Narrow Results

Approximation rate bounds for emulations of real-valued functions on intervals by deep neural networks (DNNs) are established. The approximation results are given for DNNs based on ReLU activation functions. The approximation error is measured with respect to Sobolev norms. It is shown that ReLU DNNs allow for essentially the same approximation rates as nonlinear, variable-order, free-knot (or so-called “$hp$-adaptive”) spline approximations and spectral approximations, for a wide range of Sobolev and Besov spaces. In particular, exponential convergence rates in terms of the DNN size for univariate, piecewise Gevrey functions with point singularities are established. Combined with recent results on ReLU DNN approximation of rational, oscillatory, and high-dimensional functions, this corroborates that continuous, piecewise affine ReLU DNNs afford algebraic and exponential convergence rate bounds which are comparable to “best in class” schemes for several important function classes of high and infinite smoothness. Using composition of DNNs, we also prove that radial-like functions obtained as compositions of the above with the Euclidean norm and, possibly, anisotropic affine changes of co-ordinates can be emulated at exponential rate in terms of the DNN size and depth without the curse of dimensionality.

For the spherical unit speed nonlightlike curve in pseudo-hyperbolic space and de Sitter space $\gamma$ and a given point P, we can define naturally the pedal curve of $\gamma$ relative to the pedal point P. When the pseudo-sphere dual curve germs are nonsingular, singularity types of such pedal curves depend only on locations of pedal points. In this paper, we give a complete list of normal forms for singularities and locations of pedal points when the pseudo-sphere dual curve germs are nonsingular. Furthermore, we obtain the extension results in dualities, which has wide influence on the open and closed string field theory and string dynamics in physics, and can be used to better solve the dynamics of trajectory particle condensation process.

By considering the back-reaction of the spacetime through the spherically symmetric quantum fluctuations of the background metric, Kazakov and Solodukhin removed the singularity of the Schwarzschild black hole. This regular Schwarzschild black hole has a spherical central region with a radius of the order of the Planck length. On the other hand, due to the positively accelerating expansion of the Universe, it seems that there exists a universal repulsive force known as dark energy. In the framework of quantum field theories, the quintessence field is a candidate model for investigating and modeling dark energy. Accordingly, by taking into account the quintessential matter field in the background of the Schwarzschild black hole, Kiselev gained the metric of this black hole surrounded by quintessence. By combining these two above ideas, in this study, we consider the quantum-corrected Schwarzschild black hole surrounded by quintessence to investigate null and time-like geodesics structure. Generally, this study points out that black holes are quantum-gravitational objects. We will show that the accelerated expansion of the Universe, instead of dark energy, happens because of the presence of quantum effects in this setup. Also, due to the presence of the central Planck-size sphere, the regular black hole has been possessed a shifting over radial coordinate in its inner structure.

We develop a mathematical framework for describing local features of a geometric object—such as the edges of a square or the apex of a cone—in terms of algebraic topological invariants. The main tool is the construction of a "tangent complex" for an arbitrary geometrical object, generalising the usual tangent bundle of a manifold. This framework can be used to develop algorithms for automatic feature location. We give several examples of applying such algorithms to geometric objects represented by point-cloud data sets.

In this paper, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy (DE) epoch and the early-time inflation. We will discuss the black hole (BH) solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of BHs in General Relativity (GR) can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in -gravity. Furthermore, realistic models of F(R)-gravity will be analyzed in detail. A general feature occurring in matter era will be shown, namely, the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the DE and some correction term may be required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and a unified description of the universe is evolved. In the final part of the work, we will look at the last developments in modified gravity, namely, we will investigate cosmological and BH solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" F(T)-gravity theories. A nice application to the dark matter (DM) problem will be presented.

Cosmic censorship is known to fail in some well-controlled phenomena, calling into question the predictive power of General Relativity and opening up the possibility of observing Planck-scale physics. We propose that the cosmic censorship conjecture can be amended so that its spirit prevails. Naked singularities that, classically, have zero mass are allowed. Physically, these are Planck-sized “black holes”, which evaporate in a few Planck times. General Relativity fails only for a tiny interval in time, to then quickly regain control in a Miltonian evolution that returns us to the predictive paradise of Einstein’s equations. If this refinement of the conjecture is correct, then, even though Nature does allow to expose breakdowns in the smooth fabric of spacetime, it limits them to a mostly harmless minimum.

