\[\operatorname{arccosh}z=(2(z-1))^{1/2}\*{\left(1+\sum_{n=1}^{\infty}(-1)^{n}%
\frac{1\cdot 3\cdot 5\cdots(2n-1)}{2^{2n}n!(2n+1)}(z-1)^{n}\right)},\]