A duality for Guichard nets

In this paper we study G-surfaces, a rather unknown surface class originally defined by Calapso, and show that the coordinate surfaces of a Guichard net are G-surfaces. Based on this observation, we present distinguished Combescure transformations that provide a duality for Guichard nets. Another class of special Combescure transformations is then used to construct a B\"acklund-type transformation for Guichard nets. In this realm a permutability theorem for the dual systems is proven.


Introduction
Various integrable surface classes were classically characterized by the existence of particular Combescure transformed dual surfaces: for example, isothermic surfaces via the Christoffel transforms and Guichard surfaces by the Guichard duality ( [5,8,15,18]). Based on this principle, in [27] the theory of O-surfaces was established, which provides a unified way to describe many classically known integrable surface classes.
Recently [25], a generalization of Demoulin's duality for Ω-surfaces [12] has been found, revealing that the Lie applicable class of Ω-surfaces also appears as Osurfaces. Moreover, this concept also applies to higher dimensional hypersurfaces as the example of 3-dimensional conformally flat hypersurfaces shows (cf. [23,29]).
Although triply orthogonal systems possess a rich transformation theory ( [2,3,11,17]), the concept of characterizing Combescure transformations to distinguish special subclasses of systems has not been exploited yet. In this paper we adopt this approach and present a duality for the subclass of Guichard nets that allows, together with particular associated systems, a characterization of Guichard nets in terms of their Combescure transforms.
The duality for Guichard nets presented in this paper relies on the crucial fact that the coordinate surfaces of a Guichard net are G-surfaces. This surface class was introduced by Calapso in [6] and then apparently fell into oblivion: a surface f is a G-surface if there exists a Combescure related associated surfacef such that the following relation for some ε ∈ {0, ±1} and c ∈ R \ {0} between the principal curvatures and the coefficients H 2 1 and H 2 2 of the induced metric of f holds. Note, however, that this relation is not symmetric in f andf , hence the Combescure transformf is in general not a G-surface with associated surface f .
In Section 3 we demonstrate that for a G-surface there additionally exist Combescure transforms that are again G-surfaces and allow for a dual construction. This characterization immediately reveals that G-surfaces can be interpreted in the framework of O-surfaces and contain isothermic, as well as Guichard surfaces as subclasses.
These observations will be used in Section 4 to prove how G-surfaces arise in the context of triply orthogonal systems. In particular, we show that the coordinate surfaces of a Guichard net are G-surfaces. As a consequence, there exist associated and dual surfaces for the coordinate surfaces of a Guichard net that, suitably chosen, give rise to new triply orthogonal systems. It is proven that the triply orthogonal systems formed by the dual surfaces are again Guichard nets, hence this dual construction provides a method to construct new Guichard nets from a given one.

Preliminaries
In this work we only deal with surfaces f = (f 1 , f 2 , f 3 ) : M 2 → R 3 parametrized by curvature line coordinates, that is, the coordinates are orthogonal and conjugate. Equivalently, the coordinate functions f 1 , f 2 where the induced metric of the surface is denoted by I = H 2 1 dx 2 + H 2 2 dy 2 .
2.1. Combescure transformation of a surface. Classically [9,15], two surfaces f andf are related by a Combescure transformation if they have parallel tangent planes along corresponding conjugate coordinate lines. However, note that a Combescure transformation generically preserves curvature line coordinates. Hence, in this paper we consider without loss of generality parallel tangent directions along curvature line coordinates. Analytically, any Combescure transformf of f : M 2 → R 3 is (up to translation) uniquely determined by where h, l : M 2 → R are two functions fulfilling the compatibility conditions Equivalently [15,Chap. I.4], any function ϕ : gives rise to a pair of functions h and l described by that fulfills equations (3) and therefore defines by (2) a Combescure transformation of f . Since the functions h and l are uniquely determined up to the same additive constant, a function ϕ satisfying condition (4) induces by (5) a 1-parameter family of Combescure transforms of f . Note that, in particular, the normals of f and a Combescure transformf coincide up to orientation, that is, n = δn, where δ ∈ {±1} depending on whether the product hl is positive or negative. Hence, denoting the principal curvatures of f by κ 1 and κ 2 , the principal curvaturesκ 1 andκ 2 of the Combescure transformf are given byκ We also allow the functions h or l to vanish identically, such thatf degenerates to a curve or a point. In these cases we consider the corresponding radii of principal curvature off to be zero.

