We point out that if the Hardy–Littlewood maximal operator is bounded on the space Lp(t)(ℝ), 1 < a ≤ p(t) ≤ b < ∞, t ∈ ℝ, then the well-known characterization of the spaces Lp(ℝ), 1 < p < ∞, by the Littlewood–Paley theory ...