COSTA'S SURFACE

 Surface studied by da Costa in 1982. Celso Jose da Costa (1949-....): Brazilian mathematician. https://en.wikipedia.org/wiki/Costa%27s_minimal_surface http://mathworld.wolfram.com/CostaMinimalSurface.html

 Gray parametrization: , , where  refers to the Weierstrass zeta function associated to the pair (c,0) (WeierstrassZeta[z, {c, 0}] in Mathematica), refers to the Weierstrass P function also associated to the pair (c,0)(WeierstrassP[z, {c, 0}] in Mathematica) c and  are two constants associated to the Weierstrass functions equal respectively to 189.7272... and  6.87519....

Until 1982, it was conjectured that the only complete (i.e. without boundary) non-periodic minimal surfaces without self-intersection were: the plane, the catenoid and its associated surfaces. Costa's surface, and other ones afterwards, refuted this conjecture.

It can be obtained by taking  in the Weierstrass parametrization of a minimal surface.

Costa's surface is invariant under the action of a half-turn around the axis x = y, z = 0 (in the above parametrization, y(u,v)=x(v,u) and z(v,u)=-z(u,v)), and under the action of this half-turn, the two faces are swapped.
Opposite, two half surfaces. Check that each one is the image by a half-turn of the other one
Animation of the surface the two faces of which have different colors.
Imagine water poured inside the upper funnel; it will come out by the external face of the bottom funnel...

Costa's surface is topologically equivalent to a torus minus 3 points (it is therefore of genus 1); see an animation of it here.

 It is also topologically very close to the cubic algebraic surface with the very simple Cartesian equation: , which is also invariant under the action of a half-turn that swaps the two faces. Note that the horizontal sections of the latter surface are ellipses or hyperbolas. Note also that this surface is asymptotic for large x and y to Plücker's conoid.

Costa's surface can be generalized to a surface with order n rotational symmetry, see here.

Compare Costa's surface to the finite Riemann minimal surface, which also has a plane sheet and two flared sheets, but that intersect with each other.

 Costa's surface, by Alain Esculier Costa's surface by Patrice Jeener, with his kind authorization