.0 - k*x)**(1.0/k - 1.0)*
                          exp(-(1.0 - k*x)**(1.0/k))

h != 0 and k = 0::

    kappa4.pdf(x, h, k) = exp(-x)*(1.0 - h*exp(-x))**(1.0/h - 1.0)

h = 0 and k = 0::

    kappa4.pdf(x, h, k) = exp(-x)*exp(-exp(-x))

kappa4 takes :math:`h` and :math:`k` as shape parameters.

The kappa4 distribution returns other distributions when certain
:math:`h` and :math:`k` values are used.

+------+-------------+----------------+------------------+
| h    | k=0.0       | k=1.0          | -inf<=k<=inf     |
+======+=============+================+==================+
| -1.0 | Logistic    |                | Generalized      |
|      |             |                | Logistic(1)      |
|      |             |                |                  |
|      | logistic(x) |                |                  |
+------+-------------+----------------+------------------+
|  0.0 | Gumbel      | Reverse        | Generalized      |
|      |             | Exponential(2) | Extreme Value    |
|      |             |                |                  |
|      | gumbel_r(x) |                | genextreme(x, k) |
+------+-------------+----------------+------------------+
|  1.0 | Exponential | Uniform        | Generalized      |
|      |             |                | Pareto           |
|      |             |                |                  |
|      | expon(x)    | uniform(x)     | genpareto(x, -k) |
+------+-------------+----------------+------------------+

(1) There are at least five generalized logistic distributions.
    Four are described here:
    https://en.wikipedia.org/wiki/Generalized_logistic_distribution
    The "fifth" one is the one kappa4 should match which currently
    isn't implemented in scipy:
    https://en.wikipedia.org/wiki/Talk:Generalized_logistic_distribution
    https://www.mathwave.com/help/easyfit/html/analyses/distributions/gen_logistic.html
(2) This distribution is currently not in scipy.

References
----------
J.C. Finney, "Optimization of a Skewed Logistic Distribution With Respect
to the Kolmogorov-Smirnov Test", A Dissertation Submitted to the Graduate
Faculty of the Louisiana State University and Agricultural and Mechanical
College, (August, 2004),
https://digitalcommons.lsu.edu/gradschool_dissertations/3672

J.R.M. Hosking, "The four-parameter kappa distribution". IBM J. Res.
Develop. 38 (3), 25 1-258 (1994).

B. Kumphon, A. Kaew-Man, P. Seenoi, "A Rainfall Distribution for the Lampao
Site in the Chi River Basin, Thailand", Journal of Water Resource and
Protection, vol. 4, 866-869, (2012).
:doi:`10.4236/jwarp.2012.410101`

C. Winchester, "On Estimation of the Four-Parameter Kappa Distribution", A
Thesis Submitted to Dalhousie University, Halifax, Nova Scotia, (March
2000).
http://www.nlc-bnc.ca/obj/s4/f2/dsk2/ftp01/MQ57336.pdf

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