.. _continuous-genextreme:

Generalized Extreme Value Distribution
======================================

Extreme value distributions with one shape parameter :math:`c`.

If :math:`c>0`, the support is :math:`-\infty<x\leq1/c.`  If :math:`c<0`, the support is :math:`\frac{1}{c}\leq x<\infty.`

.. math::
   :nowrap:

    \begin{eqnarray*} f\left(x;c\right) & = & \exp\left(-\left(1-cx\right)^{1/c}\right)\left(1-cx\right)^{1/c-1}\\
    F\left(x;c\right) & = & \exp\left(-\left(1-cx\right)^{1/c}\right)\\
    G\left(q;c\right) & = & \frac{1}{c}\left(1-\left(-\log q\right)^{c}\right)\end{eqnarray*}

.. math::

     \mu_{n}^{\prime}=\frac{1}{c^{n}} \sum_{k=0}^{n} \binom{n}{k} \left(-1\right)^{k}\Gamma\left(ck+1\right)\quad\text{if } cn>-1

So,

.. math::
   :nowrap:

    \begin{eqnarray*} \mu_{1}^{\prime} & = & \frac{1}{c}\left(1-\Gamma\left(1+c\right)\right)\quad c>-1\\
    \mu_{2}^{\prime} & = & \frac{1}{c^{2}}\left(1-2\Gamma\left(1+c\right)+\Gamma\left(1+2c\right)\right)\quad c>-\frac{1}{2}\\
    \mu_{3}^{\prime} & = & \frac{1}{c^{3}}\left(1-3\Gamma\left(1+c\right)+3\Gamma\left(1+2c\right)-\Gamma\left(1+3c\right)\right)\quad c>-\frac{1}{3}\\
    \mu_{4}^{\prime} & = & \frac{1}{c^{4}}\left(1-4\Gamma\left(1+c\right)+6\Gamma\left(1+2c\right)-4\Gamma\left(1+3c\right)+\Gamma\left(1+4c\right)\right)\quad c>-\frac{1}{4}\end{eqnarray*}


For :math:`c=0` the distribution is the same as the (left-skewed) Gumbel distribution, and the support is :math:`\mathbb{R}`.

.. math::
   :nowrap:

    \begin{eqnarray*} f\left(x;0\right) & = & \exp\left(-e^{-x}\right)e^{-x}\\
    F\left(x;0\right) & = & \exp\left(-e^{-x}\right)\\
    G\left(q;0\right) & = & -\log\left(-\log q\right)\end{eqnarray*}

.. math::
   :nowrap:

    \begin{eqnarray*} \mu & = & \gamma=-\psi_{0}\left(1\right)\\
    \mu_{2} & = & \frac{\pi^{2}}{6}\\
    \gamma_{1} & = & \frac{12\sqrt{6}}{\pi^{3}}\zeta\left(3\right)\\
    \gamma_{2} & = & \frac{12}{5}\end{eqnarray*}

Implementation: `scipy.stats.genextreme`