International Journal of Mathematics and Mathematical Sciences

Volume 2018, Article ID 3713248, 8 pages

https://doi.org/10.1155/2018/3713248

## A Formulation of L-Isothermic Surfaces in Three-Dimensional Minkowski Space

Department of Mathematics, University of Texas, Edinburg, TX 78540, USA

Correspondence should be addressed to Paul Bracken; ude.vgrtu@nekcarb.luap

Received 6 March 2018; Revised 11 June 2018; Accepted 16 July 2018; Published 1 August 2018

Academic Editor: Ilya M. Spitkovsky

Copyright © 2018 Paul Bracken. 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

The Cartan structure equations are used to study space-like and time-like isothermic surfaces in three-dimensional Minkowski space in a unified framework. When the lines of curvature of a surface constitute an isothermal system, the surface is called isothermic. This condition serves to define a system of one-forms such that, by means of the structure equations, the Gauss-Codazzi equations for the surface are determined explicitly. A Lax pair can also be obtained from these one-forms for both cases, and, moreover, a nonhomogeneous Schrödinger equation can be associated with the set of space-like surfaces.

#### 1. Introduction

The study of isothermic surfaces can be traced back to the work of Bianchi and Bour** [1, 2]**, as well as to Darboux** [3]**. These surfaces seem to have their origin in work by Lamé motivated by problems in heat conduction. An important subclass of isothermic surfaces was subsequently investigated by Bonnet. The study of these surfaces has seen renewed interest recently with the work of Rogers and Schief** [4–6]**. Rogers established that a Bäcklund transformation for isothermic surfaces is associated with a nonhomogeneous linear Schrödinger equation. This is largely due to the fact that the classical Gauss-Mainardi-Codazzi equations which are associated with surfaces in general are integrable in the sense they possess soliton solutions. Thus, these surfaces can be put in correspondence with solitonic solutions of certain nonlinear partial differential equations. Thus they have a strong appeal to those with interests that range from integrable equations to their associated Bäcklund transformations** [7–9]**. Thus, integrable systems theory can be applied to isothermic surfaces and used to study transformations of these surfaces as well. Consequently, isothermic surfaces constitute an important subclass of surfaces with a connection to solitons.

It is the purpose of this work to study the cases of both space-like and time-like surfaces as well as their immersion in three-dimensional Minkowski space in a unified manner by basing the approach on the structure equations of Cartan and the associated moving frame** [10]**. Suppose that is such a surface or manifold to which a first fundamental form is associated. With respect to the larger space , there exist both space-like and time-like surfaces residing in this larger space. Thus, a particular could be either one of these two types of object. This is expressed by the fact that there are two ways in which the metric or first fundamental form can be specified intrinsically on the surface. In terms of two local coordinates, the metric may be written with two positive signs, hence a positive signature, or it may be written with a negative signature or alternating signs. In the former case, the surface is referred to as space-like and in the latter case it is called time-like.

To start, let us outline the approach used here. Cartan’s equations of structure are formulated in such a way that they are adapted to the signature of the flat metric of the ambient background space . These equations are defined in terms of a set of one-forms. By selecting these one-forms in a particular way along with the appropriate choice of signs, the system of structure equations can be restricted to study one of the classes of surface already described. In fact, a set of partial differential equations can be obtained which can be used to describe each of these types of surface. Therefore, the solutions of these equations can be used to describe a corresponding type of surface immersed in .

In fact, it can be mentioned quite generally that new integrable equations have been obtained in the case of purely Euclidean space from the Cartan system by exploiting the one-to-one correspondence between the Ablowitz-Kaup-Newell-Segur (AKNS) program** [11, 12]** and the classical theory of surfaces in three dimensions. Its relationship to the problem of embedding surfaces in three-dimensional Euclidean space arises from the fact that the Gauss-Codazzi equations are in this case equivalent to Cartan’s equations of structure for . This correspondence suggests that the soliton connection can be given a deeper structure at the Riemannian level. In fact, in Euclidean space, much effort has been expended to exploit the equivalence between the AKNS systems and surface theory at the metric level in order to construct new nonlinear equations.

Once the structure equations have been formulated in this context, the one-forms can then be chosen for the case of isothermic surfaces. Classically, when the lines of curvature of a surface form an isothermal system, the surface is referred to as isothermic. The case studied here will be the one in which the third fundamental form is conformally flat in the manifold coordinates:When the surface has a third fundamental form diagonal as in (1), it is often called L-isothermic. The prefix will be omitted here. This will lead to an equation whose solutions can be used to express the three fundamental forms which characterize surface. For each type of surface, the function in (1) turns out to be specified by a nonlinear second-order equation. It will be shown finally that Lax pairs can subsequently be formulated for each of the two second-order systems. Out of these results, further equations of physical interest can be developed which are relevant to both types of surface. One way in which these auxiliary equations arise is through compatibility conditions. Moreover, an interesting result is presented by showing that there is an important link between a nonhomogeneous Schrödinger equation and a combination of surface variables which are relevant to the more physical case of space-like isothermic surfaces. Investigations into this area have appeared** [13–15]**. Here the idea is to show how these geometries can be studied consistently by using the moving frame approach by simply altering some of the parameters in the metric.

