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from __future__ import division, print_function, absolute_import
from ._ufuncs import _lambertw
def lambertw(z, k=0, tol=1e-8):
lambertw(z, k=0, tol=1e-8)
Lambert W function [1]_.
The Lambert W function `W(z)` is defined as the inverse function
of ``w * exp(w)``. In other words, the value of ``W(z)`` is
such that ``z = W(z) * exp(W(z))`` for any complex number
The Lambert W function is a multivalued function with infinitely
many branches. Each branch gives a separate solution of the
equation ``z = w exp(w)``. Here, the branches are indexed by the
integer `k`.
z : array_like
Input argument.
k : int, optional
Branch index.
tol : float, optional
Evaluation tolerance.
w : array
`w` will have the same shape as `z`.
All branches are supported by `lambertw`:
* ``lambertw(z)`` gives the principal solution (branch 0)
* ``lambertw(z, k)`` gives the solution on branch `k`
The Lambert W function has two partially real branches: the
principal branch (`k = 0`) is real for real ``z > -1/e``, and the
``k = -1`` branch is real for ``-1/e < z < 0``. All branches except
``k = 0`` have a logarithmic singularity at ``z = 0``.
**Possible issues**
The evaluation can become inaccurate very close to the branch point
at ``-1/e``. In some corner cases, `lambertw` might currently
fail to converge, or can end up on the wrong branch.
Halley's iteration is used to invert ``w * exp(w)``, using a first-order
asymptotic approximation (O(log(w)) or `O(w)`) as the initial estimate.
The definition, implementation and choice of branches is based on [2]_.
.. [1]
.. [2] Corless et al, "On the Lambert W function", Adv. Comp. Math. 5
(1996) 329-359.
The Lambert W function is the inverse of ``w exp(w)``:
>>> from scipy.special import lambertw
>>> w = lambertw(1)
>>> w
>>> w*exp(w)
Any branch gives a valid inverse:
>>> w = lambertw(1, k=3)
>>> w
>>> w*np.exp(w)
**Applications to equation-solving**
The Lambert W function may be used to solve various kinds of
equations, such as finding the value of the infinite power
tower :math:`z^{z^{z^{\ldots}}}`:
>>> def tower(z, n):
... if n == 0:
... return z
... return z ** tower(z, n-1)
>>> tower(0.5, 100)
>>> -lambertw(-np.log(0.5)) / np.log(0.5)
return _lambertw(z, k, tol)
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