ISRN Geometry

Volume 2012 (2012), Article ID 126358, 8 pages

http://dx.doi.org/10.5402/2012/126358

## A New Approach to Constant Slope Surfaces with Quaternions

^{1}Department of Mathematics, Faculty of Arts and Sciences, Bozok University, 66100 Yozgat, Turkey^{2}Department of Mathematics, Faculty of Science, Ankara University, 06100 Ankara, Turkey

Received 5 November 2011; Accepted 8 December 2011

Academic Editor: M. Przanowski

Copyright © 2012 Murat Babaarslan and Yusuf Yayli. 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 show a new method to construct constant slope surfaces with quaternions. Moreover, we give some results and illustrate an interesting shape of constant slope surfaces by using Mathematica.

#### 1. Introduction

Quaternions were invented by Sir William Rowan Hamilton as an extension to the complex number in 1843. Hamilton’s defining relation is most succinctly written as

Computing rotations is a common problem in both computer graphics and character animation. Shoemake [1] introduced an algorithm using quaternions, spherical linear interpolation (SLERP), and Bezier curves to solve this. Quaternions are used as a powerful tool for describing rotations about an arbitrary axis. Many physical laws in classical, relativistic, and quantum mechanics can be written nicely using them. They are also used in aerospace applications, flight simulators, computer graphics, navigation systems, visualizations, fractals, and virtual reality.

Kinematic describes the motion of a point or a point system depending on time. If a point moves with respect to one parameter, then it traces its 1-dimensional path, orbit curve. If a line segment or a rectangle moves with respect to one parameter, then they sweep their two- and three-dimensional paths, respectively [2]. So we are thinking of a curve as the path traced out by a particle moving in Euclidean 3-space. The position vector of the curve is very important to determine behaviour of the curve.

The Serret-Frenet formulae for a quaternionic curve in and were given by Bharathi and Nagaraj in [3].

In the last few years, the study of the geometry of surfaces in 3-dimensional spaces, in particular of product type , was developed by a large number of mathematicians. Recently, constant angle surfaces were studied in product spaces in [4] and in [5, 6], where and represent the unit 2-sphere and hyperbolic plane, respectively. The angle is considered between the unit normal of the surface and the tangent direction to .

Boyadzhiev [7] explored three-dimensional versions of these two properties: surfaces that are equiangular and those that are self-similar. He investigated the relationships among these surfaces and gave some examples. Thereafter, Munteanu [8] defined constant slope surfaces. Such surfaces are those whose position vectors make a constant angle with the normals at each point on the surface. Munteanu showed that they can be constructed by using an arbitrary curve on the sphere or an equiangular spiral.

There is also a kinematic generation of these surfaces as follows. Take a logarithmic spiral and roll its plane along a general cone such that the eye of the spiral sits in the vertex of the cone. Then the spiral sweeps out a surface with the required property.

More recently, we [9] gave some characterizations of constant slope surfaces and Bertrand curves in Euclidean 3-space. We found parametrization of constant slope surfaces for the tangent indicatrix, principal normal indicatrix, binormal indicatrix, and the Darboux indicatrix of a space curve. Furthermore we investigated Bertrand curves corresponding to parameter curves of constant slope surfaces.

By the definition of surfaces of revolution, we can see that such surfaces can be obtained by rotation matrices. Similarly, in this study, we show that constant slope surfaces can be obtained by quaternion product and the matrix representations . Subsequently, we give some results and an example of constant slope surfaces.

#### 2. Preliminaries

The algebra of quaternions is defined as the four-dimensional vector space over having a basis with the following properties: It is clear that is an associative and not commutative algebra and is identity element of .

We use the following four-tuple notation to represent a quaternion: where is the scalar component of and form the vector part and entire set of ’s is spanned by the basis quaternions:

We also write . The conjugate of is then defined as . We call a quaternion as pure if its scalar part vanishes. Summation of two quaternions and is defined as . Multiplication of a quaternion with a scalar is defined as . Quaternion product is defined in the most general form for two quaternions and as where and denote the familiar dot and cross-products, respectively, between the three-dimensional vectors and . Quaternionic multiplication satisfies the following properties: for any two quaternions and we have and the formula for the dot product . In particular, if , we obtain . If , then the quaternion is unitary. The inverse of a quaternion is given by and it satisfies the relation [10]. If is a unitary quaternion, we may write in the trigonometric form as , where .

