Continuum hypothesis

In mathematics, specifically set theory, the continuum hypothesis (abbreviated CH) is a hypothesis about the possible sizes of infinite sets. It states that there is no set whose cardinality is strictly between that of the integers and the real numbers, or equivalently, that any subset of the real numbers is finite, is countably infinite, or has the same cardinality as the real numbers. In Zermelo–Fraenkel set theory with the axiom of choice (ZFC), this is equivalent to the following equation in aleph numbers: 2^{\aleph_0}=\aleph_1, or even shorter with beth numbers: \beth_1 = \aleph_1. The continuum hypothesis was advanced by Georg Cantor in 1878, and establishing its truth or falsehood is the first of Hilbert's 23 problems presented in 1900. The answer to this problem is independent of ZFC, so that either the continuum hypothesis or its negation can be added as an axiom to ZFC set theory, with the resulting theory being consistent if and only if ZF

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L'espace contre l'architecture

Grégory Azar

This research work analyses Theo van Doesburg's Counter-constructions presented in Paris in 1923 at the "L'Effort Moderne" exhibition. These Counter-constructions stand as icons of the Modern Movement due to their role as precursors of a new "boundless" spatiality, a role first intuitively perceived by Le Corbusier or Mies van der Rohe and then theorized by Sigfried Giedion. We have attempted in this research a theoretical study of the Counter-constructions' position within the architectural field. That is to say bringing together their aspect of "spatial manifest" and the Dadaist persona of Theo van Doesburg; this alleged contradiction between the progressive and negative dimension of the Counter-constructions being the subject of this research. It is based on the following problematic: understanding what is at stake in the disjointing of the polychrome plane (as a textile surface) and the suppression of the architectural boundary under the guise of spatial continuity ("the breaking of the enclosure" mentioned by Theo van Doesburg and his interest in the notion of mathematical continuum through the theories of the fourth dimension: Henri Poincaré's Analysis situs). That amounts to studying the modalities connecting the notions of dressing and of continuum to a destructive drive, "throwing out" the inside of the house. The methodology turns towards psychoanalysis through the concepts of Verneinung [negation] (Freud) and Moi-peau [Skin-ego] (Anzieu).

EPFL2005

Geometric, variational discretization of continuum theories

Mathieu Desbrun

This study derives geometric, variational discretization of continuum theories arising in fluid dynamics, magnetohydrodynamics (MHD), and the dynamics of complex fluids. A central role in these discretizations is played by the geometric formulation of fluid dynamics, which views solutions to the governing equations for perfect fluid flow as geodesics on the group of volume-preserving diffeomorphisms of the fluid domain. Inspired by this framework, we construct a finite-dimensional approximation to the diffeomorphism group and its Lie algebra, thereby permitting a variational temporal discretization of geodesics on the spatially discretized diffeomorphism group. The extension to MHD and complex fluid flow is then made through an appeal to the theory of Euler-Poincare systems with advection, which provides a generalization of the variational formulation of ideal fluid flow to fluids with one or more advected parameters. Upon deriving a family of structured integrators for these systems, we test their performance via a numerical implementation of the update schemes on a cartesian grid. Among the hallmarks of these new numerical methods are exact preservation of momenta arising from symmetries, automatic satisfaction of solenoidal constraints on vector fields, good long-term energy behavior, robustness with respect to the spatial and temporal resolution of the discretization, and applicability to irregular meshes. (C) 2011 Elsevier B.V. All rights reserved.

2011

Lattice formulation of axion inflation. Application to preheating

José Roberto Canivete Cuissa, Daniel Garcia Figueroa

We present a lattice formulation of an interaction phi/Lambda F (F) over tilde between an axion and some U(1) gauge sector with the following properties: it reproduces the continuum theory up to O(dx(mu)(2)) corrections, it preserves exact gauge invariance and shift symmetry on the lattice, and it is suitable for self-consistent expansion of the Universe. The lattice equations of motion can no longer be solved by explicit methods, but we propose an implicit method to overcome this difficulty, which preserves the relevant system constraints down to arbitrary (tunable) precision. As a first application we study, in a comoving grid in (3 +1) dimensions, the last efolds of axion-inflation with quadratic potential and the preheating stage following afterwards. We fully account for the inhomogeneity and non-linearity of the system, including the gauge field contribution to the expansion rate of the Universe and its backreaction into the axion dynamics. We characterize in detail, as a function of the coupling, the energy transfer from the axion to the gauge field. Two coupling regimes are identified, sub- and super-critical, depending on whether the final energy fraction stored in the gauge field is below or above similar to 50% of the total energy. The Universe is very efficiently reheated for super-critical couplings, rapidly entering in a radiation dominated stage. Our results on preheating confirm previously published results.

IOP PUBLISHING LTD2019

Set theory

Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory,

Axiom of choice

In mathematics, the axiom of choice, abbreviated AC or AoC, is an axiom of set theory equivalent to the statement that a Cartesian product of a collection of non-empty sets is non-empty. Informally

Real number

In mathematics, a real number is a number that can be used to measure a continuous one-dimensional quantity such as a distance, duration or temperature. Here, continuous means that pairs of values c

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