  This paper studies the optimal testing for the nullity of the slope function
in the functional linear model using smoothing splines. We propose a
generalized likelihood ratio test based on an easily implementable data-driven
estimate. The quality of the test is measured by the minimal distance between
the null and the alternative set that still allows a possible test. The lower
bound of the minimax decay rate of this distance is derived, and test with a
distance that decays faster than the lower bound would be impossible. We show
that the minimax optimal rate is jointly determined by the smoothing spline
kernel and the covariance kernel. It is shown that our test attains this
optimal rate. Simulations are carried out to confirm the finite-sample
performance of our test as well as to illustrate the theoretical results.
Finally, we apply our test to study the effect of the trajectories of oxides of
nitrogen ($\text{NO}_{\text{x}}$) on the level of ozone ($\text{O}_3$).
