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NEW EXAMPLES OF CONSTANT MEAN CURVATURE SURFACES IN S2×R AND H2×R JOSÉM.MANZANOANDFRANCISCOTORRALBO 2 Abstract. We construct non-zero constant mean curvature H surfaces 1 in the product spaces S2×R and H2×R by using suitable conjugate 0 Plateauconstructions. Theresultingsurfacesarecomplete,havebounded 2 height and are invariant under a discrete group of horizontal transla- n tions. InS2×R(foranyH>0)orH2×R(forH>1/2),a1-parameter a familyofunduloid-typesurfacesisobtained,someofwhichareshown J to be compact in S2×R. Finally, in the case of H = 1/2 in H2×R, 4 theconstructedexampleshavethesymmetriesofatessellationofH2by 2 regularpolygons. ] G D ntroduction 1. I . h t In 1970, Lawson [Law70] established its celebrated correspondence be- a cmc m tween constant mean curvature ( from now on) surfaces in space forms: there exists a correspondence between simply connected minimal surfaces [ in a space form M3(κ) (with constant curvature κ) and constant mean curva- 3 ture H surfaces in the space M3(κ−H2). The first application that he gave v 9 was the construction of two double periodic constant mean curvature one 5 surfaces in the Euclidean 3-space. The procedure used to construct such 2 1 examples is known as the conjugate Plateau construction an it was a fruitful . method to obtain new constant mean curvature surfaces in space forms 4 0 (see, for example, [KPS88, K89, GB93, Po94]). The main steps of this con- 1 struction are the following: 1 : v (1) Solve the Plateau problem in a geodesic polygon in M3(κ). Xi (2) Consider the conjugate cmc H surface in M3(κ−H2) (or the conju- r gate minimal one in M3(κ)), whose boundary lies on some planes a ofsymmetry,sincetheinitialsurfaceisboundedbygeodesiccurves (cf. [K89, Section 1]). (3) Reflecttheresultingsurfaceacrossitsedgestogetacomplete(often embedded) constant mean curvature H surface in M3(κ−H2). 2000MathematicsSubjectClassification. Primary53C42;Secondary53C30. Key words and phrases. Surfaces, minimal, constant mean curvature, homogeneous 3- manifolds,Bergerspheres,productspaces. ResearchpartiallysupportedbytheMCyT-FederresearchprojectMTM2007-61775,the JuntaAndalucíaGrantsP06-FQM-01642andP09-FQM-4496andthegenilresearchproject PYR-2010-21. 1 2 JOSÉM.MANZANOANDFRANCISCOTORRALBO The key property of this method is that a geodesic curvature line in the initial surface becomes a planar line of symmetry in the conjugate one. This is crucial in order to extend by reflection the conjugate piece to a complete constant mean curvature surface. cmc The main problem is, once the desired symmetries in the surface have been fixed, the choice of the appropriate geodesic polygons. For instance, for conjugate minimal surfaces in the sphere or the hyperbolic space the Plateau contour has to be determined by a degree argument from a 2-parameter family of solved Plateau problems (cf. [KPS88, Po94]). In the last few years the study of constant mean curvature surfaces in the homogeneous Riemannian 3-manifolds has become an active re- search topic (see, for example, [DHM09] for a survey on recent results). In 2007, Daniel [Dan07] established a Lawson-type correspondence between constant mean curvature surfaces in the homogeneous Riemannian 3-manifolds (see Section 2.1) that opens up the possibility of extend the conjugate Plateau construction to this broadest class of 3-manifolds. Pre- viously, Hauswirth, Sa Earp and Toubiana [HST08] extended the Lawson conjugate construction between minimal surfaces in the product case, and manypapershaveappearinthissetting(see,forexample,[MR,R,MRR]). This paper has a double aim: on the one hand to extend the conjugate Plateau