Triply orthogonal systems and their Combescure transforms.
In what follows, we give a brief introduction to triply orthogonal systems and summarize some facts that will become important later. We then focus on the Combescure transformation for triply orthogonal systems and prove two results that show how these transforms can be constructed. For more details and a wide range of beautiful results in this area, we refer to the rich literature on triply orthogonal systems, for example [1,11,24,26,31].
Agreement. Throughout the paper we use the following convention for the indices in the realm of triply orthogonal systems: if only one index i or two indices i and j appear in an equation, then i = j ∈ {1, 2, 3}. If there are three indices (i, j, k) involved, then they take the values (1, 2, 3) or a cyclic permutation of them.
To avoid useless indices, we sometimes change the notations of variables: x = x 1 , x 2 = y and x 3 = z. For the partial derivatives we use the abbreviations ∂ i = ∂ xi and ∂ ij = ∂ xi ∂ xj .
Let U be an open and connected subset of where (., .) denotes the usual inner product on R 3 . The unit normals N i of the coordinate surfaces x i = const are given by the Lamé coefficients never vanish on U . The induced metric of a triply orthogonal system is then given by If f is a triply orthogonal system, then the functions (f 1 , f 2 , f 3 ) = f are solutions of the point equations of any surface family x k = const, and fulfill the Gauss equation Moreover, the Lamé coefficients H 1 , H 2 and H 3 satisfy the first and second system of Lamé's equations If we introduce Darboux's rotational coefficients then Lamé's systems (8) and (9) reduce to first order equations and the unit normals N 1 , N 2 and N 3 satisfy the system (10) Conversely, an orthogonal metric of the form (6) that satisfies Lamé's equations gives rise to a triply orthogonal system, which is uniquely determined up to Euclidean motions: to recover a parametrization f : U → R 3 from a prescribed orthogonal metric I, we first compute the rotational coefficients β ij and construct a set of suitable normals N 1 , N 2 and N 3 from system (10). Then a parametrization f : U → R 3 of the system with induced metric I is obtained by integrating Note that the integrability of the system (10) and equation (11) is guaranteed by Lamé's equations and the construction is unique up to Euclidean motions. Thus, if the ambiguity of Euclidean motions does not effect the geometry we are interested in, we talk about the induced metric instead of the explicit parametrization.
Recall that, by Dupin's Theorem, any two coordinate surfaces of two different families intersect along a curve which is a curvature line for both surfaces. This immediately reveals information about the geometry of the three families of coordinate surfaces: the induced metrics I i for the family of coordinate surfaces x i = const are given by and therefore the two principal curvatures of these coordinate surfaces read Hence, due to Lamé's equations (8), the metric coefficient H i | xi=const is a solution of the point equation (7) of the coordinate surface x i = const.
Remarkable subclasses of triply orthogonal systems emerge from induced metrics whose traces fulfill a certain condition (cf. [2,11,19]). We use the following terminology: . A χ-system is a triply orthogonal system such that the trace with respect to the Minkowski-metric of the induced metric I satisfies In particular, a 0-system is called a Guichard net.
Better insights into the geometry of a triply orthogonal system is often obtained by studying the coordinate surfaces. Thus, many geometric ideas known from surface theory were classically carried over to triply orthogonal systems by requiring a geometric property for any coordinate surface of the system: for example, systems consisting of isothermic surfaces or of constant Gaussian curvature surfaces. Also the rich transformation theory for triply orthogonal systems is based on this concept. The Combescure transformation between two triply orthogonal systems is named after its first appearance in [9]: Since the rotational coefficients β ij determine the normal directions of the coordinate surfaces of a triply orthogonal system, two systems are related by a Combescure transformation if and only if they share the same rotational coefficients (cf. [26, §5]).
The following lemma gives a criterion for three functions to define a Combescure transformation of a given parametrized triply orthogonal system: Proposition 2.3. Let f be a triply orthogonal system with Lamé coefficients (H i ) i , then Proof. Let us denote the rotational coefficients off byβ ij : Hence, β ij =β ij if and only if the equations (14) hold, which proves the claim.
Thus, for two given metrics where the functions h 1 , h 2 and h 3 satisfy equations (14), we can find two parametrizations f andf which have these metrics as induced metrics and are related by a Combescure transformation. Therefore, we also say that two such induced metrics are related by a Combescure transformation.
To conclude this section, we prove a rather technical result, which will become important later. Suppose we start with a triply orthogonal system and Combescure transform each coordinate surface. Then the following proposition gives a criterion in which cases the Combescure transformed surfaces constitute again a triply orthogonal system: be a triply orthogonal system and let ϕ 1 , ϕ 2 and ϕ 3 denote three functions that define by (5) Proof. By Proposition 2.3, any Combescure transformf of f can be described by three functions h 1 , h 2 and h 3 satisfying (14). On the other hand, on the level of coordinate surfaces, any function ϕ j gives rise to functions h j i and h j k fulfilling (cf. conditions (5)) Hence, it remains to show that the system Since ϕ i and ϕ k induce Combescure transformations, we obtain by equations (5) the integrability conditions

