Power normal distribution

The probability density function for power normal distribution is:

.. math::

f(x, k) = k \phi(x) (\Phi(-x))^{k-1}

where :math:\phi is the normal pdf, and :math:\Phi is the normal cdf, and :math:x \ge 0, :math:k > 0.

The following plots show the pdf and cdf:

.. image:: ../../../../tests/imgs/stats/continuous/powernorm/powernormpdf.png :align: center :width: 480

.. image:: ../../../../tests/imgs/stats/continuous/powernorm/powernormcdf.png :align: center :width: 480