public void TestHessian(double[] arr, params double[][] expected)
        {
            Derivative = new Derivative(TypicalMultivariateFunction);
            var vector  = new Vector(arr);
            var hessian = Derivative.GetHessian(vector);
            var got     = hessian.Items;

            for (int i = 0; i < 2; i++)
            {
                Assert.AreEqual(expected[i], got[i]);
            }
        }