In many important textbooks the formal statement of the spectral
representation theorem is followed by a process version, usually
informal, stating that any stationary stochastic process
{ξ(t), t ∈ T} is the limit in
quadratic mean of a sequence of processes
{S(n,t), t ∈ T}, each
consisting of a finite sum of harmonic oscillations with stochastic
weights. The natural issues, whether the approximation error
ξ(t) − S(n,t) is
stationary or whether at least it converges to zero uniformly in
t, have not been explicitly addressed in the literature. The
paper shows that in all relevant cases, for T unbounded the
process convergence is not uniform in t (so that
ξ(t) − S(n,t) is not
stationary). Equivalently, when T is unbounded the number of
harmonic oscillations necessary to approximate ξ(t) with a
preassigned accuracy depends on t. The conclusion is that the
process version of the spectral representation theorem should
explicitly mention that in general the approximation of
ξ(t) by a finite sum of harmonic oscillations, given
the accuracy, is valid for t belonging to a bounded
subset of the real axis (of the set of integers in the
discrete-parameter case).The author is
grateful for very useful suggestions to Francesco Battaglia, Gianluca
Cubadda, Domenico Marinucci, Enzo Orsingher, Dag Tjøstheim, and
Umberto Triacca and also to an anonymous referee and the Econometric
Theory co-editor Benedikt M. Pötscher.