Journal of Applied Mathematics

Volume 2013 (2013), Article ID 651495, 6 pages

http://dx.doi.org/10.1155/2013/651495

Research Article

On the Involute-Evolute of the Pseudonull Curve in Minkowski 3-Space

Ufuk Ozturk,1,2 Esra Betul Koc Ozturk,1,2 and Kazim Ilarslan³

¹Department of Mathematics, Faculty of Sciences, University of Çankiri Karatekin, 18100 Çankiri, Turkey

²School of Mathematics & Statistical Sciences, Arizona State University, Room PSA442, Tempe, AZ 85287-1804, USA

³Department of Mathematics, Faculty of Sciences and Arts, University of Kirikkale, 71450 Kirikkale, Turkey

Received 6 June 2013; Accepted 10 September 2013

Academic Editor: Baolin Wang

Copyright © 2013 Ufuk Ozturk et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We have generalized the involute and evolute curves of the pseudonull curves in ; that is, is a spacelike curve with a null principal normal. Firstly, we have shown that there is no involute of the pseudonull curves in . Secondly, we have found relationships between the evolute curve β and the pseudonull curve in . Finally, some examples concerning these relations are given.

1. Introduction

The general theory of curves in an Euclidean space (or more generally in a Riemannian manifold) was developed a long time ago, so now, we have a deep knowledge of its local geometry as well as its global geometry. In the theory of curves in Euclidean space, one of the important and interesting problems is the characterizations of a regular curve. And, in particular, the involute-evolute of a given curve is a well-known concept in the classical differential geometry (see [1]).

At the beginning of the twentieth century, A. Einstein’s theory opened a door for use of new geometries. One of them is simultaneously the geometry of special relativity and the geometry induced on each fixed tangent space of an arbitrary Lorentzian manifold and was introduced and some of classical differential geometry topics have been treated by the researchers. According to reference (see [2, 3]), the involute and evolute curves of the spacelike curve in with a spacelike binormal or a spacelike principal normal in Minkowski -space have been investigated. Also, Bükcü and Karacan studied the involute and evolute curves of the timelike curve in Minkowski 3-space [4].

In this paper, we have generalized the involute and evolute curves of the pseudonull curves in ; that is, is a spacelike curve with a null principal normal. Firstly, we have shown that there is no involute of the pseudonull curves in . Secondly, we have found relationships between the evolute curve and the pseudonull curve in . Finally, some examples concerning these relations are given.

2. Preliminaries

The Minkowski -space is the Euclidean -space provided with the standard flat metric given by

where is a rectangular coordinate system of . Since is an indefinite metric, recall that a vector can have one of three Lorentzian causal characters: it can be spacelike if or , timelike if , and null (lightlike) if and . In particular, the norm (length) of a nonnull vector is given , and two vectors and are said to be orthogonal, if . A lightlike vector is said to be positive (resp., negative) if and only if and a timelike vector is said to be positive (resp., negative) if and only if .

Next, recall that an arbitrary curve in can locally be spacelike, timelike, or null (lightlike), if all of its velocity vectors are, respectively, spacelike, timelike, or null (lightlike). If , the nonnull curve is said to be of unit speed (or parameterized by arc length function ).

We denote by the moving Frenet frame along the curve . Then , and are the tangent, the principal normal, and the binormal vector of the curve , respectively. Depending on the causal character of the curve , we have the following Frenet-Serret formulas.

If is a null space curve with a spacelike principal normal , then the following Frenet formulas hold

where

For a null curve, can take only two values: when is a null straight line or in all other cases.

If is a pseudonull curve, that is, is a spacelike curve with a null principal normal , then the following Frenet formulas hold:

where

For a pseudonull curve, can take only two values: when is a straight line or in all other cases.

If is a spacelike space curve with a spacelike principal normal , then the following Frenet formulas hold

where

If is a spacelike space curve with a timelike principal normal , then the following Frenet formulas hold

where

If is a timelike space curve, then the following Frenet formulas hold

where

see [5].

If the curve is nonunit speed, then

If the curve is unit speed, then

see [6].

In this study we are going to have the curve as nonline pseudonull curve or .

3. The Involute of the Pseudonull Curve

Definition 1. Let a pseudonull curve and a curve be given. For all , then the curve is called the involute of the curve , if the tangent vector of the curve at the point passes through the tangent at the point of the curve and

where and are Frenet frames of and , respectively.

This definition suffices to define this curve mate as

(see Figure 1).

Figure 1: Involute curves.

