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Stationary acceleration of Frenet curves
Journal of Inequalities and Applications volume 2017, Article number: 92 (2017)
Abstract
In this paper, the stationary acceleration of the spherical general helix in a 3-dimensional Lie group is studied by using a bi-invariant metric. The relationship between the Frenet elements of the stationary acceleration curve in 4-dimensional Euclidean space and the intrinsic Frenet elements of the Lie group is outlined. As a consequence, the corresponding curvature and torsion of these curves are computed. In Minkowski space, for the curves on a timelike surface to have a stationary acceleration, a necessary and sufficient condition is refined.
1 Introduction
Rigid body motion has attracted continuous attention since the time of Galileo and Bernoulli, and recently, the subject has generated a renewed interest in differential geometry. \(\mathrm{SE}(3)\) is the space of all rigid body motions, and the motions can be described as curves in this space [1]. In 1989, Noakes, Heinzinger and Paden [2] derived the equations for the minimum acceleration curve by using positive definite bi-invariant metrics on the rotation group \(\mathrm{SO}(3)\). By Noakes et al. [2], the specification of spline curves was extended to curves in groups associated with robotics. By using left-invariant metrics, Zefran and Kumar [3] in 1998 used the same acceleration definition for the rigid body as the covariant derivative of the motion, and so the jerk is the second covariant derivative. In 2007, Selig [4] repeated the analysis by using bi-invariant metrics on the rigid body motion group \(\mathrm{SE}(3)\). Since these metrics are not positive definite, the curves specified by differential equations are derived only stationary, not minimal. In 1990, Bottema and Roth [5] studied a number of spatial motions by using the 4D representation of \(\mathrm{SE}(3)\), one of which is the Serret-Frenet motion. Finding the curve with given curvature and torsion functions involves solving a system of differential equations given by the Serret-Frenet relations. This is not straightforward, and solutions are only known in a very few cases as studied by Lipkin [6] in 2005 and Selig [4] in 2007. In this work, the ideas of Zefran and Kumar [3] and Selig [4] are revisited. Kula et al., in [7], investigated the relations between a general helix and a slant helix. By using the Serret-Frenet frame in a 3-dimensional Lie group with a bi-invariant metric, the stationary acceleration of the spherical general helix is studied. It is proved that the normal curvature, geodesic curvature and geodesic torsion functions of the curves on a timelike surface in the Minkowsky space are linear.
2 General helix in a Lie group
Let G be a 3-dimensional Lie group with the bi-invariant metric \(\langle\cdot,\cdot \rangle\). Suppose ∇ is the corresponding Levi-Civita connection. If g denotes the Lie algebra of G, then the isomorphism \(g\simeq T_{e}\mathbf{G}\) holds, where e is the identity element of G. As is known,
hold for all \(X,Y,Z\in g\). Also, for any \(X,Y\in g\), the vector product \(X\times Y\) is defined by
Definition 2.1
General helix, see [8], Definition 1
Let \(\alpha:I\to\mathbf{G}\) be a parameterized curve, where \(I\subset{\mathbb {R}}\). Then α is called a general helix if it makes a constant angle with a left-invariant vector field.
If \(I\subset{\mathbb {R}}\) and \(\alpha:I\to\mathbf{G}\) is a curve parameterized with arc length and if the Frenet structure of α is denoted by \((t,n,b,\kappa,\tau)\), then
Theorem 2.2
Lancret, see [8], Theorem 1
A curve is a general helix in G if and only if \(\tau=c\kappa+\tau_{\mathbf{G}}\), where c is a constant.
Definition 2.3
Left shift, see [8], Definition 3
Let \(I\subset{\mathbb {R}}\) and \(\alpha:I\to\mathbf{G}\) be an arc length parameterized curve. Then a curve \(\beta:I\to g\), where g is the Lie algebra of G, for which \(\beta'(s)=dL_{\alpha^{-1}(s)}\alpha'(s)\) for all \(s\in I\), is called the left shift of α.
