“…Therefore, the only case where (1.11) is useful for such applications is when there exist sufficiently simple closed-form expressions for the λ k 's. If such is not the case, one must use different techniques (see, e.g., [18,40]), which, besides being more time-consuming for the computer than a direct approach, do not yield more than a very superficial insight concerning the specific form of the KL decomposition. Unfortunately, for most Gaussian processes of interest with respect to statistics, the values of the λ k 's are unknown, even though their existence remains guaranteed through the knowledge of R(u, v) (see, e.g., [1, p. 76]).…”