The purpose of this article is to review some recent results on the geometry of neutral signature metrics in dimension four and their twistor spaces. The following topics are considered: Neutral Kähler and hyperkähler surfaces, Walker metrics, Neutral anti-self-dual 4-manifolds and projective structures, Twistor spaces…
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48 results for “Neutral metric”
The paper studies nilpotent structures in oriented neutral vector bundles and neutral hyperKähler structures.
problem Nilpotent structures in oriented neutral vector bundles and their relation to neutral hyperKähler structures.
method Defined -nilpotent structures for Lie subgroups of related to neutral hyperKähler structures.
result Existence of complex and paracomplex structures forming neutral hyperKähler structures if and only if there exists an -nilpotent structure.
Rotational Surfaces with Constant Mean Curvature in Pseudo-Euclidean 4-Space with Neutral Metricmath.DG
Classifies surfaces with constant mean curvature in a specific type of space.
problem Classifying surfaces with constant mean curvature in a specific type of space.
method Classification based on the type of curvature (elliptic, hyperbolic, parabolic) and the rotational nature of the surfaces.
result Classification of constant mean curvature rotational surfaces.
The paper embeds CR manifolds into twistor spaces and constructs neutral hyperkähler metrics.
problem Embedding CR manifolds into twistor spaces and constructing neutral hyperkähler metrics.
method Embedding a real analytic twistor CR manifold into the twistor space of a Poincaré-Einstein metric, constructing the associated Fefferman ambient metric as a neutral hyperkähler metric.
result The construction of neutral hyperkähler metrics associated with twistor CR manifolds.
Minimal surfaces in the product of two dimensional real space forms endowed with a neutral metricmath.DG
Study minimal surfaces in product spaces with neutral metrics.
problem Characterize minimal surfaces in product spaces with neutral metrics.
method Compute totally geodesic surfaces and relate to Gordon equations; classify compact minimal surfaces.
result Provide a topological classification of compact minimal surfaces.
The paper explores neutral 4-manifolds with null boundaries using causal topology.
problem Neutral 4-manifolds with null boundaries and their topological properties.
method Neutral causal topology, foliation of null hypersurfaces, and geometric constructions.
result Neutral 4-manifolds with null boundaries and their topological properties.
Study on surfaces in pseudo-Euclidean space with neutral metric.
problem Characterizing surfaces in pseudo-Euclidean 4-space with neutral metrics.
method Defined and studied Lorentz general rotational surfaces with specific properties.
result Complete classification of various types of general rotational surfaces.
It is shown that if a compact four-dimensional manifold with metric of neutral signature is Jordan-Osserman, then it is either of constant sectional curvature or Ricci flat.
Three types of Einstein metrics are disqualified as potential local maxima.
problem Identifying local maxima of the Hilbert action in Einstein metrics.
method Analysis of three infinite families of neutrally stable homogeneous Einstein metrics.
result Three families of Einstein metrics ruled out as local maxima.
Classifies Heisenberg-invariant self-dual Einstein manifolds with explicit metrics.
problem Classifying self-dual Einstein manifolds invariant under Heisenberg group actions.
method Explicit construction of metrics and analysis of completeness.
result Einstein constants can vary and solutions exist for non-zero Ricci curvature.
Notation for spin coefficients for metrics of neutral signature in four dimensions is introduced. The utility and interpretation of spin coefficients is explored through themes in null geometry familiar from (complex) general relativity. Four-dimensional Walker geometry is exploited to provide examples and the generali…
Study stability of pseudo-Kähler and neutral Calabi-Yau manifolds, finding stability in 2D but failing in higher dimensions.
problem Stability of compact pseudo-Kähler and neutral Calabi-Yau manifolds.
method Analysis of stability through deformation theory and construction of counterexamples.
result Stability of compact pseudo-Kähler surfaces but failure in higher dimensions.
The aim of this paper is to give examples of compact neutral 4-manifolds whose Ricci tensor satisfies the relation . We present also a family of new Einstein bi-Hermitian neutral metrics on ruled surfaces of genus .
Meridian Surfaces with Constant Mean Curvature in Pseudo-Euclidean 4-space with Neutral Metricmath.DG
The study classifies meridian surfaces with constant mean curvature in a special pseudo-Euclidean space.
problem Classifying meridian surfaces with constant mean curvature in a pseudo-Euclidean space.
method Analyzing one-parameter systems of meridians of rotational hypersurfaces in a four-dimensional pseudo-Euclidean space with neutral metric.
result Complete classification of meridian surfaces with constant mean curvature, including minimal and quasi-minimal surfaces.
Study on null-projectability of Levi-Civita connections in neutral metrics.
problem Characterizing projectability of Levi-Civita connections along null parallel distributions.
method Analyzing projectability of torsion-free connections along foliations on manifolds, focusing on neutral metric signatures and mid-dimensional distributions.
result Extension of Patterson and Walker's Riemann extension metrics to null parallel distributions of any dimension.