The remaining integrability condition
then follows from the assumptions (15) and the first system of Lamé's equations of f . Conversely, suppose that h i j = h k j , then equation (16) is satisfied and we conclude that the conditions (5) hold.
Obviously, the G-condition (18) depends on the choice of particular curvature line coordinates. However, from the definition we immediately deduce that a surface given in (arbitrary) curvature line coordinates (x, y) is a G-surface if and only if there exist functions χ 1 = χ 1 (x) and χ 2 = χ 2 (y) such that As already observed in [6, §VI (89)]), a G-surface admits a special solution of its point equation: As a consequence of the construction in this proof, the function ϕ gives rise to a 1-parameter family of Combescure transformations of f such that the G-condition (18) is satisfied. Hence, any G-surface admits (up to orientation) a 1-parameter family of associated surfaces: letf 0 be an arbitrary associated surface of f , then this 1-parameter family (f c ) c∈R is given bŷ In particular, any three associated surfacesf 1 ,f 2 andf 3 satisfy the relation As many integrable surface classes, we will prove that also G-surfaces can be characterized by the existence of Combescure transforms fulfilling a special relation between their principal curvatures:

Theorem 3.3. A surface f is a G-surface if and only if there exists a non-trivial pair (f , f ) of Combescure transforms of f with principal curvatures satisfying
We will prove that the sought-after Combescure transform f of f is, for a suitable choice ofδ ∈ {±1}, obtained by integrating Indeed, a straightforward computation using the G-condition (18) shows that (22) is integrable. Since the principal curvatures of f are given by where δ ∈ {±1} depends on the orientation of the normal of f , we obtain that Hence, for the choiceδ = δ, we have constructed the sought-after pair (f , f ), which is non-trivial.
Conversely, suppose thatf and f are non-trivial Combescure transforms of f which satisfy condition (20). Then, there exist functions h , l and h , l such that and the principal curvatures of f and f read κ 1 =κ 1 /h and κ 1 =κ 1 /h , respectively. From equation (20), we then deduce that Moreover, from the compatibility conditions (3) and (23) we obtain and therefore for suitable functions ψ 1 (x) and ψ 2 (y). Since, by (21), we have h h = 1 or l l = 1, at least one of the functions ψ 1 (x) and ψ 2 (y) does not vanish.
Therefore, due to symmetry, it follows thatf is also an associated surface of f and we obtain another G-surface: Corollary and Definition 3.5. Let f be a G-surface, then the Combescure transform f defined by (20) is a G-surface, which will be called the dual surface of f with respect to the associated surfacef . Note that a dual surface depends on the choice of the associated surface: letf 0 be an associated surface with corresponding dual surface f 0 , then the dual surface f c with respect to the associated surfacef c is given by  However, note that the requirement of non-trivial Combesure transforms is indeed necessary: by Remark 3.4, any surface satisfies the criterion of an O-surface with respect to the indefinite metric induced by (24), if trivial Combescure transformations are not excluded.