Definition 2.4
Spherical curve, see [8], Definition 4
α is called a spherical curve if β lies on the unit central sphere, i.e., \(\langle\beta(s),\beta(s) \rangle=1\) for all \(s\in I\).
Theorem 2.5
See [8], Proposition 2
A curve α is the spherical general helix with \(\tau=c\kappa+\tau_{\mathbf{G}}\) if and only if
In the Lie group G, a spherical motion is determined by a unit speed space curve \(\alpha(s)\). In the Serret-Frenet motion, a point on the moving body moves along the curve and the coordinate frame on the moving body remains aligned with the tangent t, normal n and bi-normal b of the curve. Using the 4D representation of G, the motion can be specified as
where α is the curve, and the rotation matrix has the unit vectors t, n and b as columns of
Set \(\nabla_{t}x=x'\) for all \(x\in\{t,n,b\}\). Now the intrinsic Serret-Frenet formulas are
where κ and τ are the curvature and torsion functions of the curve, respectively. The Darboux vector \(\omega=\tau t+\kappa b\) has the properties
see [6], Section 10.2. This means that
can be written for the \(3\times3\) anti-symmetric matrix \(\Omega _{\omega}\), which is corresponding to ω. Since α is a unit speed curve, we have \(\alpha'=t\), and hence
Using the Serret-Frenet relations, the derivative of the velocity can be calculated as
where \(\omega'=\tau't+\kappa'b\). Hence, the second derivative of the velocity is
where \(n=b\times t\) and \(\omega''=\tau''t+(\tau'\kappa-\kappa'\tau)n+\kappa''b\). Finally, \(G^{-1}V''G\) is computed as
Since the curve α is a stationary acceleration curve,
holds, see [4]. Thus, there exist constants \(c_{1}\), \(c_{2}\), \(c_{3}\), \(c_{4}\) such that
By setting up this equation for the two unknowns κ and τ, the system of differential equations
holds, and as a consequence, the following theorem is true.
Theorem 2.6
The general spherical helix is a stationary acceleration curve in a Lie group G with the bi-invariant metric if and only if \(\kappa=1\) and τ is linear.
Proof
Let the curve α be a general spherical helix in the Lie group G. Then \(\tau=c\kappa+\tau_{\mathbf{G}}\), where c is a constant. α is a stationary acceleration curve, and due to (2.2) and (2.3), \({\kappa}'=c_{4}\) is satisfied. Therefore, \(\kappa=c_{4}s+c_{5}\), and from \(\kappa(s)=\frac{1}{\sqrt{1-c^{2}s^{2}}}\), \(c=0\) is obtained. Hence, \(\kappa=1\), and so \(c_{2}=\tau'\kappa-\kappa'\tau=\tau '_{\mathbf{G}}\). Therefore, \(\tau_{\mathbf{G}}=c_{2}s+c_{6}\), where \(c_{6}\) is a constant. Finally, \(\tau=c+\tau_{\mathbf{G}}=a+bs\), where \(a=c+c_{6}\) and \(b=c_{2}\) are constants and \(\tau''=0\). Hence, \(\kappa=1\) and τ is linear. Conversely, if \(\kappa=1\) and τ is linear, then obviously κ and τ satisfy the stationary acceleration curve condition (2.2). □
3 Spherical general helix
For \(I\subset{\mathbb {R}}\), \(\alpha:I\to S^{3}\), the unit sphere with the center at origin in \({\mathbb {R}}^{4}\) is an immersed curve in a 3-dimensional real space form. Therefore, any curve on \(S^{3}\) can also be considered to be a curve in \({\mathbb {R}}^{4}\). In this paper, the goal is to obtain the relationship between the Frenet frame \((e_{1}\mid e_{2}\mid e_{3}\mid e_{4})\) in 4-dimensional Euclidean space with the curvature functions \(k_{i}= \langle e'_{i},e_{i+1} \rangle\) for \(i=1,2,3\) and the intrinsic Frenet frame \((t\mid n\mid b)\) with the curvature \(\kappa= \langle t',n \rangle\) and torsion \(\tau= \langle n',b \rangle\) of the curve α. Set \(t=e_{1}\). By using the Gauss map of the sphere,
and
hold, see [9].
