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| [[File:Nodoid.png|thumb|Nodoid surface, a surface with constant mean curvature.]]
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| In [[differential geometry]], '''constant-mean-curvature (CMC) surfaces''' are surfaces with constant [[mean curvature]].<ref>Nick Korevaar, Jesse Ratkin, Nat Smale, Andrejs Treibergs, A survey of the classical theory of constant mean curvature surfaces in R3, 2002 [http://www.math.utah.edu/~ratzkin/papers/minicourse.pdf]</ref><ref name="lejdfors" /> This includes [[minimal surfaces]] as a subset, but typically they are treated as special case.
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| Note that these surfaces are generally different from constant [[Gaussian curvature]] surfaces, with the important exception of the [[sphere]].
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| ==History==
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| In 1841 [[Charles-Eugène Delaunay|Delaunay]] proved that the only [[surfaces of revolution]] with constant mean curvature were the surfaces obtained by rotating the [[Roulette (curve)|roulettes]] of the conics. These are the plane, cylinder, sphere, the [[catenoid]], the [[unduloid]] and [[nodoid]].<ref>C. Delaunay, Sur la surface de révolution dont la courbure moyenne est constante, J. Math. Pures Appl., 6 (1841), 309–320.</ref>
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| In 1853 J. H. Jellet showed that if <math>S</math> is a compact star-shaped surface in <math>\R^3</math> with constant mean curvature, then it is the standard sphere.<ref>J. H. Jellet, Sur la Surface dont la Courbure Moyenne est Constant, J. Math. Pures Appl., 18 (1853), 163–167</ref> Subsequently [[Aleksandr Danilovich Aleksandrov|A. D. Alexandrov]] proved that a compact embedded surface in <math>\R^3</math> with constant mean curvature <math>H \neq 0</math> must be a sphere.<ref>A. D. Alexandrov, Uniqueness theorem for surfaces in the large, V. Vestnik, Leningrad Univ. 13, 19 (1958), 5–8, Amer. Math. Soc. Trans. (Series 2) 21, 412–416.</ref> Based on this [[Heinz Hopf|H. Hopf]] conjectured in 1956 that any immersed compact orientable constant mean curvature hypersurface in <math>\R^n</math>must be a standard embedded <math>n-1</math> sphere. This conjecture was disproven in 1982 by Wu-Yi Hsiang using a counterexample in <math>\R^4</math>. In 1984 [[Henry C. Wente]] constructed the [[Wente torus]], an immersion into <math>\R^3</math> of a [[torus]] with constant mean curvature.
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| <ref>{{Citation | last1=Wente | first1=Henry C. | authorlink = Henry C. Wente | title=Counterexample to a conjecture of H. Hopf. | url=http://projecteuclid.org/euclid.pjm/1102702809 | year=1986 | journal=Pacific Journal of Mathematics | volume=121 | pages=193–243}}.</ref>
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| Up until this point it had seemed that CMC surfaces were rare; new techniques produced a plethora of examples.<ref name="coplanar">Karsten Grosse-Brauckmann, Robert B. Kusner, John M. Sullivan. Coplanar constant mean curvature surfaces. Comm. Anal. Geom. 15:5 (2008) pp. 985–1023. ArXiv math.DG/0509210. [http://arxiv.org/abs/math.DG/0509210]</ref> In particular gluing methods appear to allow combining CMC surfaces fairly arbitrarily.<ref>N. Kapouleas. Complete constant mean curvature surfaces in Euclidean three space, Ann. of. Math. (2) 131 (1990), 239–330</ref><ref>Rafe Mazzeo, Daniel Pollack, Gluing and Moduli for Noncompact Geometric Problems. 1996 arXiv:dg-ga/9601008 [http://arxiv.org/abs/dg-ga/9601008]</ref> Delaunay surfaces can also be combined with immersed "bubbles", retaining their CMC properties.<ref>I. Sterling and H. C. Wente, Existence and classification of constant mean curvature multibubbletons of finite and infinite type, Indiana Univ. Math. J. 42 (1993), no. 4, 1239–1266.</ref>
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| {{multiple image
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| | width = 150
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| | footer = Triunduloids with different neck sizes. As neck sizes are varied the asymptotic directions change.