Examples.
Using the just derived characterizations we demonstrate how various well-known surface classes arise in the context of G-surfaces.
Isothermic surfaces. Since any isothermic surface admits conformal curvature line coordinates, any scaling of f provides an associated surface as seen from the definition using ε = −1. The converse also holds: As a consequence, in the context of G-surfaces, the dual surfaces of an isothermic surface f with induced metric I = e 2ψ {dx 2 + dy 2 } are determined by Hence, if for c = 0 the associated surface degenerates to a point, the corresponding dual surface becomes the classical Christoffel dual [8] of an isothermic surface and therefore it is again isothermic. However, the other dual surfaces f c are in general not isothermic again.

Guichard surfaces. Recall [5] that a surface is a Guichard surface if and only if it fulfills Calapso's equation
for suitable curvature line coordinates, ε ∈ {±1, 0} and some c ∈ R \ {0}.
Hence, since the spherical representative of a surface is a Combescure transform that is determined by the functions h = κ 1 and l = κ 2 , any Guichard surface is a G-surface by Calapso's equation. Moreover, the spherical representative as an associated surface distinguishes this subclass:

Proposition 3.7. A G-surface f is a Guichard surface if and only if its spherical representative is an associated surface of f .
We remark that, in the case of a Guichard surface, condition (20) becomes the relation originally used to define Guichard surfaces [18], namely 1 Therefore, the dual surface f with respect to the spherical representative, constructed in the context of G-surfaces, coincides with the classical dual surface given by Guichard.
This subclass also provides examples of G-surfaces that satisfy the G-condition for ε = 0: in this case, we deduce from the G-condition and the compatibility conditions (3) that the associated surfaces are determined by functions h and l of the form where ψ 1 = ψ 1 (x).
Thus, for a Guichard surface, where one of the associated surfaces is given by the spherical representative, one of the principal curvatures of f is constant along the corresponding curvature direction. Therefore, f is a channel surface (cf. [5]). Ω-surfaces. By Demoulin's equation [12], a surface is an Ω-surface if and only if there exists a Combescure transformed dual surface f fulfilling for a suitable choice of curvature line coordinates inducing the first fundamental form I = H 2 1 dx 2 + H 2 2 dy 2 .
A reformulation of the condition (20) between the principal curvatures of a Gsurface and its associated and dual surfaces, will shed light on the relation between the dual surfaces of a G-surface and of an Ω-surface:

Lemma 3.8. Let f be a G-surface, then the principal curvatures of an associated surfacef and the corresponding dual surface f satisfy
Proof. Suppose f is a G-surface. Then the principal curvatures of a dual surface with respect to the associated surface df = h∂ x f dx + l∂ y f dy are given by Now a straightforward computation using the G-condition (18) proves the claim.
Therefore, a dual surface of a G-surface fulfills Demoulin's equation (27) if and only if Thus, we have proven: In particular, this proposition reflects that isothermic and Guichard surfaces belong to the classes of G-as well as that of Ω-surfaces. In these cases, the corresponding associated surface coincides with the associated Gauss map for Ω−surfaces, which was recently determined in [25].

