Research article Topical Sections

Evolution of null Cartan and pseudo null curves via the Bishop frame in Minkowski space R2,1

  • In the present work, we focused on studying the evolution of null Cartan and pseudo null curves using the Bishop frame in Minkowski space R2,1. We obtained the necessary and sufficient conditions for the null Cartan and pseudo null curves to be inextensible curves (the arc length is preserved). In addition, we derived the time evolution equations of the Bishop frame (TEEsBF) for these curves. Moreover, we obtained the time evolution equations of Bishop curvatures (TEEsBCs) as partial differential equations in terms of Bishop velocities. Finally, we presented some applications.

    Citation: Samah Gaber, Abeer Al Elaiw. Evolution of null Cartan and pseudo null curves via the Bishop frame in Minkowski space R2,1[J]. AIMS Mathematics, 2025, 10(2): 3691-3709. doi: 10.3934/math.2025171

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  • In the present work, we focused on studying the evolution of null Cartan and pseudo null curves using the Bishop frame in Minkowski space R2,1. We obtained the necessary and sufficient conditions for the null Cartan and pseudo null curves to be inextensible curves (the arc length is preserved). In addition, we derived the time evolution equations of the Bishop frame (TEEsBF) for these curves. Moreover, we obtained the time evolution equations of Bishop curvatures (TEEsBCs) as partial differential equations in terms of Bishop velocities. Finally, we presented some applications.



    The abbreviations that are used in this manuscript are illustrated by:

    BC(s) Bishop curvature(s)
    IF Inextensible flows
    NCC(s) Null Cartan curves
    PDE(s) Partial differential equation(s)
    PNC(s) pseudo null curves
    TEEs Time evolution equations
    TEEsBF Time evolution equation(s) for Bishop frame
    TEEsBC(s) Time evolution equation(s) for Bishop curvature(s)

    The flow of curves and surfaces in Euclidean and Minkowski spaces is a crucial issue in differential geometry. The flow of curves determines their time evolution. The evolution of curves is therefore referred to as a flow in this article. The flow of curves and surfaces has numerous applications in physics, computer graphics, and engineering. In physics, there is no strain energy generated by inextensible flows (IF) of curves. Inextensible curves and surface flows can describe the swinging motion of a cord of fixed length or the motion of a piece of paper carried by the wind. There are many physical applications in which such motions occur naturally. Chirikjian and Burdick [1] and Mochiyama et al. [2] investigated the shape control of hyper-redundant and snake-like robots.

    Computer graphics and animation depend extensively on the flow of curves [3,4,5,6], where it is used to simulate the movement of flexible objects like clothing and hair, enhancing the realism of animated characters and scenes.

    The IF of curves and surfaces have various applications in different fields of engineering like structural engineering [7], where the inextensible curves and surfaces are utilized in the design and analysis of structures like cable-supported bridges, suspension bridges, and tension systems. They help ensure the stability and load-bearing capacity of these structures, allowing engineers to create efficient and safe designs.

    To emphasize the novelty of this study, we analyze previous works on the evolution of curves in Euclidean and Minkowski spaces according to various frames, such as the Frenet frame, the Darboux frame, the modified orthonormal frame, the Bishop frame, and the quasi-frame.

    S. Gaber [8,9,10] investigated the IF of curves in 3-dimensional Euclidean space R3, spherical space S3, and three-dimensional de-Sitter space S2,1. Additionally, the TEEs of the Frenet frame and for curvatures have been constructed as a set of partial differential equations (PDEs). In [11], S. Gaber derived the TEEs of curves in R3 by employing the type1 Bishop frame. Gaber and Sorour [12] studied the IF of time-like curves according to a quasi-frame in Minkowski space R2,1. Gaber and Al Elaiw [13] discussed the flow of a null Cartan curve (NCC) characterized by velocity and acceleration functions. The binormal velocity impacts the tangential and normal velocities along the motion. Moreover, the TEEs for the Cartan frame of the null curve were constructed. The flows of an initial NCC were also utilized to create a family of inextensible NCCs.