Cosmological models with time-dependent $\mathrm{\Lambda }$ (read as $\mathrm{\Lambda }(t)$) have been investigated widely in the literature. Models that solve background dynamics analytically are of special interest. Additionally, the allowance of past or future singularities at finite cosmic time in a specific model signals for a generic test on its viabilities with the current observations. Following these, in this work we consider a variety of $\mathrm{\Lambda }(t)$ models focusing on their evolutions and singular behavior. We found that a series of models in this class can be exactly solved when the background universe is described by a spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) line element. The solutions in terms of the scale factor of the FLRW universe offer different universe models, such as power-law expansion, oscillating, and the singularity free universe. However, we also noticed that a large number of the models in this series permit past or future cosmological singularities at finite cosmic time. At last we close the work with a note that the avoidance of future singularities is possible for certain models under some specific restrictions.

This work deals with an exhaustive study of bouncing cosmology in the background of homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker spacetime. The geometry of the bouncing point has been studied extensively and used as a tool to classify the models from the point of view of cosmology. Raychaudhuri equation (RE) has been furnished in these models to classify the bouncing point as regular point or singular point. Behavior of time-like geodesic congruence in the neighborhood of the bouncing point has been discussed using the Focusing Theorem which follows as a consequence of the RE. An analogy of the RE with the evolution equation for a linear harmonic oscillator has been made and an oscillatory bouncing model has been discussed in this context.

Singularities of plane into plane mappings described by parabolic two-component systems of quasi-linear partial differential equations of the first order are studied. Impediments arising in the application of the original Whitney’s approach to such a case are discussed. Hierarchy of singularities is analyzed by the double-scaling expansion method for the simplest $2$-component Jordan system. It is shown that flex is the lowest singularity while higher singularities are given by $(k+1,k+2)$ curves which are of cusp type for $k=2n+1$, $n=1,2,3,\dots .$ Regularization of these singularities by deformation of plane into plane mappings into surface $S^{2+k}(\subset {ℝ}^{2+k})$ to plane is discussed. Applicability of the proposed approach to other parabolic type mappings is noted. We finally compare the results obtained for the parabolic case with non-generic gradient catastrophes for hyperbolic systems.

Inspired by the Bañados, Teitelboim and Zanelli (BTZ) formalism, we discuss the Maxwell-$f(T)$ gravity in $(2+1)$ dimensions. The main task is to derive exact solutions for a special form of $f(T)=T+𝜖T^2$, with $T$ being the torsion scalar of Weitzenböck geometry. To this end, a triad field is applied to the equations of motion of charged $f(T)$ and sets of circularly symmetric noncharged and charged solutions have been derived. We show that, in the charged case, the monopole-like and the $ln$ terms are linked by a correlative constant despite the known results in teleparallel geometry and its extensions.${}^{39}$ Furthermore, it is possible to show that the event horizon is not identical with the Cauchy horizon due to such a constant. The singularities and the horizons of these black holes are examined: they are new and have no analogue in the literature due to the fact that their curvature singularities are soft. We calculate the energy content of these solutions by using the general vector form of the energy–momentum within the framework of $f(T)$ gravity. Finally, some thermodynamical quantities, like entropy and Hawking temperature, are derived.

In physics, the application of the regular cases of spacelike fronts has been studied extensively; however, under the gravitational influence, the spacelike fronts can be singular. In this paper, we will consider the traveling trajectories of a set of parallel light rays as spacelike fronts with singularities in null de Sitter sphere. It is of great significance to investigate and explore the spacelike fronts which may have singularities in null de Sitter sphere. We also characterize the singularities of the nullcone dual worldsheets generated by spacelike fronts via the spherical curvature. Then we define the nullcone caustics and analyze the geometric properties of the nullcone caustics at singular points. Moreover, we establish the dual relationship between spacelike fronts and the nullcone dual worldsheets.

We study Einstein gravity minimally coupled to a scalar field in a static, spherically symmetric space–time in four dimensions. Black hole solutions are shown to exist for a phantom scalar field whose kinetic energy is negative. These "scalar black holes" have an infinite horizon area and zero Hawking temperature and are termed "cold black holes" (CBHs). The relevant explicit solutions are well known in the massless case (the so-called anti-Fisher solution), and we have found a particular example of a CBH with a nonzero potential V(ϕ). All CBHs with V(ϕ) ≢ 0 are shown to behave near the horizon, quite similarly to those with a massless field. The above solutions can be converted by a conformal transformation to Jordan frames of a general class of scalar–tensor theories of gravity, but CBH horizons in one frame are in many cases converted to singularities in the other, which gives rise to a new type of conformal continuation.