The most important property of quaternions is that they can characterize rotations in a three-dimensional space. The conventional way of representing three-dimensional rotations is by using a set of Euler angles , which denote rotations about independent coordinate axes. Any general rotation can be obtained as the result of a sequence of rotations, as given by

Let be the set obtained by rotating a regular plane curve about an axis in the plane which does not meet the curve; we shall take plane as the plane of the curve and the axis as the rotation axis. Let , be a parametrization for and denote by the rotation angle about the axis. Thus, we obtain a map from the open set into . We can see that satisfies the conditions for a parametrization in the definition of a regular surface. Thus is a regular surface which is called a surface of revolution.

A one-parameter homothetic motion of a rigid body in Euclidean 3-space is given analytically by in which and are the position vectors, represented by column matrices, of a point in the fixed space and the moving space , respectively; is an orthogonal -matrix, is a translation vector, and is the homothetic scale of the motion. Also , and are continuously differentiable functions of a real parameter [2].

The map acting on a pure quaternion is linear. Without loss of generality we choose and if then so that the matrix representation of the map is It is not difficult to check that is orthogonal: and so that the linear map represents a rotation in [11].

Now we give the characterization of constant slope surfaces as the following theorem.

Theorem 2.1. *Let be an isometric immersion of a surface in the Euclidean 3-space. Then is a constant slope surface if and only if either it is an open part of the Euclidean 2-sphere centered in the origin, or it can be parametrized by
**
where is a constant (angle) different from , and is a unit speed curve on the Euclidean sphere [8].*

#### 3. New Approach

A quaternion function defines a 2-dimensional surface in , where and . Thus, for the unitary quaternion , the matrix representation of the map is given by

We are now ready to show the main result of this study.

Theorem 3.1. *Let be an isometric immersion of a surface in the Euclidean 3-space. Then the constant slope surface can be reparametrized by , where “×” is the quaternion product, is a surface and a pure quaternion.*

*Proof. *Since and , we obtain
By using (2.4), we get
If we substitute this into the last equation, we have
Hence applying Theorem 2.1, we find that the constant slope surface is given by
This completes the proof.

As a consequence of this theorem, we get the following important corollary.

Corollary 3.2. *Let be the matrix representation of the map for the unitary quaternion . Then, for the pure quaternion , we get the constant slope surface as
**
We know that a surface of revolution can be obtained by a rotation matrix. Similarly, we view that the constant slope surface can be obtained by the matrix representation , too.*

Finally we state the following result.

Corollary 3.3. *For the homothetic motion , the constant slope surface can be written as . Therefore we have
*

Now, we give some remarks regarding our Theorem 3.1 and Corollary 3.3.

*Remark 3.4. *Theorem 3.1 says that both the points and the position vectors on the surface are rotated by through the angle about the axis .

*Remark 3.5. *Corollary 3.3 shows that the position vector of the curve is rotated by through the angle about the axis and extended through the homothetic scale .

#### 4. Example

We give an example of constant slope surfaces and draw its picture by using Mathematica.

*Example 4.1. *We consider the unit speed spherical curve
If the angle is taken then we have
Thus, by using (3.1) and (3.6), we get the following constant slope surface:
Hence, we can give Mathematica code of this constant slope surface as
and the picture of is drawn as follows (Figure 1).

#### 5. Conclusion

By the definition of surfaces of revolution, we can see that such surfaces can be obtained by rotation matrices for the position vectors of given regular plane curves. Similarly, in this study, we show that constant slope surfaces can be obtained by quaternion product and the matrix representations . Afterwards, we give some results and illustrate an example of constant slope surfaces by using quaternions and draw its picture by using Mathematica computer program.

#### Acknowledgment

The authors wish to express their gratitude to Marian Ioan Munteanu for suggesting them the subject and for helpful hints given during the preparation of this work.

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