    In [14], N. Gurbuz provided several innovative transformations and established the relationships between the motion of non-null curves and soliton equations by using the Bishop frame in Minkowski three-space. Furthermore, the differential formulae of these transformations related to the nonlinear system of the heat and repulsive-type nonlinear Schrodinger equation were derived.

    In [15], A. Ucum studied the IF of curves such as pseudo null and partially null curves in R42. Eren et al. [16] employed the modified orthogonal frame in investigating the motions of space curves and some special-ruled surfaces. Additionally, evolution equations for space curves with a modified orthogonal frame were obtained. In [17], D. Yang et al. studied a curve flow of fourth order for a smooth closed curve in R2, which arises as a nonlinear parabolic PDE. By using the evolution equations, and assuming some specific conditions, they proved that for any smooth closed initial curve in R2, the flow has a smooth solution for all time. In [18], K. Eren investigated the motion of a moving space curve using a modified orthogonal frame and derived the TEEs for curvatures.

    Ergut et al. [19] investigated the flows of inextensible spacelike curves related to the Sabban frame on S21 and derived the flows of spacelike curves in terms of PDEs. Korpinar et al. [20] constructed a novel approach for the flows of inextensible curves in three-dimensional Euclidean space by employing the Frenet frame of the given curve and provided certain characterizations for curvatures of the curve.

    Bektas et al. [21] determined the necessary and sufficient inextensibility conditions for the space-like curves in the three-dimensional lightlike cone in E41. Arroyo et al. [22] investigated the binormal flow of curves with curvatures depending on velocity and sweeping out immersed surfaces. Filaments evolving with constant torsion were constructed by using the Gauss-Codazzi equations.

    Yuzbasi et al. [23] investigated the flow of an inextensible curve on a lightlike surface in Minkowski three-space. A necessary and sufficient inextensibility condition for the curve flow was obtained. It arose as a PDE involving the curvatures. Additionally, the lightlike ruled surfaces were characterized in three-dimensional Minkowski space, and the evolution of an inextensible lightlike curve on a lightlike tangent developable surface was described.

    This article presents a novel approach for investigating the evolution of NCCs and pseudo null curves (PNCs) in Minkowski space R2,1. We derive the TEEs of NCCs and PNCs by using the Bishop frame. We obtain the necessary and sufficient conditions for NCCs and PNCs to be inextensible. We get the TEEsBCs for NCCs and PNCs as PDEs depending on Bishop velocities.

    This research article is organized as follows: In Section 2, we study the geometry of curves in Minkowski space. We introduce the NCCs in R2,1 and the relationship between the Cartan frame and Bishop frame for NCCs. In addition, we discuss the PNCs in R2,1 and the relationship between the Frenet frame and Bishop frame for PNCs. In Section 3, we study the evolution of NCCs using the Bishop frame in R2,1. In Section 4, we present an application for the evolution of NCCs using the Bishop frame. In Section 5, we discuss the evolution of the PNCs employing the Bishop frame in R2,1. In Section 6, we provide two applications for the evolution of PNCs by using the Bishop frame. Finally, we present our conclusions.

    Definition 1. [24] The Minkowski space R2,1 is defined as the real vector space R3 with the Lorentzian inner product given by: dx20+dx21+dx22 with {X=(x0,x1,x2)x0,x1,x2R}.

    Consider X=(x0,x1,x2),Y=(y0,y1,y2)R2,1 as two vectors in Minkowski space R2,1, and the following properties are defined in R2,1:

    The Lorentzian inner product: X,Y=x0y0+x1y1+x2y2.

    The vector product: X×Y=(x2y1x1y2,x2y0x0y2,x0y1x1y0).

    The non-zero vector UR2,1 is spacelike, timelike, and null (lightlike) if U,U>0, U,U<0, and U,U=0, respectively.

    Definition 2. [25,26] Consider a regular parameterized curve α:IR2,1 in Minkowski space R2,1. Let uI be the parameter of the curve, and define α=dαdu to be the tangent vector of the curve α:

    If α,α>0, then α is a spacelike curve for all uI.

    If α,α<0, then α is a timelike curve for all uI.