construction to the homogeneous Riemannian 3-manifolds by using the Daniel correspondence and, on the other hand, to obtain, apply- ing this procedure, new constant mean curvature surfaces in the product spaces S2×R and H2×R, where S2 and H2 stand for the sphere and the hyperbolic plane with curvature one and minus one. Section 2 introduces the Daniel correspondence. We must ensure, in order to extend the conjugate Plateau construction, that the key property remains true in the homogeneous case. To do that, we have to restrict ourselves to polygons consisting only of vertical or horizontal geodesics and to the case where the phase angle θ of the Daniel correspondence is equaltoπ/2,sothetargetspaceisaRiemannianproductmanifold,which makes sense since they are the only homogeneous spaces which admit totally geodesic surfaces (cf. Lemmas 1 and 2). These conditions impose a rigidityrestrictionforthepossiblegeodesicpolygonstobeconsideredthat wasnotpresentinthespaceformcaseandwhichmakestheproblemmore subtle. We finish the section with a discussion of the smooth extension of the Plateau solution by reflection over its border. Section 3 gives a brief description of those homogeneous spaces in- volved in the construction, as well as its main properties. InSection4wepresentthefirstnon-trivialexamplesofconjugatePlateau constructionshowingthatthesphericalhelicoidsintheBergerspherescor- respond to the rotationally invariant unduloids and nodoids, constructed by [HH89, PR99] in S2×R and H2×R (cf. Proposition 1). NEW EXAMPLES OF CMC SURFACES IN S2×R AND H2×R 3 Section 5 deals with the construction of a 1-parameter family of complete singly periodic cmc H surfaces in the product spaces S2 ×R and H2 ×R (H > 1/2 in the latter case), coming from a minimal surface in a Berger sphere. In this case, the corresponding surfaces can be extended by reflection to a complete surface which is invariant by a discrete 1-parameter group of isometries, consisting of rotations in S2×R or hyperbolic translations in H2×R (cf. Theorem 1). The most interesting property of these new examples is that they are all bi-multigraphs (i.e. connected surfaces made out of two multigraphs, symmetric with respect to a horizontal slice and not neccesarily embedded) and have bounded height. Furthermore, for each value µ lying between the height of the rotationally invariant sphere and the height of the rotationally invariant torus in S2 ×R (resp. cylinder invariant under hyperbolic translations in H2×R), we construct a complete cmc H surface whose height is µ. Recall that the height of the mentioned torus (resp. cylinder)isahalfofthatofthecorrespondingsphere. Moreover, weshow that in the S2×R case, there exist many compact examples for each H. Finally, Section 6 is devoted to the construction of constant mean curvature 1/2 bi-multigraphs in H2 ×R which have the symmetries of a tessellation of H2 by regular polygons (cf. Theorem 2). These surfaces come from minimal surfaces in the Heisenberg group. Recall that the value H = 1/2 is critical in the sense that cmc surfaces for H > 1/2 and H ≤ 1/2 are of different nature (e.g. cmc spheres only exist for H > 1/2). cmc Besides,wegivesomeapplicationstotheconstructionof 1/2surfaces in M×R,where M isacompactsurfacewithnegativeEulercharacteristic and constant curvature −1. The authors wish to thank professors Joaquín Pérez, Antonio Ros and Francisco Urbano for helpful discussions and for its valuable comments and suggestions during the preparation of this paper. reliminaries on homogeneous manifolds 2. P 3- ThehomogeneousRiemanniansimplyconnected3-manifoldswithisom- etry group of dimension four can be parametrized by two real numbers