and Minkowski space, which can be used to analyze the equations of motion of embedding gravity.

The Rigging Technique for Null Hypersurfaces

Starting from the main definitions, we review the rigging technique for null hypersurfaces theory and most of its main properties. We make some applications to illustrate it. On the one hand, we show how we can use it to show properties of null hypersurfaces, with emphasis in null cones, totally geodesic, totally umbilic, and compact null hypersurfaces. On the other hand, we show the interplay with the ambient space, including its influence in causality theory.

Sound in the Mediated Space

With recent advances in technological media, the artistic notion of the multimedia environment has been dramatically extended. In this collection of interviews, sound will be examined in relation to various mediated spaces, from the ubiquitous urban space of telematic cities to enclosed audiovisual spaces. The sounds present in these mediated spaces act as triggers which allow the audience to transcend the sensory experiences of the mundane. They accomplish this through the visualization of invisible auditory forces, or by creating an immersive space beyond the realm of the physical ambient space. In some cases, sounds archived and shared on media platforms direct attention to larger sociopolitical issues. Such mediated environments, artistically augmented with the power of sound, become platforms wherein audiences can experience new potentials of sensory space. Furthermore, these works also introduce possibilities of new forms of sound-based connectivity and communication.

The relative Bruce–Roberts number of a function on a hypersurface

We consider the relative Bruce–Roberts number $\mu _{\textrm {BR}}^{-}(f,\,X)$ of a function on an isolated hypersurface singularity $(X,\,0)$ . We show that $\mu _{\textrm {BR}}^{-}(f,\,X)$ is equal to the sum of the Milnor number of the fibre $\mu (f^{-1}(0)\cap X,\,0)$ plus the difference $\mu (X,\,0)-\tau (X,\,0)$ between the Milnor and the Tjurina numbers of $(X,\,0)$ . As an application, we show that the usual Bruce–Roberts number $\mu _{\textrm {BR}}(f,\,X)$ is equal to $\mu (f)+\mu _{\textrm {BR}}^{-}(f,\,X)$ . We also deduce that the relative logarithmic characteristic variety $LC(X)^{-}$ , obtained from the logarithmic characteristic variety $LC(X)$ by eliminating the component corresponding to the complement of $X$ in the ambient space, is Cohen–Macaulay.

Intersection Numbers and the Statement of the Disc Embedding Theorem

‘Intersection Numbers and the Statement of the Disc Embedding Theorem’ provides detailed definitions of some of the notions involved in the statement of the disc embedding theorem, focusing specifically on intersection numbers. The chapter begins with a detailed analysis of immersions, regular homotopies, finger moves, and Whitney moves. Then it defines intersection and self-intersection numbers for families of discs and spheres, taking values in the group ring of the fundamental group of the ambient space, with the correct relations. Then it enumerates certain properties of intersection numbers, in particular relating them to the existence of Whitney discs. This work enables the disc embedding theorem to be stated carefully.

The Topological Correctness of PL Approximations of Isomanifolds

Isomanifolds are the generalization of isosurfaces to arbitrary dimension and codimension, i.e. manifolds defined as the zero set of some multivariate multivalued smooth function $$f: {\mathbb {R}}^d\rightarrow {\mathbb {R}}^{d-n}$$ f : R d → R d - n . A natural (and efficient) way to approximate an isomanifold is to consider its piecewise-linear (PL) approximation based on a triangulation $$\mathcal {T}$$ T of the ambient space $${\mathbb {R}}^d$$ R d . In this paper, we give conditions under which the PL approximation of an isomanifold is topologically equivalent to the isomanifold. The conditions are easy to satisfy in the sense that they can always be met by taking a sufficiently fine and thick triangulation $$\mathcal {T}$$ T . This contrasts with previous results on the triangulation of manifolds where, in arbitrary dimensions, delicate perturbations are needed to guarantee topological correctness, which leads to strong limitations in practice. We further give a bound on the Fréchet distance between the original isomanifold and its PL approximation. Finally, we show analogous results for the PL approximation of an isomanifold with boundary.

Conjugate spinor field equation for massless spin-3/2 field in de Sitter ambient space

The quantum field theory in de Sitter ambient space provide us with a comprehensive description of massless gravitational field. Using the gauge-covariant derivative in the de Sitter ambient space, the gauge invariant Lagrangian density has been found.In this paper, the equation of the conjugate spinor for massless spin-$\frac{3}{2}$ field is obtained by Euler-Lagrange equation. Then the field equation is written in terms of the Casimir operator of the de Sitter group. Finally, the gauge invariant field equation is presented.

Compactifications of Cluster Varieties and Convexity

Gross–Hacking–Keel–Kontsevich [13] discuss compactifications of cluster varieties from positive subsets in the real tropicalization of the mirror. To be more precise, let ${\mathfrak{D}}$ be the scattering diagram of a cluster variety $V$ (of either type– ${\mathcal{A}}$ or ${\mathcal{X}}$), and let $S$ be a closed subset of $\left (V^\vee \right )^{\textrm{trop}} \left ({\mathbb{R}}\right )$—the ambient space of ${\mathfrak{D}}$. The set $S$ is positive if the theta functions corresponding to the integral points of $S$ and its ${\mathbb{N}}$-dilations define an ${\mathbb{N}}$-graded subalgebra of $\Gamma (V, \mathcal{O}_V){ [x]}$. In particular, a positive set $S$ defines a compactification of $V$ through a Proj construction applied to the corresponding ${\mathbb{N}}$-graded algebra. In this paper, we give a natural convexity notion for subsets of $\left (V^\vee \right )^{\textrm{trop}} \left ({\mathbb{R}}\right )$, called broken line convexity, and show that a set is positive if and only if it is broken line convex. The combinatorial criterion of broken line convexity provides a tractable way to construct positive subsets of $\left (V^\vee \right )^{\textrm{trop}} \left ({\mathbb{R}}\right )$ or to check positivity of a given subset.

Global gradient estimates for nonlinear parabolic operators

We consider a parabolic equation driven by a nonlinear diffusive operator and we obtain a gradient estimate in the domain where the equation takes place. This estimate depends on the structural constants of the equation, on the geometry of the ambient space and on the initial and boundary data. As a byproduct, one easily obtains a universal interior estimate, not depending on the parabolic data. The setting taken into account includes sourcing terms and general diffusion coefficients. The results are new, to the best of our knowledge, even in the Euclidean setting, though we treat here also the case of a complete Riemannian manifold.