Theorem 3.1
A unit speed space curve \(\alpha:I\subset{\mathbb {R}}\to S^{3}\) parameterized with arc length is a stationary acceleration curve if and only if
and
where a, b, p, q are constants and \(k_{i}\) is the ith principle curvature of the curve α for \(i=1,2\) in 4-dimensional Euclidean space.
Proof
Suppose α is a unit speed space curve on \(S^{3}\). It is known that, in the Frenet-Serret motion, a point on the moving body moves along the curve α and the coordinate frame in the moving body remains aligned with the tangent t, normal n and bi-normal b of this curve. By using the 4D representation, the motion can be specified in the form (2.1) such that the corresponding rotation matrix of motion is \(R= (t \vert n \vert b )\). The curve α is a stationary acceleration curve if and only if \(G^{-1}V''G=C\), where C is the \(4\times4\) constant matrix as in (2.3). By substituting \(k_{1}=x\) in (3.1), \(\kappa=\sqrt{x^{2}-1} \) is obtained, and from (3.1), we get \(\kappa'=c_{4}\). From
we have
Since \(\kappa\neq0\), we get \(c_{3}=0\). Hence,
On the other hand, it is clear that
are satisfied. Therefore, for \(c_{4}=a\), we obtain
and
and by using \(c_{3}=0\), we get
where C is a \(4\times4\) constant matrix such that \(k_{1}\) and \(k_{2}\) satisfy the stationary acceleration condition of α. Conversely, if \(k_{1}\) and \(k_{2}\) satisfy (3.4) and (3.5), then, from (3.1) and (3.2), we have \(\kappa=as+b\) and \(\tau=ps+q\). Thus, from (2.2), \(G^{-1}V''G\) is a \(4\times4\) constant matrix, and so α is a stationary acceleration curve. □
4 Curves on a timelike surface
The Minkowski spacetime \({\mathbb {R}}^{3}_{1}\) is the Euclidean space \({\mathbb {R}}^{3}\) with the inner product
A vector \(v\in{\mathbb {R}}^{3}_{1}\setminus\{0\}\) is spacelike, timelike or lightlike if \(\langle v,v \rangle>0\), \(\langle v,v \rangle<0\) or \(\langle v,v \rangle=0\). The vector \(v=0\) is spacelike. Also, the norm of a vector v is given by \(\Vert v \Vert =\sqrt{ \vert \langle v,v \rangle \vert }\).
Let \(X:U\to{\mathbb {R}}^{3}_{1}\) be a timelike embedding, where U is an open subset of \({\mathbb {R}}^{2}\). The tangent space \(T_{p}M\) is a timelike plane at any \(p\in X(U)\), where \(M=X(U)\). Let \(\overline{\gamma}:I\to U\) be a regular curve and define the curve \(\gamma:I\to M\subset{\mathbb {R}}^{3}_{1}\) on the timelike surface by \(\gamma(s)=X(\overline{\gamma})\). Let γ be spacelike or timelike on the timelike surface M with the unit tangent vector \(t(s)=\gamma'(s)\), where s is the arc-length parameter. Since \(M=X(U)\) is timelike, a unit spacelike normal vector field n on \(M=X(U)\) is defined by
Then \(n_{\gamma}=n\circ\gamma\) is a unit spacelike normal vector field along γ. The bi-normal vector field is defined by \((\varepsilon\circ\gamma)b=n_{\gamma}\times t\). It is known that