    If α,α=0, then α is a lightlike curve for all uI.

    Definition 3. [27] Consider a regular curve α:IRR2,1 defined on IR. The curve α is a null curve, if the tangent vector α(u) for all uI is a future-directed null vector. A curve is called a null Cartan curve (NCC), if it is parameterized by the pseudo-arc function s that is defined by:

    s(u)=u0α(u)du=u0(α(u),α(u))14du,()=ddu (2.1)

    Definition 4. [28] Consider the NCC α:IRR2,1 parameterized by the pseudo-arc s. Assume that Fc={T,N,B} is the Cartan frame at a point q along the curve α, where α(s) for all sI is a tangent null vector, N is a principal normal spacelike vector, and B is a principal binormal null vector. The Cartan frame equations along the NCC satisfy the following:

    (αTNB)=(010000100τ0100τ0)(αTNB),()=dds, (2.2)

    where τ=τ(s) is the torsion of the NCC. The Cartan frame Fc satisfies the following properties:

    T,T=0,N,N=1,B,B=0,T,B=1,N,B=N,T=0. (2.3)
    T×N=T,N×B=B,B×T=N. (2.4)

    Definition 5. [29] Let FB={T1,N1,N2} be a positively oriented Bishop frame of the NCC. The frame FB is a pseudo-orthonormal frame containing the vector fields T1,N1,N2, where T1 is the tangential vector, N1 is the relatively parallel spacelike normal vector, and N2 is a relatively parallel lightlike transversal vector. The vector fields N1 and N2 are defined to be relatively parallel; if the normal component T1=span{T1,N1} of their derivatives N1 and N2 vanish. So the vector fields N1 and N2 are collinear with N2.

    Lemma 1. [29] Let α=α(s) be a NCC in R2,1 with a pseudo-arc parameter s, and let τ=τ(s) be the torsion of the curve. The relationship between the Bishop frame FB and the Cartan frame Fc is given by:

    (T1N1N2)=(k100k2k10k222k21).(TNB), (2.5)

    where the functions k1(s)=1 and k2(s) are defined in this paper as Bishop curvatures (BCs).

    Lemma 2. [29] The Bishop frame FB of the NCC satisfies the following equations:

    dds(T1N1N2)=(k2k1000k100k2).(T1N1N2), (2.6)

    where the first BC k1(s)=1, and then

    (T1N1N2)=(k21000100k2).(T1N1N2),()=dds, (2.7)

    where the second BC k2 satisfies the Riccati differential equation: k2(s)=12k22(s)τ(s).

    Definition 6. [27] The Bishop frame FB of the NCC has the following characteristics:

    T1,T1=0,N1,N1=1,N2,N2=0,T1,N1=0,T1,N2=1,N1,N2=0. (2.8)
    T1×N1=T1,T1×N2=N1,N1×N2=N2. (2.9)

    Definition 7. [27] Consider the pseudo null curve (PNC) α:IRR2,1. Assume that at a point q on the PNC, the Frenet frame is defined by Ff={T,N,B}, where the tangent vector T is a spacelike vector, N is the principal normal vector, and B is the principal binormal vector. The vectors N and B are null vectors with the condition N,B=1. The Frenet frame equations along the PNC satisfy the following:

    dds(TNB)=(0k00τ0k0τ)(TNB), (2.10)

    where the functions k(s)=1 and τ=τ(s) are the curvature and torsion of the PNC.

    The Frenet frame for the PNC satisfies the following properties:

    T,T=1,N,N=0,B,B=0,T,B=0,N,B=1,N,T=0. (2.11)
    T×N=N,N×B=T,B×T=B. (2.12)

    Definition 8. [27] Assume that FB={T1,N1,N2} is the positively oriented pseudo-orthonormal Bishop frame of the PNC, where T1 represents the tangential vector, and the vectors N1 and N2 are relatively parallel lightlike normal vector fields. The vectors N1 and N2 are defined to be relatively parallel if the normal component T1=span{N1,N2} of their derivatives N1 and N2 is zero. So the vector fields N1 and N2 are collinear with T1.

