Abstract
Elastica and inextensible flows of curves play an important role in practical applications. In this paper, we construct a new characterization of inextensible flows by using elastica in space. The inextensible flow is completely determined for any space-like curve in de Sitter space
1 Introduction
In mathematics and physics, a de Sitter space is the analogue in Minkowski space, or spacetime, of a sphere in ordinary, Euclidean space. The n-dimensional de Sitter space, denoted dSn, is the Lorentzian manifold analogue of an n-sphere (with its canonical Riemannian metric). It is maximally symmetric, has constant positive curvature, and is simply connected for n at least 3. More recently, it has been considered as the setting for special relativity rather than using Minkowski space, since a group contraction reduces the isometry group of de Sitter space to the Poincaré group, allowing a unification of the spacetime translation subgroup and Lorentz transformation subgroup of the Poincaré group into a simple group rather than a semi-simple group. This alternate formulation of special relativity is called de Sitter relativity, [1–6].
The elastica caught the attention of many of the brightest minds in the history of mathematics, including Galileo, the Bernoullis, Euler, and others. It was present at the birth of many important fields, most notably the theory of elasticity, the calculus of variations, and the theory of elliptic integrals. The path traced by this curve illuminates a wide range of mathematical style, from the mechanics-based intuition of the early work, through a period of technical virtuosity in mathematical technique, to the present day where computational techniques dominate [16–20].
The flow of a curve or surface is said to be inextensible if, in the former case, the arc-length is preserved, and in the latter case, if the intrinsic curvature is preserved [7]. Physically, inextensible curve and surface flows are characterized by the absence of any strain energy induced from the motion. In [15], Kwon investigated inextensible flows of curves and developable surfaces in R3. Necessary and sufficient conditions for an inextensible curve flow were first expressed as a partial differential equation involving the curvature and torsion. Then, they derived the corresponding equations for the inextensible flow of a developable surface, and showed that it suffices to describe its evolution in terms of two inextensible curve flows, [15]. Flows of curves of a given curve are also widely studied, [8–14].
This study is organised as follows: Firstly, we construct a new method for inextensible flows of space-like curves in de Sitter space
2 New geometry of space-like curves in
S 1 3 -space
It is well-known that the Lorentzian space form with a positive curvature, more precisely [1], a positive sectional curvature is called de Sitter space
It is well-known that to each unit speed space-like curve
where δ(γ) = –sign(N), and κ, τ are the curvature and the torsion of a curve γ respectively and given by
with R(s) ≠ 0.
Let γ(u, w) be a one parameter family of smooth space-like curves in
Putting
which gives
Finally [18], we obtain that
The flow
Let
Now, assume that γ is arc-length parametrized curve. Then, we have
From the definition of inextensible flow, we have
Using Eq. (3), we obtain Eq. (4). This completes the proof.
Now we give the characterization of evolution of first curvature as below:
Let γ be one parameterfamily curves in de Sitter space
A differentiation in Eq. (4) and the Frenet formulas give us that
Using the formula of the curvature, we write a relation
We immediately arrive at
Another important fact is that the curvature operator R on de Sitter space
Then,
From above equations, we get
Then, we can write
Thus it is easy to obtain that
On the other hand, we have
Since, we express
Moreover, by the definition of metric tensor, we have
Then
Combining these we have
Thus, we obtain the theorem. This completes the proof.
From the above theorem, we have
Using Frenet equations, we have
Then,
Therefore
From above equation we obtain
which completes the proof.
Let γ be one parameterfamily curves in de Sitter space
Assume that
Under the assumption of space-like curve, we have
Using the formula of the curvature, we write a relation
Thus, it is seen that
By using formula of curvature, we have
Arranging the last equations, we obtain
Therefore, we can easily see that
By this way, we conclude
Thus, we obtain the theorem. The proof of theorem is completed. Now we give the characterization of evolution of second curvature as below:
Let γ be one parameterfamily curves in de Sitter space
where π1, π2, π3, are smooth functions of time and arc-length.
It is obvious from Theorem 2.6. This completes the proof.
Since δ(t) is an immersed curve, it has velocity vector V=vT and squared geodesic curvature
where
From Euler equations, we easily have
In what follows, γ: [0,1] → M is a curve of length L. Now for fixed constant λ let
For a variation γω with variation field M, we compute
This condition implies that
where
and
Thus, we can state the following.
Let γ be one parameterfamily curves in de Sitter space
where π1, π2are smooth functions of time and arc-length.
Let γ be one parameterfamily curves in de Sitter space
where π1, π2, π3are smooth functions of time and arc-length.
Firstly, we obtain
Since, we immediately arrive at
Therefore,
Now, we can obtain following equation in terms of flows.
Let γ be one parameterfamily curves in de Sitter space
The time-helix is parametrized by
Projection of γ atxyz-plane:
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