#### 2. Cartan Formulation of Structure Equations in Three-Space

A Darboux frame is established on such that the vectors are tangent to surface and is a normal vector to ; hence determines an orientation for . At a point , it is the case thatwhere denotes a position vector in (2), constitute a basis of one-forms, and index goes from to . A surface can be established by takingwith a normal vector to . The metric on the ambient background space is taken to have the following form** [13]:**so there is no loss of generality in defining the surface by means of (3). Moreover, the quantities and in (4) can assume one of the two values and . In the case in which , metric specifies a three-dimensional Euclidean space as usual. However, it is the intention here to study the cases in which only one of the quantities or is taken to be negative. On the one hand, if and the surface metric is space-like, and if and , the surface metric is time-like.

The surfaces immersed in which are studied here have first fundamental form or metric on the surface defined byThe two choices of sign for account for two classes of surface just introduced. For the basis vectors of the Darboux frame, Cartan’s structure equations must hold and they are given as follows:As in Chern** [12]**, we suppose the relative components of the frame field are and . These are differential one-forms which depend on the two independent surface coordinates . To be able to discuss the embedding problem, the second fundamental form for has to be defined as well. It is given byTo formulate and study the case of isothermic surfaces, the complete system of Cartan structure equations is required. Under the convention adopted for given in (4), these equations can be presented in the notation of Chern** [12]** asIn the case of a space-like , we take and , so a space-like metric (5) results. For the time-like case and , and a time-like metric (5) results. Each of these two cases will be studied by defining the one-forms which appear in (9)-(11) appropriately.

Theorem 1. *The Gauss-Codazzi equations (11) for embedding can be expressed in the form of Cartan’s structure equations for the group asThe one-forms , where , are defined to beThe structure constants of which appear in (13) are , .**The proof is straightforward, simply substitute the forms (13) into (12), and solve for , and . Upon carrying this out, system (11) appears directly.*

#### 3. Surfaces with Space-Like Metric

To obtain a metric which has a positive signature on surface , the parameters which appear in in (4) are set to the values and . Metric assumes the following form:The structure equations (9)-(11) then take the following form:These equations constitute the Gauss-Codazzi system for the space-like surface.

In order to study isothermic surfaces, the one-forms which are to be used in (15)-(17) are defined in such a way that the third fundamental form is proportional to a flat metric on (1). Given a coordinate chart for , the one-forms and are taken to depend on functions of the coordinate parameters asThe functions and depend on both in general. Further, define the one-forms as follows:Equations (19) and (20) imply that the first three fundamental forms of the surface can be constructed in the following way:From the fact that the mean and Gaussian curvatures are given by and in (20) can be interpreted as the principal curvatures of the surface. To obtain an expression for in terms of and from (19), let us suppose , where functions and depend on both coordinates as do and . Substituting this into (15), we can solve for and and (15) reduces to a pair of identities provided that has the following form"Clearly, forms (19), (20) clearly satisfy (16) automatically. Finally, putting the set of forms into the remaining equations in (17) produces a system which can be used to determine the two functions and . In fact, doing so produces a coupled system of partial differential equations which must hold when expressed in terms of all the relevant functions , , and .

Differentiating in (23) givesThus the first equation in (17) implies that and satisfy a second-order equation:The next two equations of (17) yield the following pair:Therefore, (25) and (26) constitute the relevant system to be studied.

The condition that the space-like surface be isothermic is that the third fundamental form be conformally flat in terms of the -coordinate system like (1). In order to ensure this, it suffices to takeThis parameterization can now be used to transform the system of (25)-(26) into a set which depends on only variable .

To this end, differentiate in (27) with respect to :Substituting (28) into the second equation of (26) simplifies to the following form:By using (27) to eliminate , (29) becomesSimilarly, differentiate with respect to to obtainSubstituting (31) into the first equation in (26) simplifies toUsing (30) and (32) in the second-order equation (25) as well as the fact that becomes an equation in terms of only the variable:To summarize then, the Gauss-Mainardi-Codazzi equations reduce to the following form under (27):Based on the results in (34), it is possible to make further links to other types of equations which are of importance in mathematics and physics. These arise by working out the compatibility conditions between them. Suppose two independent functions and and related to and in the following way:Putting these in the first equation of (34) and collecting like functions on opposite sides and multiplying by giveDoing the same thing to the second equation givesUsing the product rule on (36) and (37), the following pair of equations has been obtained:Finally, the desired compatibility condition for can be obtained by differentiating with respect to , then with respect to , and finally equating the results. After multiplying the result by , this simplifies to the following:To obtain an analogous equation for , is differentiated with respect to and with respect to . Upon equating them, one obtainsThese steps have proved the following theorem.