Theorem 2. Let be a pseudonull space curve with curvature functions and . Then, there is no involute of the curve in Minkowski -space .

Proof. Let be involute of in . We assume that is distinct from . Then we can write

where is a -function on . Differentiating (17) and by using Frenet formulas given in (4), we get

If we take the inner product with on both sides of (18), we have

Recalling definition of the involute curve couple, and, by using (5), we get

for all . Thus, since , we get ; that is, is a nonzero constant. In this case, we can write (17) as follows:

and from (18) we get

Here we notice that

and by using (5) we get

Thus, since , we get that is a null vector; that is, curve is a null curve.

Differentiating (24) with respect to and by using Frenet formulas given in (4), we have

and by using (5) we get

Since for all , we get

Thus, we obtain that is a null vector. This is a contradiction with null curve . Then, there is no involute of the curve in Minkowski -space .

4. The Evolute of the Pseudonull Curve

Definition 3. Let the pseudonull curve and a curve with the same interval be given. For all , then the curve is called the evolute of the curve , if the tangent vector of the curve at the point passes through the tangent at the point of the curve and

where and are Frenet frames and curvatures of and , respectively.

This definition suffices to define this curve mate as

Theorem 4. Let a space curve be the evolute of the pseudonull curve ; then for all

where and are Frenet frames and curvatures of and , respectively.

Proof. The tangent of the curve at the point is the line constructed by the vector . Then we can write

where and are a -function on . Differentiating (31) and by using Frenet formulas given in (4), we get

and, from ,

If we take the inner product with on both sides of (33), we have

Recalling definition of the evolute curve couple, , and by using (5), we get

for all . In this case, we can write (31) as follows:

and from (35) we get

Let and be Frenet frames and curvatures of and , respectively. If the space curve is the evolute of the pseudonull curve , then, from Theorem 4, we can write

where is a -function on .

Since this line passes through the point , the vector is perpendicular to the vector . From (36) and (37), the vector field is parallel to the vector field . Then, we have

4.1. Special Cases

If for all , then (39) is not defined. But, from (36) and (37), we get

Here we notice that

and by using (5) we get

Thus, since , we get that is a null vector; that is, the curve is a null curve. Differentiating (41) and by using Frenet formulas given in (4), we have

and from (5) we get

Since the space curve is a null curve, then from and we get

Case 1. If from (46), then we can write (41) and (44) as follows:

Differentiating (48) and by using Frenet formulas given in (2) and (4), we get

By using Frenet formulas given in (2), we get

Conclusion 1. Let a space curve be the evolute of the pseudo null curve in and let and are the curvatures of the curve . If the torsion of the curve is

for all , then and are helices and the curvatures of the curve are

for all .

Case 2. If from (46), then we can write (41) and (44) as follows:

Differentiating (54) and by using Frenet formulas given in (2) and (4), we get

By using Frenet formulas given in (2), we get

Conclusion 2. Let a space curve be the evolute of the pseudo null curve in and let and are the curvatures of the curve . If the torsion of the curve is

for all , then and are helices and the curvatures of the curve are

for all .

If , then from (39) we get

and if for all , then the curve is not defined.

4.2. Special Cases

If , then from (39) we get

From (4), (5), and (37), we get

and by using (60) we get

Case 3. If is a unit spacelike curve, then from (62) we have

Solving the differential equation (63), we get

where and are constant. Then, from (36) we can write

Case 4. If is a unit timelike curve, then from (62) we have

Solving the differential equation (65), we get

where and are constant. Then, from (36) we can write

Case 5. If is a null curve, then from (62) we have

Solving of the differential equation (65), we get

where is constant. This is a contraction with . Then, the curve is not defined.

5. Example

Let be a unit speed pseudonull curve in the Minkowski -space with parameter equation (see Figure 2)

Figure 2: The pseudonull curve in .

Then we have the Frenet frame vectors as follows:

where is spacelike vector and and are null vectors for all . Moreover the curvatures and of are as follows:

Let the space curve with the same interval be the evolute of the pseudonull curve . Then we can write

(1) From (40), the curve is given by (see Figure 3)

Figure 3: The evolute curve of the pseudonull curve in .

(2) From (65), the curve is given by (see Figure 4)

Figure 4: The evolute curve of the pseudonull curve in .

(3) From (68), the curve is given by (see Figure 5)

Figure 5: The evolute curve of the pseudonull curve in .

Acknowledgment

The authors would like to thank the referee for his/her helpful suggestions and comments which significantly improved the first version of the paper.

References

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