Error in tanh for large complex argument (Trac #1726)

[numpy-gitbot]

Original ticket http://projects.scipy.org/numpy/ticket/1726 on 2011-01-28 by trac user mbakker, assigned to unknown.

tanh computes nans for large complex argument:

In [85]: tanh(1000+0j)
Out[85]: (nan+nan*j)

while the correct answer is 1.0

Solution: Program tanh as:

tanh(z) = (1.0 - exp(-2.0_z)) / (1.0 + exp(-2.0_z))

[numpy-gitbot]

trac user mbakker wrote on 2011-01-28

As Pauli pointed out the suggested solution has problems for z -> -inf.
Need to check what others do.

[numpy-gitbot]

trac user mbakker wrote on 2012-04-07

tanh is computed correctly for large complex argument by cmath. We may look there for the correct approach.