Theorem 2. *The compatibility conditions for functions and defined in terms of and by (35) are specified in terms of the following Moutard equations:*

Normally, there exists a close connection between the Moutard equation and a transformation called the fundamental transformation between surfaces. We show that a Lax pair exists for the second-order system in (34). Let , be unit space-like tangent vectors to in .

Theorem 3. *Let , be unit tangent vectors and a unit normal to the space-like surface . Define the following matrix system which depends on function :**In (42), is a spectral parameter. The zero curvature condition for system (42) is satisfied if and only if function satisfies the second-order equation of (34), namely,*

*Proof. *It suffices to differentiate the first matrix equation in (42) with respect to , and the second with respect to and require that the results agree identically. In other words, (42) is equivalent to the first-order system:Condition under (44) reduces toSimplifying this, the spectral parameter disappears and equality holds exactly when the function satisfies the second-order equation, . Similarly, the condition isThis holds whenever satisfies this partial differential equation. Finally, simply reduces towhich is an identity.

To write the position vector of the surface, it is useful to define the new variable in terms of , as follows:Taking to be complex, the following complex derivatives are defined:In terms of and these derivatives, (44) can be abbreviated to the following form:The position vector of the space-like surface will be obtained by integration of the following equation:To this end, introduce the scalar quantity:which can be regarded as the distance from the origin to the tangent plane on the space-like surface at the point . Differentiating with respect to and , we find thatThe position vector of the space-like surface therefore admits a decomposition of the following form:To obtain in terms of and , differentiate with respect to and substitute (50):Replacing the first derivatives from (44), this derivative simplifies toComparing this result with (51), this procedure allows us to write and in terms of and :It has been found that the position vector of the space-like surface is given by where the real function is a solution of the following equation:Finally, it can be shown that (58) is equivalent to an inhomogeneous Schrödinger equation. To do so, a new variable is introduced and defined asDifferentiating both sides of with respect to givesand, after a second time, we haveSubstituting this second derivative on the left of (58), the following second-order equation for results after dividing out isIntroducing the potential function which is defined in terms of as , (62) assumes the following form:

#### 4. Surfaces with Time-like Metric

To obtain a metric for this case with a time-like structure on it must be that and in (4). The metric then assumes the following form:Structure equations (9)-(11) then differ by signs and are given byThe one-forms and required to define the first fundamental form (5) are taken to bewhere , are functions of the coordinates . Furthermore, the one-forms and take the following form:Based on the one-forms (69)-(70), the three fundamental forms for can be written asSince both and differ from the previous case, has to be determined again, and it is given byEquation (66) is satisfied automatically by this system of forms as well. The remaining three equations (67) can now be computed exactly as before. The conclusion is that a second-order equation results, namely,as in the previous case, and the pairBoth equations in (74) are seen to be identical to their corresponding counterparts in (74). In this case as well, the equations in (73) and (74) can be written in such a way that the fundamental form is conformally flat assuming the form (1), Since the steps are identical to the previous case, the results are summarized as follows:

These are exactly analogous to (34), the first two being identical to those of the space-like case. The second-order equation differs by signs from the case (34). Since the first two equations are exactly the same, similar functions and can be introduced which are related to and as in (35). All the steps which lead to Theorem 2 are unchanged as they involve only the first two equations and are independent of the second-order equation. Thus, a version of Theorem 2 can be formulated here as well. The Lax pair however has to be different since the second-order equation is different.

Theorem 4. *Let , be unit tangent vectors to time-like surface and a unit normal vector to . Define the following matrix system in terms of function asThe compatibility condition in for this system holds if and only if function satisfies the second-order equation in (75), namely,**The proof of Theorem 4 goes exactly as the proof of (42). To illustrate, the details for the equations will be given. Differentiating the first matrix equation by and the second by and substituting (76) for the first derivatives, it is found thatThis will be satisfied provided that satisfies the second-order equation in (75). A similar result is found to hold for the equation and holds as an identity.*

Again, if is defined exactly as in the previous case, then, in terms of complex derivatives, and using the equations of (76), the system corresponding to (50) isTaking to have the same form (51) and given by (54), then differentiating (54) with respect to and comparing to (51), it is found thatSupposing has the form (35), then the first equation in (80) becomes a second-order partial differential equation for the function , namely,

#### 5. Conclusions and Summary

In the Cartan framework, we can discuss isothermic surfaces in Minkowski three-space for both space-like and time-like cases. As Theorems 2 and 4 show, the classical Gauss-Mainardi-Codazzi system associated with isothermic surfaces is integrable in the modern solitonic sense. Bäcklund transformations will exist for both types of surface. The appearance of the Moutard equations (41) in both cases is remarkable, and, subsequently, Sturm-Liouville or Schrödinger equation (62). This leads to the final proposition.

Proposition 5. *Let and satisfy the compatibility condition and let be a real solution of the inhomogeneous Schrödinger equation (63). Then with , (54) provides a position vector for a space-like isothermic surface.*

#### Data Availability

This is a theoretical work; no data was involved.

#### Conflicts of Interest

The author declares that they have no conflicts of interest.

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