Space is the only quantity which can be directly observed. Its main characteristics, in its 3-dimensional Euclidean form, are describable by a Clifford algebra. There have been attempts to construct physics on the basis of this parameter alone, involving extensions to extra dimensions and non-Euclidean forms of geometry. However, it seems that adding a second Clifford algebra, identical in structure to the first, but commutative to it, and dual to it in that it contains no extra physical information, with the duality determined by a zero totality condition, gives us the mathematical construction that we need for both quantum mechanics and other forms of physics. The results can be explored from many angles, for example, Clifford algebra, group theory, geometry and topology, to produce significant results in fundamental physics.

We discuss a recently proposed limiting curvature theory of gravity and its application to the problem of singularities inside black holes. In this theory the growth of the curvature is suppressed by specially chosen inequality constraints included in the gravity action. We consider a case of a spherically symmetric four-dimensional black hole and demonstrated that imposed curvature constraints modify a solution in the black hole interior. Instead of forming the curvature singularity the modified metric describes a space which is exponentially expanding in one direction and oscillating the other two directions. The space–time is complete and its polynomial curvature invariants are uniformly bounded.

We study topological properties of Ind-groups $\mathrm{\text{Aut}}(K[x_1,\dots ,x_n])$ and $\mathrm{\text{Aut}}(K〈x_1,\dots ,x_n〉)$ of automorphisms of polynomial and free associative algebras via Ind-schemes, toric varieties, approximations, and singularities. We obtain a number of properties of $\mathrm{\text{Aut}}(\mathrm{\text{Aut}}(A))$, where $A$ is the polynomial or free associative algebra over the base field $K$. We prove that all Ind-scheme automorphisms of $\mathrm{\text{Aut}}(K[x_1,\dots ,x_n])$ are inner for $n\ge 3$, and all Ind-scheme automorphisms of $\mathrm{\text{Aut}}(K〈x_1,\dots ,x_n〉)$ are semi-inner. As an application, we prove that $\mathrm{\text{Aut}}(K[x_1,\dots ,x_n])$ cannot be embedded into $\mathrm{\text{Aut}}(K〈x_1,\dots ,x_n〉)$ by the natural abelianization. In other words, the Automorphism Group Lifting Problem has a negative solution. We explore close connection between the above results and the Jacobian conjecture, as well as the Kanel-Belov–Kontsevich conjecture, and formulate the Jacobian conjecture for fields of any characteristic. We make use of results of Bodnarchuk and Rips, and we also consider automorphisms of tame groups preserving the origin and obtain a modification of said results in the tame setting.

Using singularity theory tools we study the time averaged problem for control systems and profit densities on the circle. We show that the optimal strategy always can be selected within periodic cyclic motions and stationary strategies. When the problem depends additionally on a one dimensional parameter, we analyze generic transitions between these two types of strategies under change of the parameter and classify all generic singularities of the averaged profit as a function of the parameter.

In this paper, we present a model for the late-time evolution of the universe where a dark energy-dark matter interaction is invoked. Dark energy is modeled through an holographic Ricci dark energy component. The model is embedded within an induced gravity braneworld model. For suitable choices of the interaction coupling, the big rip and little rip induced by the holographic Ricci dark energy, in a relativistic model and in an induced gravity braneworld model, are removed. In this scenario, the holographic dark energy will have a phantom like behavior even though the brane is asymptotically de Sitter.

It is well-known that stress singularities occur at the obtuse corners of skew plates, especially when the skew angles are large. Owing to the stress singularities, accurate bending results, vibration frequencies and buckling loads of highly skewed plates are difficult to obtain accurately. In this paper, the mesh-free least squares-based finite difference (LSFD) method is proposed for solving the free vibration and buckling problems of highly skewed plates. As such vibration and buckling results are scarce in the open literature, the method was verified by comparing the LSFD solutions with existing ones having a skew angle θ ≤ 70°, or by carrying out convergence studies. The vibration and buckling results for plates with very large skew angle (θ = 80°) are presented for the first time. The close agreement observed in the comparison studies and the good convergence behavior of the LSFD solutions provide the confidence that these vibration and buckling results predicted by the LSFD method are of good accuracy.

In this paper, we study the mimetic theory and derive a new spherically symmetric black hole solution. The asymptotic behavior of this solution behaves as a flat spacetime. This black hole is characterized by the fact that it has different components of $g_{tt}$ and $g^{rr}$. Nevertheless, both of these components have a coinciding Killing and event horizons. Furthermore, this black hole has curvature singularities which are stronger than those of the known black hole solutions in general relativity. This feature can be shown by calculating some invariants of curvature. We study the stability of the perturbation and the related anti-evaporation of the Nariai spacetime.

The nature of gravitational singularities, long mysterious, has now become clear through a combination of mathematical and numerical analysis. As the singularity is approached, the time derivative terms in the field equations dominate, and the singularity behaves locally like a homogeneous oscillatory spacetime.