class DoublesCurveNelsonSiegel extends DoublesCurve {

  @Override
  public double getDyDx(final double x) {
    final double x1 = x / _parameters[3]; //TODO untested
    final double eP = epsilonP(-x1);
    return -(_parameters[1] * eP + _parameters[2] * (eP - Math.exp(-x1))) / _parameters[3];
  }

  protected  DoublesCurveNelsonSiegel();
  public  DoublesCurveNelsonSiegel(final String name, final double beta0, final double beta1, final double beta2, final double lambda);
  public  DoublesCurveNelsonSiegel(final String name, final double[] parameters);

}