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Sunday, 7 May 2017

Surfaces of revolution with constant mean curvature (I)

We calculate the equation of a surface of revolution with constant mean curvature H. Without loss of generality, we suppose that the profile curve is a planar curve in the xz-plane and that the z-axis is the rotational axis. Also, suppose that the curve is a graph on the z-axis, that is, a parametrization of the profile curve is (f(z),0,z), zI. Then a parametrization of the rotational surface is X(t,s)=(f(t)cos(s),f(t)sin(s),t), tI,s[0,2π]. Thus H satisfies 
f(1+f2)3/2+1f1+f2=2H.
The key of this equation is that because H is constant, it is possible to obtain a first integral of this equation (which is of second order). Indeed, multiplying by ff we have fff(1+f2)3/2+f1+f2=2Hff,which can be written as 
(f1+f2)=(Hf2). Therefore there exists cR such that 
f(z)1+f(z)2=Hf(z)2+c.
For example, the sphere and the cylinder can be obtained from (*). For the sphere, take c=0. Then we have 
f=1H1f2H2,
or
f1f2H2=1H. By integrating, we have
1H21H2f2=1Hx.
Definitively, f(z)=1H2z2 which is a circle of radius 1/|H|, and the surface is a sphere of radius 1/|H|.

For the cylinder, we have to come back to the initial equation for H. If  f(z)=r, then  H=1/(2r).  

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