Research Article | Open Access

# Singularities of Indefinite Hypersurfaces and Indefinite Surfaces along Timelike Curves in Nullcone with Index Two

**Academic Editor:**Gestur Ã“lafsson

#### Abstract

In this paper, a class of indefinite hypersurfaces and a class of indefinite surfaces generated by timelike curves located in nullcone in 4-dimensional semi-Euclidean space with index 2 are discussed. Using the unfolding theory in singularity theory, the singularities of the indefinite hypersurfaces and the indefinite surfaces are classified and the different kinds of singularities are estimated by means of a geometric invariant . Meanwhile, the definition of osculating nullcone is presented; the study shows that the differential geometric invariant of timelike curves measured also the order of the contact between a timelike curve and a osculating nullcone . Finally, some relevant counterexamples are indicated.

#### 1. Introduction

Semi-Euclidean 4-space with index 2 has significant physical background, for example, antiâ€“de Sitter spacetime , as a subspace of semi-Euclidean 4-space with index 2, is a solution of Einsteinâ€™s field equations for an empty universe with a negative cosmological constant. Very recently, some new results concerning the non-Euclidean geometry were established by physicists and geometers [1â€“11]; for example, S. Izumiya et al. investigated the regular curve in the embedded unit spheres in the de Sitter space or the light cone via the theory of Legendrian duality [6]. L. Kong and D. Pei [8] investigated the singularities of lightlike surfaces and focal surfaces of spacelike curves in hyperbolic spacetime sphere, and some geometry properties of the spacelike curves are obtained. The second author and his collaborators presented abundant research results concerning the singularities of submanifolds in semi-Euclidean space [12â€“15]. In [13], they investigated the null developables of timelike curves that lie on nullcone in 3-dimensional semi-Euclidean space with index 2 and classified the singularities of the null developables of timelike curves. It is shown that the null developable is locally diffeomorphic to the cuspidal edge or swallowtail which are two classes of topologically stable singularities. The study also shows that the differential geometric invariants of timelike curves measured the order of the contact between a timelike curve and a conic . As the generalization and the further exploration on previous study [13], increasing the dimension of semi-Euclidean space from three to four, in the current study we devote ourselves to explore the singularities of a class of indefinite hypersurfaces and a class of indefinite surfaces which are generated by timelike curves in nullcone in 4-dimensional semi-Euclidean space with index 2. In comparison with [13], the current study primarily discusses indefinite hypersurfaces and indefinite surfaces instead of null surfaces and the indefinite hypersurfaces and the indefinite surfaces show more kinds of singularities under certain equivalent conditions, for example, , , . Moreover, the new definition* osculating nullcone* is presented and the locus of center points of osculating nullcone is exactly the critical value set of the indefinite surface discussed in this paper. This will undoubtedly give the indefinite surface more geometric meanings.

Methods of the unfolding theory in singularity theory which are quite useful for the study of the geometry of hypersurfaces and surfaces are adopted in this paper. To allow a useful study of the singularities of the indefinite hypersurfaces and the indefinite surfaces located in 3-dimensional nullcone, we consider distance squared function denoted by on a nullcone timelike curve . These functions are the unfolding of these singularities in the local neighborhood of , and these functions only depend on the germs that they are unfolding. In this paper, we create these functions by varying a fixed point in the distance squared function to obtain a family of functions. We show that these singularities are -versally unfolded by the family of the distance squared functions. Noticing that the indefinite hypersurface is exactly the discriminant set of the order one of the four-parameter unfolding, the indefinite surface is the discriminant set of the order two of the four-parameter unfolding, while the locus of the center points of the osculating nullcone for timelike curve is the discriminant set of the order three of the four-parameter unfolding. If the singularity of is -type and if the corresponding four-parameter unfolding is -versal, then, applying Bruceâ€™s theory to it (see [16, 17]), we know that the discriminant set (i.e., the indefinite hypersurface) of order one of the four-parameter unfolding is locally diffeomorphic to , , or under certain conditions, the discriminant set (i.e., the indefinite surface) of order two of the four-parameter unfolding is locally diffeomorphic to , , or under certain conditions, and the discriminant set (i.e., the curve) of order three of the four-parameter unfolding is locally diffeomorphic to a line or -cusp under certain conditions. Thus, we have completed the classification of the singularities of the indefinite hypersurface and the indefinite surface .