    Definition 9. The FB of the PNC satisfies the following properties:

    T1,T1=1,N1,N1=0,N2,N2=0,T1,N2=0,N1,N2=1,N1,T1=0. (2.13)
    T1×N1=N1,N1×N2=T1,N2×T1=N2. (2.14)

    Theorem 3. [30] Let α be a PNC in R2,1 parameterized by the arc-length parameter s with the curvature k(s)=1 and the torsion τ(s). The relationship between the Bishop frame {T1,N1,N2} and the Frenet frame {T,N,B} is given by:

    (i) Case (1):

    [T1N1N2]=[10001k2000k2][TNB]. (2.15)

    In this case, the equations of the FB for the PNC are given by:

    dds[T1N1N2]=[0k2k1k100k200][T1N1N2], (2.16)

    where the functions k1(s)=0 and k2(s)=c0eτ(s)ds,c0R+0, are the first and second BCs.

    (ii) Case (2):

    [T1N1N2]=[10000k101k10][TNB] (2.17)

    In this case, the equations of the FB for the PNC are given by:

    dds[T1N1N2]=[0k2k1k100k200][T1N1N2] (2.18)

    where the the first and second BCs are k1(s)=c0eτ(s)ds, c0R0, and k2(s)=0.

    Let α:I=[0,l]×[0,t]R2,1 be the family of NCCs according to Bishop frame, where l and t are the parameters of the initial curve and time, respectively. Let u be the parameter variable of the NCC, where 0ul. Define the pseudo-arc-length parameter of the NCC by:

    s(u,t)=u0αuu(u,t)du=u0σdu,σ(u,t)=(αuu(u),αuu(u))14,σ=su. (3.1)

    The time evolution of the NCC with the Bishop frame is characterized by the velocities w1,w2, and w3 as follows:

    αt=w1T1+w2N1+w3N2. (3.2)

    In this paper, we call w1,w2, and w3 the Bishop velocities.

    Theorem 4. The NCC is inextensible with the Bishop frame FB if the Bishop velocities satisfy the following conditions:

    w2=w3,s+k2w3,()s=s,w1=12(w2,s+k2w2,sds). (3.3)

    Proof. From (3.1), we have

    σ4=αuu,αuu. (3.4)

    If the NCC with a Bishop frame is inextensible, then st=σt=0, by taking the t-derivative of (3.4), and then:

    0=αuut,αuu. (3.5)

    Since αu=σαs=σT1, by using (2.7), then we get:

    αuu=σ(σs+σk2)T1+σ2N1. (3.6)

    Taking the u-derivative of (3.2), then we obtain:

    αtu=σ(w1,s+k2w1)T1+σ(w2,s+w1)N1+σ(w3,s+w2k2w3)N2.

    For simplicity, we put:

    Ω1=w1,s+k2w1,Ω2=w2,s+w1,Ω3=w3,s+w2k2w3. (3.7)

    Then, we have

    αtu=σ(Ω1T1+Ω2N1+Ω3N2). (3.8)

    Taking the u-derivative of (3.8), we get:

    αtuu=σ(σsΩ1+σΩ1,s+σk2Ω1)T1+σ(σsΩ2+σΩ2,s+σΩ1)N1+σ(σsΩ3+σΩ3,s+σΩ2σk2Ω3)N2. (3.9)

    Since αut=αtu, then αuut=αtuu. Substitute from (3.6) and (3.9) into (3.5), and using the properties of the Bishop frame for NCC that are defined by (2.8), then

    (σs+σk2)(σsΩ3+σΩ3,s+σΩ2σk2Ω3)=σ(σsΩ2+σΩ1+σΩ2,s). (3.10)

    Equate the coefficients of σ2s, σσs, and σ2 on both sides of equation (3.10), and then we respectively obtain:

    Ω3=0,Ω3,s=0,k2Ω3,s+k2Ω2k22Ω3=Ω2,s+Ω1.

    Explicitly, by using (3.7), we get:

    w2=(w3,s+k2w3),w1=12(w2,s+s0k2w2,sds).

    Hence, the theorem holds.