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| | image1 = Triunduloid.png
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| | alt1 = Triunduloid
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| | caption1 = Equal neck sizes
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| | image2 = Asymmetric trinoid.png
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| | alt2 = Unequal neck size triunduloid
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| | caption2 = Unequal neck sizes
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| | image3 = Triunduloid with nodoid end.png
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| | alt3 = Triunduloid with nodoid end
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| | caption3 = With nodoid end
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| }}
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| Meeks showed that there are no embedded CMC surfaces with just one end in <math>\R^3</math>.<ref>Meeks W. H., The topology and geometry of embedded surfaces of constant mean curvature, J. Diff. Geom. 27 (1988) 539–552.</ref> Korevaar, Kusner and Solomon proved that a complete embedded CMC surface will have ends asymptotic to unduloids.<ref>Korevaar N., Kusner R., Solomon B., The structure of complete embedded surfaces with constant mean curvature, J. Diff. Geom. 30 (1989) 465–503.</ref> Each end carries a <math>n(2\pi-n)</math> "force" along the asymptotic axis of the unduloid (where n is the circumference of the necks), the sum of which must be balanced for the surface to exist. Current work involves classification of families of embedded CMC surfaces in terms of their [[moduli space]]s.<ref>John M. Sullivan, A Complete Family of CMC Surfaces. In Integrable Systems, Geometry and Visualization, 2005, pp 237–245. [http://torus.math.uiuc.edu/jms/Papers/triund/jms-fuk.pdf]</ref> In particular, for <math>k \geq 3</math> coplanar ''k''-unduloids of genus 0 satisfy <math>\sum_{i=1}^k n_i \leq (k-1)\pi</math> for odd ''k'', and <math>\sum_{i=1}^k n_i \leq k\pi</math> for even ''k''. At most ''k'' − 2 ends can be cylindrical.<ref name="coplanar" />
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| ==Generation methods==
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| ===Representation formula===
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| Like for minimal surfaces, there exist a close link to harmonic functions. An oriented surface <math>S</math> in <math>\R^3</math> has constant mean curvature if and only if its [[Gauss map]] is a [[harmonic function]].<ref name="lejdfors">Carl Johan Lejdfors, Surfaces of Constant Mean Curvature. Master’s thesis Lund University, Centre for Mathematical Sciences Mathematics 2003:E11 [http://www.matematik.lu.se/matematiklu/personal/sigma/students/Karl-Johan-Lejdfors-MSc.pdf]</ref> Kenmotsu’s representation formula<ref>K. Kenmotsu, Weierstrass Formula for Surfaces of Prescribed Mean Curvature, Math. Ann., 245 (1979), 89–99</ref> is the counterpart to the [[Weierstrass–Enneper parameterization]] of minimal surfaces:
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| Let <math>V</math> be an open simply connected subset of <math>\C</math> and <math>H</math> be an arbitrary non-zero real constant. Suppose <math>\phi: V \rightarrow \C</math> is a harmonic function into the Riemann sphere. If <math>\phi_{\bar z} \neq 0</math> then <math>X : V \rightarrow R</math> defined by
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| :<math>X(z) = \Re \int_{z_0}^z X_z(z)\,dz</math>
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| with | |
| :<math>X_z(z)=\frac{-1}{H(1+\phi(z)\bar\phi(z))^2} \left \{(1-\phi(z)^2, i(1+\phi(z)^2), 2\phi(z)) \frac{\bar{\partial\phi}}{\partial \bar z}(z) \right \}</math>
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| for <math>z \in V</math> is a regular surface having <math>\phi</math> as Gauss map and mean curvature <math>H</math>.
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| For <math>\phi(z)=-1/\bar z</math> and <math>H=1</math> this produces the sphere. <math>\phi(z)=-e^{iz}</math> and <math>H=1/2</math> gives a cylinder.
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| ===Conjugate cousin method===
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| Lawson showed 1970 that each CMC surface in <math>\R^3</math>has an isometric "cousin" minimal surface in <math>\mathbb{S}^3</math>.<ref>Lawson H.B., “Complete minimal surfaces in S3”, Annals of Math. 92 (1970) 335–374.</ref><ref>Karsten Grosse-Brauckmann, Robert B Kusner, John M Sullivan. Triunduloids: Embedded constant mean curvature surfaces with three ends and genus zero. J. Reine Angew. Math, 564, pp. 35–61 2001 arXiv:math/0102183v2 [http://arxiv.org/abs/math/0102183v2]</ref> This allows constructions starting from geodesic polygons in <math>\mathbb{S}^3</math>, which are spanned by a minimal patch that can be extended into a complete surface by reflection, and then turned into a CMC surface.
