Abstract: | Suppose that (X 0, X 1) is a Banach couple, X 0 ∩ X 1 is dense in X 0 and X 1, (X0,X1)θq (0 < θ < 1, 1 ≤ q < ∞) are the spaces of the real interpolation method, ψ ∈ (X 0 ∩ X 1), ψ ≠ 0, is a linear functional, N = Ker ψ, and N i stands for N with the norm inherited from X i (i = 0, 1). The following theorem is proved: the norms of the spaces (N0,N1)θ,q and (X0,X1)θ,q are equivalent on N if and only if θ ? (0, α) ∪ (β∞, α0 ∪ (β0, α∞) ∪ (β, 1), where α, β, α0, β0, α∞, and β ∞ are the dilation indices of the function k(t)=K(t,ψ;X 0 * ,X 1 * ). |