public void TestSecondPartial(double[] arr, double[] expected)
        {
            Derivative = new Derivative(TypicalMultivariateFunction);
            var vector       = new Vector(arr);
            var joinedResult = new double[4];

            for (int i = 0; i < 2; i++)
            {
                for (int j = 0; j < 2; j++)
                {
                    joinedResult[2 * i + j] = Derivative.GetSecondPartialDerivative(vector, i, j);
                }
            }

            Assert.AreEqual(expected, joinedResult);
        }