42,210 research outputs found
Characterization of curves that lie on a geodesic sphere or on a totally geodesic hypersurface in a hyperbolic space or in a sphere
The consideration of the so-called rotation minimizing frames allows for a
simple and elegant characterization of plane and spherical curves in Euclidean
space via a linear equation relating the coefficients that dictate the frame
motion. In this work, we extend these investigations to characterize curves
that lie on a geodesic sphere or totally geodesic hypersurface in a Riemannian
manifold of constant curvature. Using that geodesic spherical curves are normal
curves, i.e., they are the image of an Euclidean spherical curve under the
exponential map, we are able to characterize geodesic spherical curves in
hyperbolic spaces and spheres through a non-homogeneous linear equation.
Finally, we also show that curves on totally geodesic hypersurfaces, which play
the role of hyperplanes in Riemannian geometry, should be characterized by a
homogeneous linear equation. In short, our results give interesting and
significant similarities between hyperbolic, spherical, and Euclidean
geometries.Comment: 15 pages, 3 figures; comments are welcom
Characterization of manifolds of constant curvature by spherical curves
It is known that the so-called rotation minimizing (RM) frames allow for a
simple and elegant characterization of geodesic spherical curves in Euclidean,
hyperbolic, and spherical spaces through a certain linear equation involving
the coefficients that dictate the RM frame motion (da Silva, da Silva in
Mediterr J Math 15:70, 2018). Here, we shall prove the converse, i.e., we show
that if all geodesic spherical curves on a Riemannian manifold are
characterized by a certain linear equation, then all the geodesic spheres with
a sufficiently small radius are totally umbilical and, consequently, the given
manifold has constant sectional curvature. We also furnish two other
characterizations in terms of (i) an inequality involving the mean curvature of
a geodesic sphere and the curvature function of their curves and (ii) the
vanishing of the total torsion of closed spherical curves in the case of
three-dimensional manifolds. Finally, we also show that the same results are
valid for semi-Riemannian manifolds of constant sectional curvature.Comment: To appear in Annali di Matematica Pura ed Applicat
Characterization of Spherical and Plane Curves Using Rotation Minimizing Frames
In this work, we study plane and spherical curves in Euclidean and
Lorentz-Minkowski 3-spaces by employing rotation minimizing (RM) frames. By
conveniently writing the curvature and torsion for a curve on a sphere, we show
how to find the angle between the principal normal and an RM vector field for
spherical curves. Later, we characterize plane and spherical curves as curves
whose position vector lies, up to a translation, on a moving plane spanned by
their unit tangent and an RM vector field. Finally, as an application, we
characterize Bertrand curves as curves whose so-called natural mates are
spherical.Comment: 8 pages. This version is an improvement of the previous one. In
addition to a study of some properties of plane and spherical curves, it
contains a characterization of Bertrand curves in terms of the so-called
natural mate
Rotation minimizing frames and spherical curves in simply isotropic and pseudo-isotropic 3-spaces
In this work, we are interested in the differential geometry of curves in the
simply isotropic and pseudo-isotropic 3-spaces, which are examples of
Cayley-Klein geometries whose absolute figure is given by a plane at infinity
and a degenerate quadric. Motivated by the success of rotation minimizing (RM)
frames in Euclidean and Lorentzian geometries, here we show how to build RM
frames in isotropic geometries and apply them in the study of isotropic
spherical curves. Indeed, through a convenient manipulation of osculating
spheres described in terms of RM frames, we show that it is possible to
characterize spherical curves via a linear equation involving the curvatures
that dictate the RM frame motion. For the case of pseudo-isotropic space, we
also discuss on the distinct choices for the absolute figure in the framework
of a Cayley-Klein geometry and prove that they are all equivalent approaches
through the use of Lorentz numbers (a complex-like system where the square of
the imaginary unit is ). Finally, we also show the possibility of obtaining
an isotropic RM frame by rotation of the Frenet frame through the use of
Galilean trigonometric functions and dual numbers (a complex-like system where
the square of the imaginary unit vanishes).Comment: 2 figures. To appear in "Tamkang Journal of Mathematics
Deterministic Seasonality in Dickey-Fuller Tests: Should We Care?
This paper investigates the properties of Dickey-Fuller tests for seasonally unadjusted quarterly data when deterministic seasonality is present but it is neglected in the test regression. While for the random walk case the answer is straightforward, an extensive Monte Carlo study has to be performed for more realistic processes and testing strategies. The most important conclusion is that the common perception that deterministic seasonality has nothing to do with the long-run properties of the data is incorrect. Further numerical evidence on the shortcomings of the general-to-specific t-sig lag selection method is also presented.unit root; Dickey-Fuller tests; similar tests; seasonality; Monte Carlo
The Order of Integration for Quarterly Macroeconomic Time series: a Simple Testing Strategy
Besides introducing a simple and intuitive definition for the order of integration of quarterly time series, this paper also presents a simple testing strategy to determine that order for the case of macroeconomic data. A simulation study shows that much more attention should be devoted to the practical issue of selecting the maximum admissible order of integration. In fact, it is shown that when that order is too high, one may get (spurious) evidence for an excessive number of unit roots, resulting in an overdifferenced series.
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