The paper constructs examples of compactifications for Einstein metrics.
problem Compactifying Einstein metrics with specific properties.
method Constructing projective and --projective compactifications.
result Neutral signature Einstein metrics can be compactified canonically.
The paper studies null hypersurfaces in 4-manifolds with a specific metric structure.
problem Characterizing geometric properties of null hypersurfaces in 4-manifolds.
method Analyzes hypersurfaces null with respect to a neutral metric derived from a Riemannian Einstein metric and an almost paracomplex structure.
result Shows that totally geodesic null hypersurfaces imply Ricci-flatness of the ambient Einstein metric and provides necessary conditions for other types of null hypersurfaces.
The study classifies meridian surfaces with specific curvature properties in a special 4D space.
problem Characterizing surfaces with parallel mean or normalized mean curvature vectors.
method Classification of meridian surfaces based on curvature properties.
result Existence of surfaces with parallel normalized mean curvature but not mean curvature.
Researchers find non-diagonal Einstein metrics in various signatures.
problem Finding non-diagonal four-dimensional cohomogeneity-one Einstein metrics in different signatures.
method Explicitly seeking and constructing new examples of non-diagonal Einstein metrics, particularly in neutral signature.
result Construct new examples of neutral signature non-diagonal Bianchi type VIII Einstein metrics with self-dual Weyl tensor.
The paper classifies metrics on a Heisenberg group's cotangent bundle.
problem Classifying Riemannian metrics on a Heisenberg group's cotangent bundle.
method Left-invariant Riemannian metrics classification and uniqueness proof.
result Complex structure is unique and pseudoKähler metrics are Ricci flat.
While the Lorenzian and Riemanian metrics for which all polynomial scalar curvature invariants vanish (the VSI property) are well-studied, less is known about the four-dimensional neutral signature metrics with the VSI property. Recently it was shown that the neutral signature metrics belong to two distinct subclasses:…
The Reactive Beta Modelq-fin.RM
A new beta model reduces bias in market neutral strategies.
problem Bias in beta estimation for market neutral strategies.
method Derive a metric of correlation with leverage effect to identify market beta and volatility changes.
result Empirical test confirms the reactive beta model's ability to reduce bias.
The study classifies special Lorentz surfaces in a 4-space with neutral metric.
problem Classifying meridian surfaces in a pseudo-Euclidean 4-space.
method Constructing and classifying meridian surfaces with specific properties.
result There exist meridian surfaces with parallel normalized mean curvature vector field but not parallel mean curvature vector.
Study of pure spinors on neutral manifolds with applications to supersymmetric solutions.
problem Characterizing pure spinors and their properties on neutral manifolds.
method Using the theory of real spinorial forms and differential systems, the square of pure spinors is analyzed.
result Non-pure spinors correspond to specific structures in signature (4,4), and parallel spinors are characterized by differential systems.
Study classifies gradient almost Ricci solitons in Lorentzian and neutral signatures.
problem Classifying gradient almost Ricci solitons in different signatures.
method Proved local isometry and constructed examples.
result Found that gradient almost Ricci solitons are locally isometric to specific types of manifolds.
We study the neutral Kähler metric on the space of time-like lines in Lorentzian , which we identify with the total space of the tangent bundle to the hyperbolic plane. We find all of the infinitesimal isometries of this metric, as well as the geodesics, and interpret them in terms of the Lorentzian metr…
We study the totally null surfaces of the neutral Kaehler metric on certain 4-manifolds. The tangent spaces of totally null surfaces are either self-dual (-planes) or anti-self-dual (-planes) and so we consider -surfaces and -surfaces. The metric of the examples we study, which include the spaces of oriente…
Study null conformal Killing vector fields on complex surfaces.
problem Characterize pseudo-Hermitian surfaces with null vector fields.
method Analyze topological types and use vector fields to define para-hyperhermitian structures.
result Classify compact four-manifolds with orthogonal null Killing vector fields.
We present a simple explicit construction of hyper-Kaehler and hyper-symplectic (also known as neutral hyper-Kaehler or hyper-parakaehler) metrics in 4D using the Bianchi type groups of class A. The construction underlies a correspondence between hyper-Kaehler and hyper-symplectic structures in dimension four.
Characterizes neutral deformation modes of minimal surfaces.
problem Understanding the energy content of deformation modes of minimal surfaces.
method Analyzes the energy content of stretching, drilling, and bending modes of minimal surfaces.
result All isometries of a minimal surface are globally neutral and give rise to soft elasticity.
Benchmark-Neutral Pricingq-fin.MF
Paper introduces benchmark-neutral pricing for long-term contracts.
problem High prices of long-term contracts under risk-neutral pricing.
method Uses growth optimal portfolio as numeraire and new pricing measure.
result Identifies minimal possible prices for contingent claims.
