# How do I convert a complex number to a point on the Riemann sphere?

How do I convert a complex number to a point on the Riemann sphere? Where coordinates on the sphere are $(x,y,z)$ constrained $x^2+y^2+z^2=1$?

There is a formula on Wikipedia going the other direction

$$\zeta = \frac {x + iy} {1-z}$$

## 1 Answer

This is known as "stereographic projection".

In the plane, you can get $(X,Y) = (\frac{x}{1-z},\frac{y}{1-z})$. From the plane to the sphere you get $(x,y,z) = (\frac{2X}{1+X^2+Y^2},\frac{2Y}{1+X^2+Y^2},\frac{X^2+Y^2-1}{1+X^2+Y^2})$.

Then, you just take complex numbers to be $X+iY$.

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