Half-logistic distribution

The probability density function for the Half-logistic distribution is:

.. math::

f(x) = \frac{ 2 e^{-x} }{ (1+e^{-x})^2 }
         = \frac{1}{2} \text{sech}(x/2)^2

where :math:x \ge 0. The following plots show the pdf and cdf:

.. image:: ../../../../tests/imgs/stats/continuous/halflogistic/halflogisticpdf.png :align: center :width: 480

.. image:: ../../../../tests/imgs/stats/continuous/halflogistic/halflogisticcdf.png :align: center :width: 480