κ andτ andtheyareoftendenotedbyE(κ,τ)intheliterature(see[Dan07]). Moreover, every E(κ,τ) has a fibration over a simply connected constant curvature κ surface whose vertical field ξ is Killing and τ represents the bundle curvature. In particular, E((cid:101),0) are the product spaces S2×R or H2×R if (cid:101) = 1 or (cid:101) = −1, respectively. We will say that a geodesic in E(κ,τ) is horizontal if its tangent vector is orthogonal to ξ and vertical if its tangent vector is co-linear with ξ. For our purposes, the following property will be essential: Givenaverticalorhorizontalgeodesic,thereexistsauniqueinvo- lutive isometry of E(κ,τ) which fixes each point in the geodesic. 4 JOSÉM.MANZANOANDFRANCISCOTORRALBO Thisisometrywillbecalledageodesic reflectionwithrespectto the geodesic. 2.1. TheDanielcorrespondence. Aclassicaltoolinthestudyofcmcsur- faces in space forms is the Lawson correspondence, which establishes an isometric one-to-one local correspondence between constant mean curvature surfaces in different space forms (cf. [Law70, Section 14]). This correspondence was generalized by Daniel [Dan07, Theorem 5.2] to the context of the E(κ,τ) spaces. More explicitly, given E = E(κ,τ) and E∗ = E(κ∗,τ∗), such that κ− 4τ2 = κ∗ −4(τ∗)2 and given θ,H,H∗ ∈ R satisfying H+iτ = eiθ(H∗ + iτ∗), the following statement holds: Let φ : Σ → E an isometric constant mean curvature H immersion of a simply connected surface Σ. There exists a isometric immersion φ∗ : Σ → E∗ of cmc H∗ such that: (a) ν∗ = ν (b) T∗ = eθJT. (c) S∗ = eθJ(S−H·id)+H∗·id where eθJ, ν = (cid:104)N,ξ(cid:105), T = ξ −νN and S are the positive ori- ented rotation of angle θ in the tangent plane to Σ, the angle function, the tangent part of the vertical field and the shape operator for φ, and N is a unit normal vector field to the immersion. The elements ν∗, T∗ and S∗ are the corresponding ones for φ∗. The immersion φ∗ is unique up to an ambient isometry in E∗ and is called a sister immersion of φ. Aswepointedoutintheintroduction,weareinterestedinapplyingthat correspondence between a minimal surface in some E(κ,τ) and a cmc H surface in M2((cid:101))×R = E((cid:101),0), so the parameter θ must be π (there is 2 no loss of generality in considering θ to be positive) and some additional properties appear (see Lemma 1 below). Hence, κ must be 4H2+(cid:101) and τ must be equal to H, and this yields the relations (2.1) ν∗ = ν, T∗ = JT, S∗ = JS+H·id, between the sister surfaces. In addition, we obtain the following scheme of possible configurations: Initial E(κ,τ) Surfaces in H2×R Surfaces in S2×R κ > 0 E(4H2+(cid:101),H) cmc H > 1/2 cmc H > 0 κ = 0 E(0,1/2) cmc H = 1/2 - κ < 0 E(4H2−1,H) cmc 0 < H < 1/2 - as well as, for H = τ = 0, the families of associate minimal surfaces in S2×R or H2×R (notice that θ is free in that case). NEW EXAMPLES OF CMC SURFACES IN S2×R AND H2×R 5 The following result will allow us to precise what the horizontal and vertical geodesicscontained ina minimal surfacebecome in thesister surface for the cases in the table above. Observe that the choice θ = π is 2 fundamental in the proof. Lemma 1. Given (cid:101) ∈ {−1,1} and H ≥ 0, let φ : Σ (cid:35) E(4H2 +(cid:101),H) be a isometric minimal immersion of a simply connected Riemannian surface Σ and suppose φ∗ : Σ (cid:35) M2((cid:101))×R is its sister cmc H immersion. Given a smooth curve α : [a,b] → Σ, (a) if φ(α) is a horizontal geodesic, then φ∗(α) is contained in a vertical plane, which the immersion meets orthogonally. (b) If φ(α) is a vertical geodesic, then φ∗(α) is contained in a horizontal plane, which the immersion meets orthogonally. Proof. The first part of this lemma was proved by Torralbo (cf. [Tor10a, Proposition 3]) but, for completeness, we include the complete proof. We will follow the above notation and consider γ = φ(α) and γ∗ = φ∗(α) = (β,h) ⊆ Σ ⊆ M2((cid:101))×R, where there is no loss of generality in considering α tobeparametrizedbyitsarclength. Moreover, itispossible to immerse isometrically M2((cid:101))×R in ⊆ R3 ×R if (cid:101) = 1 or R3 ×R if 1 (cid:101) = −1 with unit normal along γ given by (β,0). We will start by proving item (a). We claim that Jγ(cid:48) is constant, where ∗ Jγ(cid:48) is considered to be a curve in R4 or R4. ∗ 1 (cid:10)(Jγ∗(cid:48))(cid:48),γ∗(cid:48)(cid:11) = −(cid:10)Jγ(cid:48),∇γ(cid:48)γ(cid:48)(cid:11) = 0, (as γ is a geodesic) (cid:10)(Jγ(cid:48))(cid:48),Jγ(cid:48)(cid:11) = 0, (as Jγ(cid:48) has length 1) ∗ ∗ ∗ (cid:10)(Jγ(cid:48))(cid:48),N∗(cid:11) = −(cid:10)Jγ(cid:48), dN∗(γ(cid:48))(cid:11) = (cid:10)Jγ(cid:48),S∗γ(cid:48)(cid:11) = (cid:10)Jγ(cid:48),−Sγ(cid:48)+τγ(cid:48)(cid:11) = ∗ ∗ ∗ ∗ ∗ = −(cid:10)γ(cid:48),Sγ(cid:48)(cid:11) = 0, (as γ is an asymptote line) where we take into account the relation (2.1). Thus, the tangent part of (Jγ(cid:48))(cid:48) to M2((cid:101))×R vanishes, so (Jγ(cid:48))(cid:48) is proportional to (β,0). On the ∗ ∗ other hand, as γ is an horizontal curve in E(4H2 +(cid:101),H), we know that (cid:104)γ(cid:48),ξ(cid:105) = (cid:104)γ(cid:48),T(cid:105) = 0 so γ(cid:48) is proportional to JT and, since it has unit √ length, we can suppose that, up to a sign, γ(cid:48) = JT/ 1−ν2. Therefore, √ √ T∗ = γ(cid:48) 1−ν2 and (0,1) = T∗+νN∗ = 1−ν2γ(cid:48) +νN∗. ∗ ∗ This last relation implies that (cid:104)Jγ(cid:48),(0,1)(cid:105) = 0. Hence, ∗ (cid:10)(Jγ(cid:48))(cid:48),(β,0)(cid:11) = −(cid:10)Jγ(cid:48),(β(cid:48),0)(cid:11) = h(cid:48)(cid:10)Jγ(cid:48),(0,1)(cid:11) = 0, ∗ ∗ ∗ where we have used that 0 = (cid:104)Jγ(cid:48),γ(cid:48)(cid:105) = (cid:104)Jγ(cid:48),(β(cid:48),0)(cid:105)+(cid:104)Jγ(cid:48),h(cid:48)(0,1)(cid:105), ∗ ∗ ∗ ∗ and the claim is proved. In fact, we have proved that Jγ(cid:48) = (v,0) ∈ R3 ×R for some fixed ∗ v ∈ TM2((cid:101)) ⊂ R3. Taking this into account, (cid:104)γ∗,(v,0)(cid:105)(cid:48) = (cid:10)γ∗(cid:48),(v,0)(cid:11) = (cid:10)γ∗(cid:48),Jγ∗(cid:48)(cid:11) = 0 which implies that (cid:104)γ∗,(v,0)(cid:105) is constant, but (cid:104)γ∗,(v,0)(cid:105) = (cid:104)β,v(cid:105) = 0 as β is normal to M2((cid:101)) and v is tangent. 6 JOSÉM.MANZANOANDFRANCISCOTORRALBO All this information says that γ∗ lies in the vertical plane P = {(p,t) ∈ M2((cid:101))×R : (cid:104)p,v(cid:105) = 0}. Moreover, the immersion φ∗ is orthogonal to P since the tangent plane along γ∗ is spanned by {γ∗(cid:48),Jγ∗(cid:48) = (v,0)}. Let us now prove item (b). Observe first that, if γ is a vertical geodesic, then ν = 0 along it, so ξ∗ = T∗+νN∗ = T∗ along γ∗. Thus, (cid:10)γ(cid:48),ξ∗(cid:11) = (cid:10)γ(cid:48),T∗(cid:11) = (cid:10)γ(cid:48),JT(cid:11) = 0. ∗ ∗ The last equality follows from the fact that γ(cid:48) is vertical whereas JT is horizontal (T is vertical along γ since γ is a vertical curve contained in the surface). Finally, notice that 0 = (cid:104)γ(cid:48),ξ∗(cid:105) = (cid:104)(β(cid:48),h(cid:48)),(0,1)(cid:105) = h(cid:48), so h ∗ is constant along γ∗, i.e. this curve is contained in a horizontal slice. As ν = 0 along γ∗ the surface intersects that slice orthogonally. (cid:3) 2.2. Smooth extension of surfaces bordered by geodesics. Let Σ be a minimal surface immersed in E(κ,τ) and γ a vertical or horizontal geodesic of E(κ,τ) contained in ∂Σ. Then, it is possible to extend Σ by geodesic reflection around γ (we recall that every geodesic reflection, when- ever the geodesic is horizontal or vertical, is an