protected double rotational_derivative(double r)
{
DerivFunction df = (x) => this.sagitta(x);
DerivResult result = Derivatives.central_derivative(df, r, 1e-4);
return(result.result);
}
protected Vector2 base_derivative(Vector2 xy)
{
//double abserr;
DerivFunction dxf = (x) => this.sagitta(new Vector2(x, xy.y()));
DerivFunction dyf = (y) => this.sagitta(new Vector2(xy.x(), y));
DerivResult result = Derivatives.central_derivative(dxf, xy.x(), 1e-6);
double dx = result.result;
result = Derivatives.central_derivative(dyf, xy.y(), 1e-6);
double dy = result.result;
// TODO what do we do about error?
return(new Vector2(dx, dy));
}