G-systems.
Although it is quite popular to distinguish surface classes by the existence of particular Combescure transforms, there are only few results in this direction in the realm of triply orthogonal systems (for example, the Combescure transforms of the confocal quadrics [26]).
However, it is a prominent question to analyse systems consisting of particular coordinate surfaces: for example, systems with isothermic coordinate surfaces [11, Chap. III-V] or Bianchi's systems consisting of coordinate surfaces with constant Gaussian curvature (cf. [22,26]). Clearly, these systems are all built from G-surfaces (cf. Propositions 3.6 and 3.7).
Here, we are interested in both aspects and investigate triply orthogonal systems consisting of G-surfaces that admit Combescure transforms constituted by the associated and dual surfaces. In particular, it will turn out that Guichard nets belong to this class of triply orthogonal systems.

Definition 3.10. A triply orthogonal system f that admits non-trivial Combescure transformsf and f such that the principal curvatures of the coordinate surfaces satisfy
will be called a G-system. The Combescure transformsf and f are said to be an associated system and a dual system, respectively.
If we describe the Combescure transformf of a G-system f by we deduce, using similar arguments as in the proof of Theorem 3.3 that there exist Hence, the coordinate surfaces of a G-system are G-surfaces andf consists of the corresponding associated surfaces.
The corresponding dual system f is then given by and therefore the coordinate surfaces of f are dual surfaces of the coordinate surfaces of f . In particular, the existence of a dual system f just follows from the existence of the associated systemf .
In summary, we have proven
induce the corresponding Combescure transformations that transform any coordinate surface to the associated surfaces. Moreover, Proposition 2.4 guarantees that, if suitably chosen, the associated surfaces constitute again a triply orthogonal system: by the Guichard condition and (32), we obtain and therefore the functions ϕ 1 , ϕ 2 and ϕ 3 induce a 1-parameter family of Combescure transformed systems. By construction, these are the associated systems of the Guichard net f . Thus, equation (30) describes the corresponding dual systems of f and a Guichard net is indeed a G-system.
To gain more insights into the geometry of Guichard nets, we explicitly construct its associated systems: where the functions h 1 , h 2 and h 3 are determined by From Proposition 4.2, we obtain, similar to the case of G-surfaces, the following relation between the associated systems: iff 0 is an associated system induced by the functions h 1 , h 2 and h 3 satisfying system (34), then the 1-parameter familyf c of associated systems are induced by the functions h ic : Hence, we obtainf c =f 0 + cf. As a consequence, we conclude that the dual systems of a Guichard net are given by The characterization of Guichard nets given in Theorem 4.3 then shows, that the dual systems provide a 1-parameter family of (generically) new Guichard nets: Proof. Let f be a Guichard net with an associated systemf and the corresponding dual system f . We will show that hence, a Combescure transformation defined by the functions −h 1 , −h 2 and −h 3 satisfies the conditions (35) of Theorem 4.3 and therefore the dual system f is a Guichard net.
To prove this relation, we use ε i ε j = −ε k , formula (36) and the relations Then we obtain which completes the proof.

