y = a * cos(b * x) + b * sin(a * x)

public override void GetGradient(double x, Vector<double> parameters, ref Vector<double> gradient)
{
    gradient[0] = (Math.Cos(parameters[1] * x) + parameters[1] * Math.Cos(parameters[0] * x));
    gradient[1] = (-parameters[0] * Math.Sin(parameters[1] * x) * x + Math.Sin(parameters[0] * x));
}

y = b + a1 * g1^2 / ((x - x1)^2 + g1^2) + a2 * g2^2 / ((x - x2)^2 + g2^2)