The inverse function to the cosine.

If y is the cosine of theta, then theta is the arccosine of y.

arccos(z) is equal to i × ln(z + √(z² − 1)) since cos(φ) = cosh(i × φ).

The arccosine is real for -1 ≤ x ≤ 1 only. Values are then between 0 and π. (I had originally written defined instead of real, thanks to IWhoSawTheFace for pointing out this ridiculous error to me)

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