Guichard nets with cyclic associated systems.
A triply orthogonal system is cyclic if two of the coordinate surface families consist of channel surfaces. In this subsection we employ the geometric consequences for a Guichard net if all its associated systems are cyclic. It will turn out that these Guichard nets are rather special and contain a family of parallel or totally umbilic coordinate surfaces.
To begin, we recall some properties of such systems (cf. [16,26]) and remark some immediate relations to their associated systems.
If the coordinate surfaces of a Lamé family are parallel, then the other two families are developable surfaces formed by the normal lines along the curvature lines of the parallel surface family. Thus, two of the rotational coefficients vanish. Conversely, if for example β ji = β ki = 0, then we conclude from (10) and (11) that the coordinate surfaces x i = const are parallel.
Consequently, a triply orthogonal system contains a family of parallel coordinate surfaces if and only if the corresponding coordinate surfaces of a (hence all) Combescure transforms are also parallel.
Moreover, taking into account construction (34) for the associated systems, we deduce two characterizations for the Guichard nets under consideration.
In the following paragraphs, the functions h 1 , h 2 and h 3 fulfill the system (34) and therefore induce an associated system of the Guichard net.  Proof. Recall that a triply orthogonal system is x i -cyclic, where the x i -trajectories provide the circular direction, if and only if Thus, suppose that the associated systems are x i -cyclic and denote the Lamé coefficients of them byĤ c i = (h i + c)H i , where c ∈ R. Then, we obtain for c = 0 : which shows that the corresponding Guichard net is x i -cyclic. Therefore, we deduce from the circularity of the cyclic associated systems that As a consequence of the Corollaries 4.5 and 4.6, the coordinate surfaces Conversely, assume that the Guichard net has a family of parallel coordinate surfaces, then, by Corollary 4.5, the corresponding coordinate surfaces of the associated systems are also parallel and therefore the associated systems are cyclic.
In the other case, if the coordinate surfaces x i = const of the Guichard net are totally umbilic, then the Guichard net is a cyclic system (cf. [20, §4]) and, by Corollary 4.6, we obtain h i = h i (x j , x k ). Thus, the principal curvatures κ ji and κ ki do not depend on x i and therefore the associated systems are x i -cyclic.
Therefore, the associated systems of a Guichard net are triply orthogonal systems consisting of Dupin cyclides if and only if all coordinate surface families of the Guichard net are totally umbilic or parallel.

