code: def gradient(self, ts):
          
          gradient = self._jmodel.gradient(_py2java(self._ctx, Vectors.dense(ts)))
          return _java2py(self._ctx, gradient)
docs: Find the gradient of the log likelihood with respect to the given time series.
          
          Based on http://www.unc.edu/~jbhill/Bollerslev_GARCH_1986.pdf
          
          Returns an 3-element array containing the gradient for the alpha, beta, and omega parameters.