    Theorem 5. Consider the motion of an inextensible NCC with a Bishop frame, and then the TEEsBFs are:

    (T1N1N2)t=(w1,s+k2w1w2,s+w1000w2,s+w100(w1,s+k2w1))(T1N1N2), (3.11)

    and the TEEsBC for k2 is given by:

    k2,t=(w1,s+k2w1),s. (3.12)

    Proof. Since

    αts=(w1T1+w2N1+w3N2)s,

    then

    αts=(w1,s+k2w1)T1+(w2,s+w1)N1+(w3,s+w2k2w3)N2. (3.13)

    Since the curve is inextensible, by using the first condition of (3.3) and substituting into (3.13), we have

    αts=(w1,s+k2w1)T1+(w2,s+w1)N1. (3.14)

    Since αs=T1, αst=T1,t, and the curve is inextensible, so the commutative law st()=ts() is satisfied. So αts=αst, hence

    T1,t=(w1,s+k2w1)T1+(w2,s+w1)N1. (3.15)

    Taking the s-derivative for (3.15), we have

    T1,ts=((w1,s+k2w1)s+k2(w1,s+k2w1))T1+((w2,s+w1)s+(w1,s+k2w1))N1+(w2,s+w1)N2. (3.16)

    Taking the s-derivative of the second condition of (3.3), and substituting in (3.16), we have

    T1,ts=((w1,s+k2w1)s+k2(w1,s+k2w1))T1+k2(w2,s+w1)N1+(w2,s+w1)N2. (3.17)

    Since T1,s=k2T1+N1, then by taking the t-derivative, we have

    T1,st=(k2,t+k2(w1,s+k2w1))T1+k2(w2,s+w1)N1+N1,t. (3.18)

    Since Tts=Tst, then we obtain the TEEsBC for k2 and the time evolution equation of the normal vector N1,t:

    k2,t=(w1,s+k2w1)s,

    and

    N1,t=(w2,s+w1)N2.

    The evolution equation of the second curvature (k2,t) explains how the non-constant curvature of the NCC bends and twists over time based on the Bishop velocities. This equation offers valuable insights into the dynamic behavior of NCCs and provides theoretical advancements and practical improvements in modeling geometric phenomena.

    Assume that N2,t can be written as follows:

    N2,t=c11T1+c12N1+c13N2. (3.19)

    Since N2 is a lightlike transversal vector, N2,N2=0, then N2,t,N2=0, hence c11=0. Using N1,N2=0, then N1,N2,t=N1,t,N2, hence c12=0.

    Since T1,N2=1, then T1,N2,t=T1,t,N2, so

    c13=(w1,s+k2w1).

    Hence, we have

    N2,t=(w1,s+k2w1)N2.

    Hence, the theorem holds.

    We begin by considering the nonlinear partial differential equation (NPDE) (3.12) and transforming it into an ordinary differential equation using a suitable traveling wave transformation defined as:

    k2(s,t)=k2(ζ),ζ=sλt,

    where λ denotes the velocity of the traveling wave. This transformation simplifies the NPDE (3.12) into the following ODE:

    λdk2dζ+ddζ(dw1dζ+k2w1)=0. (4.1)

    Upon integrating (4.1), we obtain:

    λk2+dw1dζ+k2w1=0. (4.2)

    For simplicity, the constant of integration is set to zero. Furthermore, under the traveling wave transformation, ζ=sλt, the inextensibility conditions (3.3) are transformed into the following system of ODEs:

    w2=dw3dζ+k2w3,w1=12(dw2dζ+k2dw2dζdζ). (4.3)

    Application 1. Consider the normal velocity w3=1. By utilizing Eq (4.3), we can determine the tangential velocity w1 and the normal velocity w2 as follows:

    w2=k2,w1=12(k2+12k22),()=ddζ. (4.4)

    the given ODE (4.2) that describes the second Bishop curvature takes the following form:

    k2=12k32+2λk2. (4.5)

    To solve this equation, we assume that

    k2=dk2dζ=Y(k2(ζ)),k2=YdYdk2. (4.6)

    Substituting from (4.6) into (4.5), hence:

    YdYdk2=12k32+2λk2.