Bäcklund-type transformations of Guichard nets
Classically [2, §4], two triply orthogonal systems form a Ribaucour pair if any two corresponding coordinate surfaces are Ribaucour transformations of each other; that is, two corresponding surfaces envelop a common sphere congruence so that curvature lines correspond.
In this paper we are interested in Ribaucour transformations that preserve the Guichard condition of a Guichard net (cf. [2,3,23]).
To begin with, we summarize some facts about the Ribaucour transformation of triply orthogonal systems in general, which will then be used for the particular case of Guichard nets and their associated systems.
More details and results in this realm can be found in the two classical monographs [2] and [3].
where the data of the Ribaucour transform reads Here, N i denotes the unit normals of the coordinate surfaces x i = const of f . Note that, by Lamé's equations, for three given functions γ 1 , γ 2 and γ 3 satisfying (37) there exists a 1-parameter family of suitable functions ϕ, which define a 1-parameter family of Ribaucour transforms. Since the radii of the enveloped Ribaucour sphere congruences for the coordinate surface family x i = const are given by the 1-parameter family of functions ϕ indeed determines different Ribaucour transformations.
As observed by Bianchi in [2, §8], any Ribaucour transformation of a triply orthogonal system can be decomposed into two Combescure transformations and an inversion in the unit sphere. Since this construction will turn out to be a crucial tool in the transformation of Guichard nets, we recall it here in detail. Suppose f is a Ribaucour transformation of a triply orthogonal system f described by the functions γ 1 , γ 2 , γ 3 and ϕ. If we denote the unit normals of the coordinate surfaces x i = const of f by N i , then the function Moreover, the functionφ := 1 2 (A − 1) fulfills condition (37) with respect to the induced metricĪ and therefore the functions (γ 1 , γ 2 , γ 3 ,φ) determine a Ribaucour transformationf off :f Thus, this Ribaucour transformation is an inversion ι in the unit sphere and the induced metric off = (ι • C 1 )(f ) is given bȳ Since the metric coefficients ofĪ fulfill the transformed system (ι • C 1 )(f ) is a Combescure transformation of f and we indeed gain the sought-after decomposition of the Ribaucour transformation into two Combescure transformations and an inversion.
The constructed systems (f ,f ) will be called the induced Combescure transformations of the Ribauocur pair (f, f ).
Conversely, a Combescure transformed systemf of f induces a 1-parameter family of Ribaucour transforms of f : suppose thatf is an arbitrary Combescure transform of f and let N i denote the common unit normals of its coordinate surfaces x i = const. Then the functions γ i :=f · N i and a solution ϕ of the integrable system (39) where H i are the metric coefficients of f , define a Ribaucour transform f of f . Since the function ϕ is, by the equations (39), uniquely defined up to an additive constant, we obtain a 1-parameter family of induced Ribaucour transforms.
Furthermore, since the Ribaucour transformed rotational coefficients β ij only depend on the functions γ 1 , γ 2 and γ 3 , two Ribaucour transforms that are induced by the same systemf share the same rotational coefficients. Hence, we obtain the following lemma (cf. Definition 2.2): Proof. By [3, §11], we know that a Ribaucour pair (f, f ) of Guichard nets is generated by the data (γ 1 , γ 2 , γ 3 , ϕ) fulfilling Therefore, the Lamé coefficients (θ 1 , θ 2 , θ 3 ) of the induced Combescure transform satisfy the condition Moreover, taking into account the change of the induced metric after an inversion in the unit sphere, completes the proof.
Conversely, we can utilize this fact to construct Ribaucour pairs of Guichard nets and obtain in this way a Bäcklund-type transformation for Guichard nets: where ϕ := 1 α 2 {H 1H1 + H 2H2 − H 3H3 }. This Ribaucour transformation is said to be a Bäcklund-type transformation of f with respect to the systemf .
Proof. Letf be a Combescure related α 2 |f | 2 -system of the Guichard net f . Then, by using the differentiated Guichard condition, we conclude that the 1-parameter family of induced Ribaucour transformations of f are defined by the functions γ i :=f · N i and ϕ λ := 1 α 2 {H 1H1 + H 2H2 − H 3H3 } + λ, λ ∈ R, which satisfy the conditions (37). Therefore, these Ribaucour transforms are described by Moreover, the Lamé coefficients satisfy Thus, an induced Ribaucour transform determined by (44) is again a Guichard net if and only if λ = 0.
By Proposition 5.2 and Theorem 5.3, we have reduced the Bäcklund-type transformation for Guichard nets to the existence of Combescure transformed α 2 |f | 2systems of the Guichard nets. Thus, after an α 2 |f | 2 -system is determined, the Bäcklund-type transformation of any Combescure related Guichard net is obtained by simple computations from (43).
Analogous proofs as given for Proposition 5.2 and Theorem 5.3 lead to a Bäcklundtype transformation for the class of λ-systems, where λ ∈ R is a constant: where ϕ := 1 α 2 {H 1H1 + H 2H2 − H 3H3 }. Based on these observations, we prove a permutability theorem for the associated and dual systems of a Guichard net and the Bäcklund-type transformation: Proof. Firstly observe that, due to Lemma 5.1, the systems R(f ), R(f c ) and R(f c ) are again Combescure transformations of each other, because they are induced by the same α 2 |f | 2 -system. Furthermore, let us denote the Lamé coefficients of the Guichard net f and an associated systemf with the corresponding dual system f by (H i ) i , (Ĥ i ) i and (H i ) i , respectively. Thus, by Theorem 4.1, the Lamé coefficients satisfy and we obtain the following relations (47) These functions, together with the functions γ i = f · N i ,γ i =f · N i and γ i = f · N i , determine the induced Ribaucour transforms R(f ), R(f c ) and R(f c ) (see Theorem 5.3 and Corollary 5.4). A straightforward computation using the relations (47) then shows that the Lamé coefficients satisfy the conditions Thus, by Theorem 4.1, the systems R(f c ) and R(f c ) form an associated and a dual system of R(f ) and the claim is therefore proven.