isometry of E(κ,τ)). This extension is smooth in view of [DHKW92, Theorem 4]. Moreover, taking into account Lemma 1 and the data (ν,T,S) of a minimal surface invariant with respect to a vertical or horizontal geodesic reflection, it is easy to prove the following result that establishes the behaviour of this symmetries with respect to the Daniel correspondence. Lemma 2. Let φ : Σ (cid:35) E(4H2 +(cid:101),H) be a minimal immersion of a simply connected surface and φ∗ : Σ (cid:35) M2((cid:101))×R its sister immersion. Then: (i) If φ(Σ) is invariant by a horizontal (resp. vertical) geodesic reflection then φ∗(Σ) is invariant by a reflection over a vertical (resp. horizontal) plane. (ii) The axis of reflection in the original surface corresponds to the curve where the sister immersion meets de plane of reflection. Σ Anotherinterestingsituationthatwilloftenappeariswhen∂ contains two different vertical or horizontal geodesics meeting at some point p ∈ Σ ∂ . Then, the surface can be extended by reflection over both geodesics producing, in each step, a new vertical or horizontal geodesic passing through p. If the angle between the two geodesics is π for some k ∈ k N, then a surface is produced after 2k reflections, which is smooth in every point except possibly at p. Nevertheless, if such a surface is locally embedded around the point p then, thanks to the removable singularity result [CS85, Proposition 1], it will be also smooth at p. odels for homogeneous spaces 3. M For our purposes we will restrict ourselves to the construction of minimal surfaces in E(κ,τ) spaces where τ (cid:54)= 0. In what follows we will only consider the cases where κ ≥ 0 (i.e. either the Berger spheres or the NEW EXAMPLES OF CMC SURFACES IN S2×R AND H2×R 7 Heisenberg group) so cmc H > 0 in S2×R and H ≥ 1/2 in H2×R will be produced. In this section we will introduce briefly the aforementioned homogeneous spaces, focusing on the properties needed in the paper. 3.1. The Berger spheres. A Berger sphere is a 3-sphere S3 = {(z,w) ∈ C2 : |z|2+|w|2 = 1} endowed with the metric 4 (cid:20) (cid:18)4τ2 (cid:19) (cid:21) g(X,Y) = (cid:104)X,Y(cid:105)+ −1 (cid:104)X,V(cid:105)(cid:104)Y,V(cid:105) , κ κ where (cid:104), (cid:105) stands for the usual metric on the sphere, V = J(z,w) = (z,w) (iz,iw), for each (z,w) ∈ S3 and κ, τ are real numbers with κ > 0 and τ (cid:54)= 0. From now on, we will denote the Berger sphere (S3,g) as S3(κ,τ). b We note that if κ = 4τ2 then S3(κ,τ) is, up to homotheties, the round b sphere. The Berger spheres are examples of E(κ,τ) for κ > 0 and τ (cid:54)= 0 (cf. [Tor10b] for a detailed description). The Hopf fibration Π : S3(κ,τ) → S2(κ), where S2(κ) stands for the √ b 2-sphere of radius 1/ κ, given by (cid:18) (cid:19) 2 1 Π(z,w) = √ zw¯, (|z|2−|w|2) , κ 2 is a Riemannian submersion whose fibers are geodesics. The vertical unit Killingfieldisgivenbyξ = κ V. Itiseasytocheckthatboththehorizontal 4τ and vertical geodesic are great circles. It is interesting to remark that the length of every vertical geodesic is 8τπ/κ, whereas the length of every √ horizontal geodesic is 4π/ κ. 3.2. The product spaces. As it has been pointed out before, the only homogeneous spaces with isometry group of dimension four and zero bundle curvature are the Riemannian products M2(κ) × R = E(κ,0), where M2(κ)standsforthesimplyconnectedsurfacewithconstantcurva- ture κ. The Riemannian submersion coincides with the natural projection Π : M2(κ)×R → M2(κ). Totally geodesic surfaces of M2(κ)×R are either vertical planes, i.e. the product of a geodesic of M2(κ) with the real line (they are topological cylinders if κ > 0 and planes if κ < 0) or horizontal planes (also called slices), i.e. M2(κ)×{t }, t ∈ R. It is well known that the reflection over a 0 0 horizontal or vertical plane is an ambient isometry. cmc Σ If a surface meets a horizontal or vertical plane orthogonally, we can smoothly extend this surface by reflecting over this plane. This is a consequence of the continuation result of Aronszajn [Aron57] for elliptic pde ’s joint with the fact that the reflection over horizontal and vertical planes are ambient isometries. 