# Helicoid

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The helicoid is one of the first minimal surfaces discovered. Its name derives from its similarity to the helix: for every point on the helicoid there is a helix contained in the helicoid which passes through that point.

The helicoid is also a ruled surface, meaning that it is a trace of a line. Alternatively, for any point on the surface, there is a line on the surface passing through it.

The helicoid and the catenoid are parts of a family of helicoid-catenoid minimal surfaces.

The helicoid is shaped like the Archimedes' screw, but extends infinitely in all directions. It can be described by the following parametric equations in Cartesian coordinates:

[itex] x = \rho \cos \theta, \ [itex]
[itex] y = \rho \sin \theta, \ [itex]
[itex] z = \alpha \theta, \ [itex]

where both ρ and θ range from negative infinity to positive infinity.

The helicoid is homeomorphic to the plane [itex] \mathbb{R}^2 [itex]. To see this, let alpha decrease continuously from its given value down to zero. Each intermediate value of α will describe a different helicoid, until α = 0 is reached and the helicoid becomes a plane (a plane is a degenerate helicoid).

A plane can be turned into a helicoid by choosing a line on the plane (call it an axis) then twisting the plane around that axis.

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