class_docstr: Power variance function
  
  Parameters
  ----------
  power : float
      exponent used in power variance function
  
  Methods
  -------
  call
      Returns the power variance
  
  Notes
  -----
  Formulas
     V(mu) = numpy.fabs(mu)**power
  
  Aliases for Power:
  mu = Power()
  mu_squared = Power(power=2)
  mu_cubed = Power(power=3)
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docstr: Derivative of the variance function v'(mu)
  
  May be undefined at zero.
docstr_tok: ["Derivative","of","the","variance","function","v","'","(","mu",")","May","be","undefined","at","zero","."]
code: def deriv(self, mu):
      """
      Derivative of the variance function v'(mu)
  
      May be undefined at zero.
      """
  
      der = self.power * np.fabs(mu) ** (self.power - 1)
      ii = np.flatnonzero(mu < 0)
      der[ii] *= -1
      return der
  
file_path: statsmodels/statsmodels/genmod/families/varfuncs.py
imports: import numpy
label_desc: Use image node_id 3 for calling the Power obj's underlying member method code with example usage: obj.deriv(mu) and returns: der