3.3. The Heisenberg group. The Heisenberg group Nil is a Lie group 3 whose Lie algebra consists of the upper-triangular nilpotent 3×3 real 8 JOSÉM.MANZANOANDFRANCISCOTORRALBO matrices. It can be modelled by R3, endowed with the metric ds2 = dx2+dy2+(cid:0)1(ydx−xdy)+dz(cid:1)2, 2 where (x,y,z) are the usual coordinates of R3. It corresponds to κ = 0, τ = 1 in the description of E(κ,τ). The projection Π : Nil → R2 given 2 3 by Π(x,y,z) = (x,y) is a Riemannian submersion and ∂ is a unit vertical z Killing vector field. AllverticalandhorizontalgeodesicsinNil areEuclideanstraightlines, 3 not necessarily linearly parametrized. Moreover, every non-vertical Eu- clideanplaneisminimalandanytwoofthemarecongruentbyanambient isometry. Vertical Euclidean planes are also minimal in Nil . 3 pherical helicoids and their correspondent sister surfaces 4. S The previous section helped us to understand the Daniel correspondence between minimal surfaces in the Berger spheres or the Heisenberg group and cmc surfaces in M2((cid:101))×R. In this section we are going to il- lustratetheconstructionmethod. Forthatpurpose,wefocusontheBerger sphere case, i.e. we are going to work in S3(4H2+(cid:101),H) and analyse what b the correspondent to the so-called spherical helicoids are. The latter form a 1-parameter family of well known minimal immersions in the round 3-sphere, introduced by Lawson in [Law70] and given by Φ : R2 → S3 ⊂ C2 c (x,y) (cid:55)→ (cid:0)cos(x)eicy,sin(x)eiy(cid:1). All these immersions are minimal in S3(κ,τ), for any κ and τ. In fact, b theyaretheonlyimmersionsinthe3-spherethatareminimalwithrespect to all the Berger metrics (cf. [Tor10a, Proposition 1]). Remark 1. (1) We can restrict the parameter c to the interval [−1,1] since the sur- Φ Φ faces and are congruent up to a reparametrization, i.e. 1/c c (L◦Φ )(π −x,cy) = Φ (x,y), where L(z,w) = (w,z). 1/c 2 c Φ Φ (2) is the minimal sphere, is the Clifford torus, and they are 0 1 the only embedded surfaces of the family. Observe that a Clifford torus is nothing but the lift by the Hopf projection of a geodesic in S2(κ). Moreover, given a point p ∈ S3(κ,τ) and a horizontal vector b u at p there exist a unique Clifford torus passing through p with tangent plane at p orthogonal to u. (3) Forevery c,thesurface Φ isinvariantbythe1-parametergroupof c (cid:16) (cid:17) isometries t → eict 0 . 0 eit NEW EXAMPLES OF CMC SURFACES IN S2×R AND H2×R 9 Consider now the polygon Λ , c (cid:54)= −1 (the case c = −1 will be treated c in the following section), consisting of the curves (see Figure 1): h (t) = (cid:0)cos(t),sin(t)(cid:1) = Φ (t,0), t ∈ (cid:2)0, π(cid:3), 1 c 2 (cid:16) (cid:17) h2(t) = (cid:0)cos(t)e2(i1π+cc),sin(t)e2(1iπ+c)(cid:1) = Φc t, 2(1π+c) , t ∈ (cid:2)0, π2(cid:3), (cid:104) (cid:105) v (t) = (eict,0) = Φ (0,t), t ∈ 0, π , 1 c 2(1+c) (cid:104) (cid:105) v (t) = (0,eit) = Φ (cid:0)π,t(cid:1), t ∈ 0, π . 