Investigating $f(R)$ gravity and cosmologies

The f (R) theory of gravity is an extended theory of gravity that is based on general relativity in the simplest case of $f(R) = R$. This theory extends such a function of the Ricci scalar into arbitrary functions that are not necessarily linear, i.e. could be of the form $f(R) = \alpha R^{2}$. The action for such a theory would be $S_{EH} = \frac{1}{2k} \int f(R) + L^{m}\; d^{4}x\sqrt{−g}$, where $S_{EH}$ is the Einstein-Hilbert action for our theory, $g$ is the determinant of the metric tensor $g_{\mu \nu}$ and $L^{m}$ is the Lagrangian density for matter. In this paper, we will look at some of the physical implications of such a theory, and the importance of such a theory in cosmology and in understanding the geometric nature of such f (R) theories of gravity.

Continuous-Curvature Trajectory Planning

Continuous‐curvature paths play an important role in the area of driving robots: as vehicles usually cannot changethe steering angle in zero‐time, real trajectories must not have discontinuities in the curvature profile. Typical continuous‐curvature paths are thus built of straight lines, arcs and clothoids. Due to the geometric nature of clothoids, some questions in the area of trajectory planning are difficult the answer – usually we need approximations here. In this paper we describe a full approach for continuous‐curvature trajectory planning for mobile robots – it covers a maneuver‐based planning with Viterbi optimization and geometric approximations required to construct the respective clothoid trajectories.

On the constancy theorem for anisotropic energies through differential inclusions

In this paper we study stationary graphs for functionals of geometric nature defined on currents or varifolds. The point of view we adopt is the one of differential inclusions, introduced in this context in the recent papers (De Lellis et al. in Geometric measure theory and differential inclusions, 2019. arXiv:1910.00335; Tione in Minimal graphs and differential inclusions. Commun Part Differ Equ 7:1–33, 2021). In particular, given a polyconvex integrand f, we define a set of matrices $$C_f$$ C f that allows us to rewrite the stationarity condition for a graph with multiplicity as a differential inclusion. Then we prove that if f is assumed to be non-negative, then in $$C_f$$ C f there is no $$T'_N$$ T N ′ configuration, thus recovering the main result of De Lellis et al. (Geometric measure theory and differential inclusions, 2019. arXiv:1910.00335) as a corollary. Finally, we show that if the hypothesis of non-negativity is dropped, one can not only find $$T'_N$$ T N ′ configurations in $$C_f$$ C f , but it is also possible to construct via convex integration a very degenerate stationary point with multiplicity.

Complex geodesics in convex domains and ℂ-convexity of semitube domains

In this paper the complex geodesics of a convex domain in ℂ n are studied. One of the main results provides a certain necessary condition for a holomorphic map to be a complex geodesic for a convex domain in ℂ n . The established condition is of geometric nature and it allows to find a formula for every complex geodesic. The ℂ-convexity of semitube domains is also discussed.

FROM THE METAPHYSICS OF EUCLIDES - TO GEOMETRIC IDEAS IN PHYSICS THROUGH THE AGE (GEOMETRIC IDEAS IN PHYSICS EXPAND THE HORIZONS OF THE KNOWLEDGE OF THE WORLD)

This work consists of two parts. In the first part, a historical analysis is made with modern comments on the importance of a deep study of stable knowledge, experience and traditions of a geometric nature about the structure of the world accumulated by our civilization, which have passed thousands of years of testing. In addition to mathematics, in physics, the tradition of geometric research methods comes from Archimedes, through the work of Leonardo da Vinci, Galileo Galilei, René Descartes, Isaac Newton and other scientists. This trend is now stronger than ever. The second part briefly and summarizes the stages of how and what we have come to on this path.

A Björling representation for Jacobi fields on minimal surfaces and soap film instabilities

We develop a general framework for the description of instabilities on soap films using the Björling representation of minimal surfaces. The construction is naturally geometric and the instability has the interpretation as being specified by its amplitude and transverse gradient along any curve lying in the minimal surface. When the amplitude vanishes, the curve forms part of the boundary to a critically stable domain, while when the gradient vanishes the Jacobi field is maximal along the curve. In the latter case, we show that the Jacobi field is maximally localized if its amplitude is taken to be the lowest eigenfunction of a one-dimensional Schrödinger operator. We present examples for the helicoid, catenoid, circular helicoids and planar Enneper minimal surfaces, and emphasize that the geometric nature of the Björling representation allows direct connection with instabilities observed in soap films.

On the Geometry of Stiffness and Compliance Under Concatenation

Eigentwists and eigenwrenches capture the stationary stiffness behavior of compliant mechanisms and can be related to a mechanism’s primary kinematic behavior. The nature of concatenation of multiple mechanism building blocks is not well understood. In this paper, we consider the mechanics of concatenation and develop design rules that capture the geometric nature of concatenation in terms of eigenwrenches and eigentwists. The rules are illustrated through mechanisms from the literature and an example design problem. The design rules have potential to provide intelligent guidance for systematic building block synthesis of compliant mechanisms.

Geometry of vaulted systems in the treatises by Guarino Guarini

<span>Guarini first develops a rigorous and systematic discourse on vaulted systems. In three treatises: Architettura Civile, Euclides adauctus, and Modo di misurare le fabriche, he described the geometric nature of the vaults, the principles of geometry and their practices in the stereotomy, and the measurement of the vaults’ surfaces and volumes, respectively. In this paper, moving from previous studies, the significant relationships between the Architettura Civile and Modo di misurare le fabriche are deepened, also in light to the theoretical bases established in the Euclides adauctus. Graphical analyses and reconstructive digital models, linking the texts to the original diagrams and drawings of the treatises, allow to highlight the role of geometry in Guarini’s theorization and the logic of shapes’ composition at the basis of his inventions, and constitute a knowledge base to recognize and interpret the geometric structure of the vaults in Baroque built heritage.</span>

Recovering the cosmological constant from affine geometry

A gravity theory without masses can be constructed in Minkowski spaces using a geometric Minkowski potential. The related affine spacelike spheres can be seen as the regions of the Minkowski spacelike vectors characterized by a constant Minkowski gravitational potential. These spheres point out, for each dimension [Formula: see text], spacetime models, the de Sitter ones, which satisfy Einstein’s field equations in absence of matter. In other words, it is possible to generate geometrically the cosmological constant. Even if a lot of possible parameterizations have been proposed, each one highlighting some geometric and physical properties of the de Sitter space, we present here a new natural parameterization which reveals the intrinsic geometric nature of cosmological constant relating it with the invariant affine radius coming from the so-called Minkowski–Tzitzeica surfaces theory.