The point at infinity

To remedy this, we extend the euclidean plane E to the extended plane E⁺
by adding a single point at infinity Å.

Then, for any circle D with centre P, we define i_D(P) to be Å,
and i_D(Å) to be P.

Thus, inversion is defined on E⁺, and still has order 2.

Finally, if L is a line on E, then we define the extended line L⁺ to be the line L together with Å

We can justify this by looking at stereographic projection.

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