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| ===Discrete numerical methods===
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| [[Discrete differential geometry]] can be used to produce approximations to CMC surfaces (or discrete counterparts), typically by minimizing a suitable energy functional.<ref>Smith, J. 2003. Three Applications of Optimization in Computer Graphics. PhD thesis, Robotics Institute, Carnegie Mellon University, Pittsburgh, PA [http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.10.1682]</ref><ref>Hao Pan, Yi-King Choi, Yang Liu, Wenchao Hu, Qiang Du, Konrad Polthier, Caiming Zhang, Wenping Wang, Robust modeling of constant mean curvature surfaces. ACM Transactions on Graphics (TOG) – SIGGRAPH 2012 Conference Proceedings. Volume 31 Issue 4, July 2012 Article No. 85</ref>
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| ==Applications==
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| CMC surfaces are natural for representations of [[soap bubbles]], since they have the [[Young–Laplace equation|curvature corresponding to a nonzero pressure difference]]. In the [[Weaire–Phelan structure]] CMC surfaces form the walls of the bubbles.
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| Besides macroscopic bubble surfaces CMC surfaces are relevant for the shape of the gas-liquid interface on a [[superhydrophobic]] surface.<ref>E.J. Lobaton, T.R. Salamon. Computation of constant mean curvature surfaces: Application to the gas-liquid interface of a pressurized fluid on a superhydrophobic surface. Journal of Colloid and Interface Science. Volume 314, Issue 1, 1 October 2007, Pages 184–198</ref>
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| Like [[triply periodic minimal surface]]s there has been interest in periodic CMC surfaces as models for [[block copolymers]] where the different components have a nonzero interfacial energy or tension. CMC analogs to the periodic minimal surfaces have been constructed, producing unequal partitions of space.<ref>D. M. Anderson, H. T. Davis, L. E. Scriven, J. C. C. Nitsche, Periodic Surfaces of Prescribed Mean Curvature in Advances in Chemical Physics vol 77, eds. I. Prigogine and S. A. Rice, John Wiley & Sons, 2007 , p. 337–396</ref><ref>Meinhard Wohlgemuth, Nataliya Yufa, James Hoffman, and Edwin L. Thomas. Triply Periodic Bicontinuous Cubic Microdomain Morphologies by Symmetries. Macromolecules 2001, 34, 6083–6089 [https://secure.msri.org/about/sgp/jim/papers/morphbysymmetry/text/levelset.pdf]</ref> CMC structures have been observed in ABC triblock copolymers.<ref>Samuel P. Gido , Dwight W. Schwark , Edwin L. Thomas , Maria do Carmo Goncalves, Observation of a non-constant mean curvature interface in an ABC triblock copolymer, Macromolecules, 1993, 26 (10), pp 2636–2640</ref>
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| In architecture CMC surfaces are relevant for [[air-supported structures]] such as inflatable domes and enclosures, as well as a source of flowing organic shapes.<ref>Helmut Pottmann, Yang Liu, Johannes Wallner, Alexander Bobenko, Wenping Wang. Geometry of Multi-layer Freeform Structures for Architecture. ACM Transactions on Graphics (TOG) – Proceedings of ACM SIGGRAPH 2007 Volume 26 Issue 3, July 2007 Article No. 65 [http://www.dmg.tuwien.ac.at/pottmann/2007/plwbw_freeform_07/paper_docs/freeform.pdf]</ref>
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| ==See also==
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| * [[Double bubble conjecture]]
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| * [[Minimal surface]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| * CMC surfaces at the Scientific Graphics Project [https://secure.msri.org/about/sgp/jim/geom/cmc/index.html]
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| * GeometrieWerkstatt surface gallery [http://www.mathematik.uni-tuebingen.de/ab/Differentialgeometrie/gallery/index.html]
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| * GANG gallery of CMC surfaces [http://www.gang.umass.edu/gallery/cmc/]
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| * Noid, software for computing ''n''-noid CMC surfaces [http://www-sfb288.math.tu-berlin.de/~nick/Noid/]
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| [[Category:Differential geometry of surfaces]]
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