[SciPy-user] exponential integral behaviour near -20

[David Joyner]

Hi:

A user reported a bug on the sage-support list (Sage, www.sagemath.org, includes scipy) for the exponential 
integral. This seems to illustrate the idea:

>>> import scipy.special, math
>>> -scipy.special.exp1(-complex(19.9999990))
(25615628.4058083-3.1415926535897931j)
>>> -scipy.special.exp1(-complex(19.9999991))
(25615624.582086116+3.1415926535897931j)

Any ideas on what would cause this?

- David Joyner