where \(\varepsilon\circ\gamma=\operatorname{sgn}(t)\) which equals 1 when γ is spacelike and equals −1 when γ is timelike. When \(\varepsilon(\gamma(s))=1\), the semi-orthonormal frame is \((b(s)\mid n_{\gamma}(s)\mid t(s) )\), and when \(\varepsilon(\gamma(s))=-1\), the semi-orthonormal frame is \((t(s)\mid b(s)\mid n_{\gamma }(s) )\). Therefore, we have
For
the Frenet equations are
and these are called normal curvature, geodesic curvature and geodesic torsion, respectively [9]. Now we suppose \(t'(s)\neq0\). The Darboux vector field in two cases \(\varepsilon(\gamma(s))=\pm1\) is
Therefore,
Also, we have
and
where
Let \(\Omega_{\omega}\) be the anti-symmetric 3×3 matrix corresponding to the Darboux vector field ω, so
and
By using the 4D representation of \(\mathrm{SE}(3)\), the motion can be specified as
where
and
From the properties of the Darboux vector, we can write \(R'=\Omega _{\omega}R\). Hence, we have
Therefore,
and thus
By using the standard formulas for the scalar and vector products of t, \(n_{\gamma}\) and b, we can write
where \(A_{1}\), \(A_{2}\) and \(A_{3}\) are as mentioned above. From \(G^{-1}V''G=C\), where C is a \(4\times4\) constant matrix, we obtain
Then
Also, from
we can obtain \(\tau_{g}(s)=as+b\). Hence, we have the following result.
Theorem 4.1
A curve on the timelike surface in Minkowski space is a stationary acceleration curve if and only if its normal curvature, geodesic curvature and geodesic torsion are linear functions.
5 Curves on Minkowski spacetime
The Minkowski spacetime \({\mathbb {R}}^{4}_{1}\) is the Euclidean space \({\mathbb {R}}^{4}\) with the inner product
A vector \(v\in{\mathbb {R}}^{4}_{1}\setminus\{0\}\) is spacelike, timelike or lightlike if \(\langle v,v \rangle>0\), \(\langle v,v \rangle<0\) or \(\langle v,v \rangle=0\). The vector \(v=0\) is spacelike. Also, the norm of a vector v is given by \(\Vert v \Vert =\sqrt{ \vert \langle v,v \rangle \vert }\). Let α be a unit speed timelike or spacelike curve with \((e_{1}\mid e_{2}\mid e_{3}\mid e_{4} )\) as the Frenet frame in \({\mathbb {R}}^{4}_{1}\) and set \(\langle e_{i},e_{i} \rangle=\varepsilon_{i}\in\{-1,1\} \), \(i=1,2,3,4\). We can define the curvature functions by \(k_{i}= \langle e'_{i},e_{i+1} \rangle\) for \(i=1,2,3\). Therefore, the Frenet equations are
Also, the vector \(x\times y\times z\) is defined by
here \(\{i,j,k,l\}\) is the canonical basis of \({\mathbb {R}}^{4}_{1}\) and
Then, for any \(t\in{\mathbb {R}}^{4}_{1}\), we can write \(\langle t,x\times y\times z \rangle=\det(t,x,y,z)\). Thus, \(x\times y\times z\) is semi-orthogonal to x, y and z. A normal curve in \({\mathbb {R}}^{4}_{1}\) is a curve whose position vector always lies in its normal space \(e^{\perp}_{1}=\{w\in{\mathbb {R}}^{4}_{1}: \langle w,e_{1} \rangle=0\}\).
