Let m, p, q ∈ (0, ∞) and let u, v, w be nonnegative weights. We characterize validity of the inequalityfor all t > 0.By the characterization of linearity of Λ spaces [4, Theorem 1.4], CL m,p (u, v) is not a linear set. The Λ spaces are not always linear sets either, as seen above. The Γ spaces are at least linear spaces thanks to sublinearity of the mapping f → f * * , the functional · Γ p (v) however does not have to be a norm (see [10,15]). Nevertheless, the term "space" is used to describe all these structures, for the sake of simplicity.Suppose that · X : M → [0, ∞] is a functional such that λf X = |λ| f X and f X ≤ g X for all λ ∈ [0, ∞) and f, g ∈ M such that |f | ≤ |g| a.e. on R n . Let · Y be another functional with the same properties. Let X, Y be two function "spaces" given by (1) holds for all f ∈ M is called the optimal constant. Hence, if X is not embedded in Y , the optimal constant in (1) is infinite. Assume that X is moreover rearrangement-invariant, i.e., that f X = h X whenever f, h ∈ M are such that f * = h * on (0, ∞). Then the associated space of X, denoted X ′ , is defined asIf X is a Banach function space (see [2]), then X ′ is a Banach function space as well. However, to define X ′ in the way described above is possible even for a more general X, though without claiming space-like properties of X ′ . For more details, see [2].As it can be observed from the previous definition, embeddings into Λ spaces play a rather significant role since the associate "norm" g X of a function g ∈ M is equal to the optimal