The paper is organized as follows. We begin in Section 2 by establishing the theoretical frame of the explicit differential geometry on curves in , defining a class of indefinite hypersurfaces and a class of indefinite surfaces generated by timelike curves in nullcone in , calculating the singular sets of the indefinite hypersurfaces and the indefinite surfaces, and presenting the main results in this paper. In Section 3 we present a distance squared function of a timelike curve that lies on nullcone and establish the equivalent relations between -singularity of the distance squared function and geometric invariant which can characterize the types of singularities of the indefinite hypersurfaces and the indefinite surfaces via differential calculations. In Section 4, we give the proof of the main theorem and complete the classifications of singularities of the indefinite hypersurfaces and the indefinite surfaces by using some general results from the singularity theory for families of function germs. In this Section 5, using transversality theorem as the vehicle, we consider generic properties of timelike curves which lie on . To better illustrate our results, we give an example of the indefinite hypersurface and the indefinite surface and draw their graphs in 3-dimensional projection space.

#### 2. Preliminaries

Let denote the 4-dimensional semi-Euclidean space with index 2, that is to say, the manifold with a flat metric of signature (-,-,+,+), for any vectors and in ,We also define the* pseudovector product* of , , and as follows:where is the canonical basis of We state that a vector is* spacelike, null,* or* timelike* if is positive, zero, or negative, respectively. The* norm* of a vector is defined as , and we call a* unit vector* if For a vector and a real number , we define the hyperplane with pseudonormal vector by We call a Lorentz hyperplane, a semi-Euclidean hyperplane with index 2, or a null hyperplane if is timelike, spacelike, or null, respectively.

Let be a smooth curve in (i.e., for any ), where is an open interval. For any , the curve is called* spacelike curve*,* null curve*, or* timelike curve* if all its velocities are , , or , respectively. We call the* nonnull curve* if is a timelike curve or a spacelike curve.

The arc-length of a nonnull curve , measured from , is . Then the parameter s is determined such that for the nonnull curve, where . Therefore, we say that a nonnull curve is parameterized by arc-length; throughout the remainder in this paper, we denote the parameter of as the arc-length parameter.

Letâ€™s define a three-dimensional submanifold in semi-Euclidean 4-spacewhere ; we call a nullcone at the vertex .

There exist spacelike curves, timelike curves, and null curves in ; among them, the null curves had been investigated by the second author [15]; in this paper, we will restrict our attention to the case of timelike curve in ; the case of space curve would be discussed in future work. Let be a timelike regular curve; we consider a basis for all , such that . We choose the , and then there exists a null transversal vector such that . Moreover, defining , we have a Frenet type frame of along satisfyingandIt is easy to see that and have the same positive orientation. In order to study the timelike curve that lies on nullcone, we have the following fundamental formulae to use:and we call (6) the Frenet type equations, whereand and are called* cone curvature function* of the curve . The Frenet type frame is also called the* asymptotic orthonormal frame* on along the curve in .

Before stating the main theorem we define several geometric objects involved in this paper. Let be a unit speed timelike curve with ; firstly, we define an indefinite hypersurface in along in :such thatwith . Next, we also define an indefinite surface given bywith . Last but not least, letâ€™s define a curve given byBy applying the formulae expressed in (7), we can calculateAt each of the regular points of ; that is, the formulaholds at ; the tangent hyperplane , , can be Lorentz, semi-Euclidean, or null. Similar calculation shows that the tangent plane of can also be Lorentz, semi-Euclidean, or null. This is the cause why we call an indefinite hypersurface and an indefinite surface.

It is obvious that if , then the four vectors are linearly dependent; this situation is trivial. We shall assume throughout the whole paper that all the maps and manifolds are and unless the contrary is explicitly stated. In this paper, one of our main tasks is to study the singularities of , ; we state the following proposition.

Proposition 1. *Let be a regular timelike curve in the nullcone with . One can state the following:**(i) is a singularity of if and only if .**(ii) is a singularity of if and only if and .*

*Proof. *. It has been calculated in (13) and (14) thatThe three vectors above are linearly dependent if and only if that is, if and only if ; thus, assertion is true.

We calculateThe two vectors above are linearly dependent if and only if , that is, if and only if and . Thus, assertion is proven.

We remark the relationships between , , and as follows.

*Remark 2. *By Proposition 1, the images of are the critical value sets of , and the images of are the critical value sets of .

*Definition 3. *Let be a submersion and be a timelike curve with . One says that and have -point contact for if the function satisfies , One also says that and have at least -point contact for if the function satisfies .