Theorem 5.1
See [10], Theorem 3.1
Let α be a unit speed timelike or spacelike curve with non-lightlike vector fields \(e_{2}\), \(e_{3}\), \(e_{4}\), lying in \({\mathbb {R}}^{4}_{1}\). Then α is congruent to a normal curve if and only if
Theorem 5.2
See [10], Formula (3.3)
Let α be a unit speed timelike or spacelike normal curve with non-lightlike vector fields \(e_{2}\), \(e_{3}\), \(e_{4}\), lying in \({\mathbb {R}}^{4}_{1}\). Then its position vector satisfies the equation
Let M be a hypersurface in \({\mathbb {R}}^{4}_{1}\) with the induced Levi-Civita connection ∇ of \({\mathbb {R}}^{4}_{1}\). Let \(\alpha:I\to M\) be a non-lightlike immersed unit speed curve in M, and let us denote the Frenet frame by \((t\mid n\mid b)\). The Frenet equations are
where \(\varepsilon_{X}= \langle X,X \rangle\) and κ, τ are curvature and torsion functions, respectively, and t, n, b satisfy the equations
The Darboux vector field is \(\omega=-\varepsilon_{b}\tau t-\varepsilon _{n}\kappa b\). Therefore, we have
Also, we have
and
If \(R=(t\mid n\mid b)\) is the rotation matrix and \(\Omega_{\omega}\) is the corresponding \(3\times3\) anti-symmetric matrix to the Darboux vector ω, then
where \(\alpha'=t\). Therefore, for the motion
from the properties of the Darboux vector, we have
Hence,
and
where C is a \(4\times4\) constant matrix, which is the necessary and sufficient condition for non-lightlike immersed curves \(\alpha:I\to M\) in the hypersurface \(M\subset{\mathbb {R}}^{4}_{1}\) to be an acceleration curve. Then, by \(G^{-1}V''G=C\), we obtain
where a, b, p, q are constants and s is the arc-length parameter of the curve α.
Let \((e_{1}\mid e_{2}\mid e_{3}\mid e_{4})\) be the Frenet frame of the normal curve α as a unit speed timelike or spacelike normal curve with non-lightlike vector fields \(e_{2}\), \(e_{3}\), \(e_{4}\), lying in \({\mathbb {R}}^{4}_{1}\). Let the curvature functions of α be \(k_{1}\), \(k_{2}\), \(k_{3}\). Then
where \(\langle v,v \rangle=\varepsilon\) with v the unit normal vector field to the hypersurface M in \({\mathbb {R}}^{4}_{1}\) and \(\langle e_{i},e_{i} \rangle=\varepsilon_{i}\in\{-1,1\}\), \(i=1,2,3,4\). Also, by using the Gauss map [11] and Theorem 5.1, we can write
and from Theorem 5.2, we can write
The bi-normal vector is
Therefore,
Then \(\langle b',\alpha \rangle=0\). Finally,
Hence, we have proved the following result.
Theorem 5.3
Let M be a hypersurface in \({\mathbb {R}}^{4}_{1}\) and \(I\subset{\mathbb {R}}\). Let \(\alpha:I\to M\) be a unit speed timelike or spacelike normal curve with non-lightlike vector fields \(e_{2}\), \(e_{3}\), \(e_{4}\), lying in \({\mathbb {R}}^{4}_{1}\). Then α is an acceleration curve in M if and only if
and
where a, b, p, q are constants and \(k_{i}\) is the ith principle curvature of the curve α for \(i=1,2\) in \({\mathbb {R}}^{4}_{1}\) and \(\langle v,v \rangle=\varepsilon\) with v the unit normal vector field to the hypersurface M in \({\mathbb {R}}^{4}_{1}\) and \(\langle e_{3},e_{3} \rangle=\varepsilon_{3}\).
6 Results and discussion
In this paper, it is proved that the general spherical helix is the stationary acceleration curve in a Lie group with a bi-invariant metric if and only if its curvature is unit and torsion is linear. The relationship between the Frenet elements of the stationary acceleration curve in 4-dimensional Euclidean space and the intrinsic Frenet elements of the Lie group is obtained. In other words, the necessary and sufficient conditions for stationary acceleration of unit speed spherical curves are studied, and as a consequence, the corresponding curvature and torsion of these curves are derived.
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The first author would like to thank the University of Mohaghegh Ardabili for financial support.
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Abazari, N., Bohner, M., Sağer, I. et al. Stationary acceleration of Frenet curves. J Inequal Appl 2017, 92 (2017). https://doi.org/10.1186/s13660-017-1354-7
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DOI: https://doi.org/10.1186/s13660-017-1354-7