Study finds all 4D neutral manifolds.
problem Classifying neutral manifolds in four dimensions.
method Examined homogeneous semi-symmetric neutral manifolds.
result Identified all four-dimensional neutral manifolds.
A global twistor correspondence is established for neutral self-dual conformal structures with alpha-surface foliation when the structure is close to the standard structure on S^2 times S^2. We need to introduce some singularity for the alpha-surface foliation such that the leaves intersect on a fixed two sphere. In th…
Minimal Lorentz surfaces in pseudo-Euclidean 4-space are characterized by specific curvature conditions.
problem Characterizing minimal Lorentz surfaces in a specific geometric space.
method Introducing canonical parameters and solving a system of partial differential equations.
result Minimal Lorentz surfaces of general type are uniquely determined by two invariant functions.
A set of canonical parahermitian connections on an almost paraHermitian manifold is defined. ParaHermitian version of the Apostolov-Gauduchon generalization of the Goldberg-Sachs theorem in General Relativity is given. It is proved that the Nijenhuis tensor of a Nearly paraKähler manifolds is parallel with respect to t…
Study on surfaces in neutral space forms with zero mean curvature.
problem Characterizing surfaces with zero mean curvature in neutral space forms.
method Analyzing curvature and normal connection properties of time-like conformal immersions.
result Conditions for surfaces with zero mean curvature in neutral space forms.
We study surfaces in TN that are area-stationary with respect to a neutral Kaehler metric constructed on TN from a riemannian metric g on N. We show that holomorphic curves in TN are area-stationary, while lagrangian surfaces that are area-stationary are also holomorphic and hence totally null. However, in general, are…
A new ERM framework ensures neutral classifiers in machine learning.
problem Ensuring fairness and neutrality in machine learning predictions.
method Neutralized Empirical Risk Minimization (NERM) framework.
result Theoretical and empirical neutrality bounds for classifiers.
Generalized connections, spinors, and integrability of generalized structures on Courant algebroidsmath.DG
Characterizes integrability of generalized structures on Courant algebroids.
problem Integrability of generalized structures on Courant algebroids.
method Characterization via torsion-free generalized connections and Dirac generating operators.
result Criterion for integrability of generalized almost Hermitian structures and hyper-Hermitian structures.
We exhibit Osserman metrics with non-nilpotent Jacobi operators and with non-trivial Jordan normal form in neutral signature (n,n) for any n which is at least 3. These examples admit a natural almost para-Hermitian structure and are semi para-complex Osserman with non-trivial Jordan normal form as well; they neither sa…
New formula for portfolio risk management using conditional PDEs.
problem Optimal diversification and risk management of portfolios.
method Closed-form formula for conditional probability, Gaussian copulas, conditional risk-neutral PDE.
result Dynamic monitoring of portfolio volatilities and weights from PDEs.
Optimizes risk-neutral probabilities for derivative pricing.
problem Deriving bounds on derivative values under multiple risk-neutral scenarios.
method Convex optimization over the set of risk-neutral probability distributions.
result Tractable finite-dimensional optimization problems for pricing.
The paper shows how to calculate risk-neutral default probabilities from bid and ask CDS quotes.
problem Calculating risk-neutral default probabilities from market quotes.
method Using conic finance framework and Poisson process to formulate and solve the calibration problem.
result A unique solution for risk-neutral default probabilities and implied liquidity.
Risk-Neutral Market Simulationq-fin.CP
Simulates risk-neutral markets using neural spline flows.
problem Creating realistic risk-neutral market simulations.
method Developed a low-dimensional martingale representation and used neural spline flows for sampling.
result The calibrated simulator is closest to historical data with respect to Kullback-Leibler divergence.
Generative model uses DDPMs for risk-neutral derivative pricing.
problem Derivative pricing using arbitrage-free models.
method Developed a framework using DDPMs to generate risk-neutral asset price dynamics.
result Empirically validated the method for both European and path-dependent derivatives.
We study the geodesic flow on the normal line congruence of a minimal surface in induced by the neutral Kähler metric on the space of oriented lines. The metric is lorentz with isolated degenerate points and the flow is shown to be completely integrable. In addition, we give a new holomorphic description …
In this paper we construct a family of examples of self-dual Einstain metrics of neutral signature, which are not Ricci flat, nor locally homogenous. Curvature of these manifolds is studied in details. These are obtained by the para-quaternionic reduction. We compare our examples with the orbifolds $\oo$ given by Galic…
The local structure of the manifolds named in the title is described. Although curvature homogeneous, they are not, in general, locally homogeneous. Not all of them are Ricci-flat, which answers an existence question about type III Jordan-Osserman metrics, raised by Diaz-Ramos, Garcia-Rio and Vazquez-Lorenzo (2006).