In this paper,we shall introduce the concept of the Bessel (Riesz) potential Köthe function spaces
Xs and give some dual estimates for a class of operators determined by a semi-group in the spaces
Lq (
-T,T;Xs) (
Lq(
-T,T;)).Moreover,some time-space
Lp -Lp' estimates for the semi-group exp(
emt(-Δ)
m/2) and the operator
A:=∫
0t exp(i(
t-τ)(-Δ)
m/2)·
dτ in the Lebesgue-Besov spaces
Lq(-T,T;) are given.On the basis of these results,in a subsequent paper we shall present some further applications to a class of nonlinear wave equations.