    By integrating this equation, we get

    Y(k2(ζ))=k22k22+8λ+c1. (4.7)

    For simplicity, the constant of integration is set to zero. Since Y(k2(ζ))=dk2dζ, then we rewrite (4.7) as follows:

    dk2dζ=k22k22+8λ.

    By integrating this equation, then we obtain:

    k2(ζ)=8λcsch(2λ(ζ+c1)).

    Therefore, we have obtained the second Bishop curvature in the form:

    k2(s,t)=8λcsch(2λ(sλt+c1)). (4.8)

    The second Bishop curvature is illustrated in Figure 1.

    Figure 1.  The evolution of the second Bishop curvature of the NCC with blue, red, and black curves at t=0.1,1.4, and 2.9, respectively, for s[0,3],t[0,3].

    Substituting from (4.8) into the two systems (2.7) and (3.11), and solving them numerically, we can then visualize the evolution of the NCC at various values of time in Figure 2.

    Figure 2.  The evolution of the NCC with blue, red, and black curves at t=0.1,1.4, and 2.9, respectively, for λ=0.3,c1=5, s[0,3], and t[0,3].

    Let α:J=[0,l]×[0,t]R2,1 be the family of PNCs with Bishop frame FB, where l is the parameter of the initial curve and t represents the time parameter. Let u be the parameter variable of the PNC, where 0ul. Let s be the parameter of the arc length of the PNC and it is defined by:

    s(u,t)=u0αu(u,t)du=u0g(u,t)du.g2(u,t)=αu(u,t)2=|αu,αu|. (5.1)

    The law describing the motion of the PNC with Bishop frame FB is given by:

    αt=ϖ1T1+ϖ2N1+ϖ3N2, (5.2)

    where ϖ1,ϖ2, and ϖ3 are the tangential, normal, and binormal velocities, and we call them the Bishop velocities.

    Theorem 6. The PNC with the Bishop frame is inextensible if the Bishop velocities ϖ1,ϖ2, and ϖ3 satisfy the following condition:

    ϖ1,s=k1ϖ2+k2ϖ3. (5.3)

    Proof. If the PNC with the Bishop frame is inextensible, then g(u,t)t=0, by taking the t-derivative of (5.1), and then:

    2ggt=0=αut,αu. (5.4)

    Using

    αu=gαs=gT1, (5.5)

    since αut=αtu, and by taking the u-derivative of (5.2) and using (2.16), hence we obtain:

    αtu=g(ϖ1,sk1ϖ2k2ϖ3)T1+g(ϖ2,s+k2ϖ1)N1+g(ϖ3,s+k1ϖ1)N2. (5.6)

    Substituting from (5.5) and (5.6) into (5.4), and using the properties of the Bishop frame for the PNC that are defined by (2.13), we have

    ϖ1,sk1ϖ2k2ϖ3=0.

    Hence, the theorem holds.

    Lemma 7. Consider the motion of the PNC with the Bishop frame. The curve is inextensible if the Bishop velocities ϖ1,ϖ2, and ϖ3 satisfy the following condition:

    Case 1: In the case where the motion of the PNC with the Bishop frame defined by (2.16) with k1=0, then the curve is inextensible if the Bishop velocities ϖ1,ϖ2, and ϖ3 satisfy:

    ϖ1,s=k2ϖ3. (5.7)

    Case 2: In the case where the motion of the PNC with the Bishop frame defined by (2.18) with k2=0, then the curve is inextensible if the Bishop velocities ϖ1,ϖ2, and ϖ3 satisfy:

    ϖ1,s=k1ϖ2. (5.8)

    Theorem 8. Consider the flow of the inextensible PNCs with a Bishop frame. Then the TEEsBFs are:

    (T1N1N2)t=(0ϖ2,s+k2ϖ1ϖ3,s+k1ϖ1(ϖ3,s+k1ϖ1)00(ϖ2,s+k2ϖ1)00)(T1N1N2). (5.9)

