Given a stationary continuous-time process f(t), the Hilbert–Schmidt operator Aτ can be defined for every finite τ. Let λτ,i be the eigenvalues of Aτ with descending order. In this article, a Hilbert space
$$\mathcal {H}_f$$
ℋ
f
and the (time-shift) continuous one-parameter semigroup of isometries
$$\mathcal {K}^s$$
K
s
are defined. Let
$$\{v_i, i\in \mathbb {N}\}$$
{
v
i
,
i
∈
ℕ
}
be the eigenvectors of
$$\mathcal {K}^s$$
K
s
for all s ≥ 0. Let
$$f = \displaystyle \sum _{i=1}^{\infty }a_iv_i + f^{\perp }$$
f
=
∑
i
=
1
∞
a
i
v
i
+
f
⊥
be the orthogonal decomposition with descending |ai|. We prove that limτ→∞λτ,i = |ai|2. The continuous one-parameter semigroup
$$\{\mathcal {K}^s: s\geq 0\}$$
{
K
s
:
s
≥
0
}
is equivalent, almost surely, to the classical Koopman one-parameter semigroup defined on L2(X, ν), if the dynamical system is ergodic and has invariant measure ν on the phase space X.