ArcCos[z] gives the arc cosine of the complex number z.
Mathematical function (see Section A.3.10).
All results are given in radians.
For real z between and , the results are always in the range to .
ArcCos[z] has branch cut discontinuities in the complex z plane running from to and to .
See The Mathematica Book: Section 1.1.3 and Section 3.2.6.