    In addition, the TEEsBCs for k1 and k2 are:

    k1,t=(ϖ3,s+k1ϖ1),s,k2,t=(ϖ2,s+k2ϖ1),s. (5.10)

    Proof. Since αs=T1, then

    αst=T1,t. (5.11)

    Taking the s-derivative of (5.2), we have

    αts=(ϖ1,sk1ϖ2k2ϖ3)T1+(ϖ2,s+k2ϖ1)N1+(ϖ3,s+k1ϖ1)N2. (5.12)

    Since the curve is inextensible, then the condition (5.3) is satisfied. Hence,

    αts=(ϖ2,s+k2ϖ1)N1+(ϖ3,s+k1ϖ1)N2. (5.13)

    Since the curve is inextensible, then αts=αst, so from (5.11) and (5.13), we obtain:

    T1,t=(ϖ2,s+k2ϖ1)N1+(ϖ3,s+k1ϖ1)N2. (5.14)

    Applying the s-derivative of (5.14), we get

    T1,ts=(k1(ϖ2,s+k2ϖ1)k2(ϖ3,s+k1ϖ1))T1+(ϖ2,s+k2ϖ1),sN1+(ϖ3,s+k1ϖ1),sN2. (5.15)

    Since T1,s=k2N1+k1N2, by taking the t-derivative of this equation, we get:

    T1,st=k2,tN1+k1,tN2+k2N1,t+k1N2,t. (5.16)

    Since T1,ts=T1,st, then from (5.15) and (5.16), we obtain that the TEEsBCs for k1 and k2 are:

    k1,t=(ϖ3,s+k1ϖ1),s,k2,t=(ϖ2,s+k2ϖ1),s, (5.17)

    and

    k2N1,t+k1N2,t=(k1(ϖ2,s+k2ϖ1)+k2(ϖ3,s+k1ϖ1))T1. (5.18)

    The PDEs (5.17) describe the evolution of curvatures (k1,t,k2,t), detailing how Bishop velocities affect the behavior of non-constant curvatures of the PNC (how the curve bends and twists over time).

    Assume

    N1,t=a11T1+a12N1+a13N2,N2,t=b11T1+b12N1+b13N2. (5.19)

    Since N1 and N2 are lightlike vectors, N1,N1=0 and N2,N2=0. Then by applying the derivative and using (2.13), we get:

    a13=0,b12=0.

    Since N1,T1=0 and N2,T1=0, hence, by applying the t-derivative, we get:

    a11=(ϖ3,s+k1ϖ1),b11=(ϖ2,s+k2ϖ1).

    Using the condition N1,N2=1, then by taking the t-derivative, we get:

    b13=a12.

    Hence, we have

    N1,t=(ϖ3,s+k1ϖ1)T1+a12N1,N2,t=(ϖ2,s+k2ϖ1)T1a12N2. (5.20)

    Substitute from (5.20) in (5.18), then a12=0. Hence, we get

    N1,t=(ϖ3,s+k1ϖ1)T1,N2,t=(ϖ2,s+k2ϖ1)T1. (5.21)

    Hence, the theorem holds.

    Lemma 9. Consider the flows of the inextensible PNC with the Bishop frame that is defined by (2.16) and (2.18) with conditions (5.7) and (5.8), respectively. Then, the TEEsBFs are:

    Case 1: In the case of the motion of the inextensible PNC with Bishop frame (2.16) with k1=0 and condition (5.7), we have

    (T1N1N2)t=(0ϖ2,s+k2ϖ1ϖ3,sϖ3,s00(ϖ2,s+k2ϖ1)00)(T1N1N2). (5.22)

    The TEEBC is:

    k2,t=(ϖ2,s+k2ϖ1),s,ϖ3,ss=0. (5.23)

    Case 2: In the case of the motion of the inextensible PNC with Bishop frame (2.18) with k2=0 and condition (5.8), we have

    (T1N1N2)t=(0ϖ2,sϖ3,s+k1ϖ1(ϖ3,s+k1ϖ1)00ϖ2,s00)(T1N1N2). (5.24)

    The TEEsBC is:

    k1,t=(ϖ3,s+k1ϖ1),s,ϖ2,s=0. (5.25)