In order to characterize the singularities of the indefinite hypersurfaces, the indefinite surfaces, and the curves, we introduce a nullcone invariant defined to beThe main result in the paper is as follows.

Theorem 4. *Let be a unit speed timelike curve that lies on nullcone with . For and the nullcone , one has the following:**(1)â€‰â€‰ and have at least 2-point contact for .**(2) and have 3-point contact for if and only if**Under the condition, the hypersurface at is locally diffeomorphic to , and at is locally diffeomorphic to .**(3) and have 4-point contact for if and only if andUnder the condition, the hypersurface at is locally diffeomorphic to , at is locally diffeomorphic to , and at is locally diffeomorphic to a line.**(4) and have 5-point contact for if and only if , , andUnder the condition, the hypersurface at is locally diffeomorphic to , at is locally diffeomorphic to , and at is locally diffeomorphic to -cusp.*

#### 3. Unfoldings of Functions of One-Variable and -Singularity

In this section, let us first recall the following definition (see [16] and references therein): Let be the germ of a function. One calls an* r-parameter unfolding* of , where . One says that has -singularity at if for all and if ; One also says that has -singularity at if for all .

Furthermore, we define a family of functions on a timelike curves which is useful to the study of geometric invariants of timelike curve. We also study the geometric properties of the indefinite hypersurfaces and the indefinite surfaces of a timelike curve in nullcone. We define the distance squared function of a timelike curve that lies on nullcone bywhere is independent of ; we denote that for any fixed vector in . Using (7) and making a simple calculation, we can state the following facts.

Proposition 5. *Suppose that one has a unit speed timelike curve that lies on nullcone with ; then**(i) if and only if there exist some such that and**(ii) if and only if there exist and such that**(iii) if and only if and such that**(iv) if and only if**(v) if and only if and**(vi) if and only if and*

*Proof. * Because , from , there exist with such that Thus, assertion holds.

Under this condition , if , then we have such that and ; thus, assertion holds.

When , we compute thatimplies that ; hence, and , and, in combination with , it is easy to deduce that if and only if and such that ; that is, assertion holds.

From , we calculate thatIf , then and ; hence, Thus, assertion holds.

Suppose that holds; we notice thatHence, if and only if andThus, assertion holds.

Assume that holds; we calculate thatThus, assertion holds.

Corollary 6. *Suppose that we have a unit speed timelike curve that lies on nullcone with ; then**(i) has -singularity at if and only if there exist , , and such that**(ii) has -singularity at if and only if , , and such that**(iii) has -singularity at if and only if and**(iv) has -singularity at if and only if , , and*

#### 4. Singularities of Indefinite Hypersurfaces, Indefinite Surfaces, and Curves

In this section, we use some general results from the singularity theory for families of function germs ([16, 17]). Let be an unfolding of , where has at We denote the -jet of the partial derivative at by , for Then, is called a *-versal unfolding *if the matrix of coefficients has rank . Under the same conditions as above, is called a *-versal unfolding *if the matrix of coefficients has rank , where .

We now introduce the following important sets concerning the unfolding:which is called a* discriminant set of order *. Of course, , and is the set of singular points of . In order to understand the geometric properties of the discriminant set of order , we introduce an equivalence relationship among the unfoldings of functions. Let and be the -parameter unfoldings of and , respectively. We state that and are --equivalent if there exists a diffeomorphism germ of the form such that . Straightforward calculations yield the following proposition.

Proposition 7. *Let and be the -parameter unfoldings of and , respectively. If and are --equivalent by a diffeomorphism germ of the form , then are set germs.*

Theorem 8. *Let be an -parameter unfolding of with -singularity at . Suppose is a versal unfolding of . Then, is -equivalent to one of the following unfoldings:**(a) .**(b) .**(c) .**(d) .*

We review the following result [16].

Theorem 9. *Let be an -parameter unfolding of , which has singularity at . Suppose that is a versal unfolding.**(i) If , then is locally diffeomorphic to and .**(ii) If , then is locally diffeomorphic to , is locally diffeomorphic to , .**(iii) If , then is locally diffeomorphic to , is locally diffeomorphic to , is locally diffeomorphic to , and .**(iv) If , then is locally diffeomorphic to , is locally diffeomorphic to , is locally diffeomorphic to , is locally diffeomorphic to , and .*

We remark that all of diffeomorphisms in the above assertions are diffeomorphism germs. We, respectively, call a *-cusp* (see Figure 1),