Calculates the inverse digamma function using Newton–Raphson. Works for
y > -500. Uses Newton–Raphson with relative tolerance of eps^0.25.
Arguments
- y
Values to invert.
Details
The number of iterations are few, 1 for most input values, especially those
that are large. The starting value is the lower bound found by Batir (2017).
References
Batir, N. (2017). Inequalities for the inverses of the polygamma functions. arXiv. http://arxiv.org/abs/1705.06547