    Application 2. Consider the constant tangential velocity ϖ1=a, and then the binormal velocity ϖ3=0. Assume that the normal velocity ϖ2=logk2(s,t). Therefore, we have

    k2,t=(k2,sk2+ak2),s. (6.1)

    By solving the PDE (6.1), we obtain the second Bishop curvature in the form:

    k2(s,t)=C21aC1C2(1+tanh(C1s+C2t+C3)). (6.2)

    The second Bishop curvature is illustrated in Figure 3. Substituting from (6.2) into (2.16) and (5.22) and solving the two systems (2.16) and (5.22) numerically for k1=0 and k2(s,t)=C21aC1C2(1+tanh(C1s+C2t+C3)), then we can visualize the evolution of the PNC at various values of time (see Figure 4).

    Figure 3.  The evolution of the second Bishop curvature of the PNC with blue, red, and black curves at t=0.4,1.4, and 2.9 for C1=0.1,C2=1,C3=0.01,a=0.1, s[0,2], and t[0,3].
    Figure 4.  The evolution of the PNC with blue, red, and black curves at t=0.4,1.4, and 2.9 for C1=0.1,C2=1,C3=0.01,a=0.1, s[0,2], and t[0,3].

    Application 3. Consider the constant tangential velocity, ϖ1=a, and then the binormal velocity ϖ3=0. Assume that the normal velocity is ϖ2=k2(s,t), and we have

    k2,t=(k2,s+ak2),s. (6.3)

    By solving the PDE (6.3), then we obtain the second Bishop curvature in the form:

    k2(s,t)=1+eC1s+t(aC1+C2)+C3. (6.4)

    The second Bishop curvature is illustrated in Figure 5. Substituting from (6.4) into the two systems (2.16) and (5.22) and solving them numerically for k1=0 and k2(s,t)=1+eC1s+t(aC1+C2)+C3, then we can visualize the evolution of (PNC) at various values of time (see Figure 6).

    Figure 5.  The evolution of the second Bishop curvature of the PNC with blue, red, and black curves at t=0.4,1.4, and 2.9, respectively, for C1=0.3,C2=0.09,C3=0.01,a=0.1, s[0,2], and t[0,3].
    Figure 6.  The evolution of the PNC with blue, red, and black curves at t=0.4,1.4, and 2.9, respectively, for C1=0.3,C2=0.09,C3=0.01,a=0.1, s[0,2], and t[0,3].

    In the present paper, we study the evolution of null Cartan curves (NCCs) and pseudo null curves (PNCs) according to the Bishop frame. The new results are listed as follows:

    ● We obtained the necessary and sufficient inextensibility conditions for the NCCs (Theorem 4 by Eq (3.3)) and for the PNC according to the Bishop frame (Theorem 6 by Eq (5.3) and Lemma 7 by Eqs (5.7) and (5.8)).

    ● We derived the TEEsBF for the NCCs (Theorem 5 by Eq (3.11)) and the PNC in R2,1 (Theorem 8 by Eq (5.9) and Lemma 9 by Eqs (5.22) and (5.24)).

    ● We derived the TEEsBC of the NCCs (Theorem 5 by Eq (3.12)) and the PNCs as (PDEs) in terms of Bishop velocities (Theorem 8 by Eq (5.10) and Lemma 9 by Eqs (5.23) and (5.25)).

    ● The evolution of curvatures for both null Cartan curves (NCCs) and pseudo null curves (PNCs) were calculated and plotted for some applications. Additionally, the evolution of NCCs and PNCs is visualized.

    The authors sincerely thank the reviewers for their insightful comments and constructive suggestions. Their valuable feedback has been instrumental in refining the content and improving the quality of this paper.

    Samah Gaber: Investigation, Writing the original draft, Writing the review and editing, Software; Abeer Al Elaiw: Investigation, Writing the original draft, Writing the review and editing. All authors have read and agreed to the published version of the manuscript.

    The authors declare they have not used Artificial Intelligence (AI) tools in creating this article.

    The authors declare no conflicts of interest.



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