2 c 2 2(1+c) It is easy to check that, for every H, the curve h (t) = Φ (t,θ), θ ∈ θ c [0, π ], is a horizontal geodesic and v , v are vertical ones. More- 2(1+c) 1 2 over, we can recover the whole surface Φ (R2) by geodesic reflection of c the piece Φ ([0, π]×[0, π ]) over the edges of Λ . c 2 2(1+c) c Considernowthesisterimmersion Φ∗ : [0, π]×[0, π ] → M2((cid:101))×R c 2 2(1+c) and denote by h∗ and v∗, j = 1,2, the corresponding curves. In view of θ j Lemma 1, h∗ are contained in a vertical plane of symmetry, P , while v∗ is θ θ j contained in a slice M2((cid:101))×{p }, j = 1,2. j In order to know the behaviour of these curves, one can compute their curvature as curves in the vertical or horizontal plane where they lie. To do that, let us observe firstly that, since the sister surface intersects the slice and the vertical plane where the curves h∗ and v∗ meet orthogonally, j j the curvatures of these two curves (supposed to be parametrized by arc length) are given by kM2((cid:101))×{pj} = (cid:68)S∗(v∗)(cid:48),(v∗)(cid:48)(cid:69) = H−(cid:68)Sv(cid:48),Jv(cid:48)(cid:69), v∗ j j j j j kPθ = (cid:10)S∗(h∗)(cid:48),(h∗)(cid:48)(cid:11) = H−(cid:10)Sh(cid:48),Jh(cid:48)(cid:11), h∗ θ θ θ θ θ where S∗ is the shape operator of the sister immersion Φ∗ and the second c equality holds since the shape operator S of Φ its related with S∗ by c S∗ = JS+H·Id [cf. (2.1)]. Φ Finally, as we know explicitlythe shape operator of , straightforward c computationsshowthat v∗ areconstantcurvaturecurvesin M2((cid:101))×{p }, j j j = 1,2. On the other hand, all the curves h∗ has the same curvature since θ Φ the immersion is invariant by a 1-parameter group of isometries that c transform each h into another h . θ1 θ2 Hence, foreverypointof v∗ thereexistsaverticalplaneofsymmetryso j the sister surface must be rotationally invariant. Proposition 1. The sister surface of the spherical helicoid Φ is a rotationally c invariant surface. More precisely, (i) The sister surface of the minimal sphere (i.e. Φ ) is the constant mean cur- 0 vature H sphere. (ii) The sister surface of the Clifford torus (i.e. Φ ) is the vertical cylinder, i.e. 1 the product of a constant curvature 2H curve of M2((cid:101)) with the real line. 10 JOSÉM.MANZANOANDFRANCISCOTORRALBO (iii) The sister surface of Φ for 0 < c < 1 is an unduloid (cf. [PR99, Lemma c 1.3]). (iv) The sister surface of Φ for −1 < c < 0 is a nodoid (cf. [PR99, Lemma c 1.3]). Figure 1. Polygon Λ (c (cid:54)= −1) in the Berger sphere (left) c and its sister contour in M2((cid:101))×R (right) for c > 0 (solid line) and c < 0 (dashed line) Proof. On the one hand, the previous argument shows that the sister surface of Φ must be a rotationally invariant cmc H surface in M2((cid:101))×R. c On the other hand, the first assertion is trivial and the second one is easy because the Clifford torus has vanishing constant angle which remains invariant under the Daniel correspondence. Finally, it remains to prove (iii) and(iv). Butthisisaconsequenceofadeepanalysisofthecurvatureofthe (cid:3) curves h , which will be omitted since it is long and straightforward. θ Remark2. Thepreviousargumentshowsthateveryminimalsurfacewhich is ruled by horizontal geodesics becomes, via the Daniel correspondence, cmc a surface invariant by a 1-parameter group of isometries. In the round sphere case, the corresponding surfaces, via the Lawson cmc correspondence, to the spherical helicoids are the Delaunay rotationally invariant examples of R3. This was proved in [GB93, Theorem 2.1]. 5. Constant mean curvature surfaces in S2×R and H2×R In this section we will construct, for each H > 0 (resp. H > 1/2), a 1- parameter family of constant mean curvature H surfaces in S2×R (resp. H2×R). To do that we first build a 1-parameter family of minimal surfaces Σ in the Berger sphere S3(4H2 +(cid:101),H), (cid:101) ∈ {−1,1}, solving the λ b Γ Plateau problem over an appropriate geodesic polygon (see figure 2). λ The desired surfaces will be the corresponding cmc H surfaces.