@Test
public void testAddDifferentiable() {
DifferentiableUnivariateFunction sin = new Sin();
DifferentiableUnivariateFunction c = new Constant(4);
DifferentiableUnivariateFunction m = new Minus();
DifferentiableUnivariateFunction inv = new Inverse();
final double a = 123.456;
Assert.assertEquals(- 1 / (a * a) -1 + Math.cos(a),
FunctionUtils.add(inv, m, c, sin).derivative().value(a),