The Haar system constitutes an unconditional basis for a separable rearrangement invariant (symmetric) space E if and only if the multiplier determined by the sequence λ nk = (−1) n , k = 0, 1, for n = 0 and k = 0, 1, . . . , 2 n for n ≥ 1, is bounded in E. If the Lorentz space Λ(ϕ) differs from L 1 and L ∞ then there is a multiplier with respect to the Haar system which is bounded in Λ(ϕ) and unbounded in L ∞ and L 1 .1. The Haar system is the following system of functions: χ 0 0 (t) = 1 andwhere 1 ≤ k ≤ 2 n , n = 0, 1, . . . . We denote by Ω the set of indices (n, k) corresponding to the Haar functions. The formula m = 2 n + k establishes a one-to-one correspondence between Ω and the set N of naturals and enables us to use the single index Haar system {χ m }. Every sequence λ = (λ 1 , λ 2 , . . . ) generates the multiplier Λ that is defined at the polynomials with respect to the Haar system as follows: Λ m c m χ m = m λ m c m χ m . By the classical Paley-Marcinkiewicz Theorem [1, Theorem 3 and Corollary 2] or [2, Section 2, p. 5] the multiplier Λ is bounded in L p (1 < p < ∞) if sup m |λ m | < ∞. Moreover, if 1 < p < ∞ then Λ Lp ≈ λ l∞ = sup m |λ m |,where the sign ≈ stands for two-sided estimates with constants independent of λ. This article studies the spaces of multipliers in some pairs of function spaces. The multipliers with respect to the Haar system were studied in [3-7] and many other articles.
2.We now give the relevant definitions. A Banach function space E on [0, 1] with the Lebesgue measure is called rearrangement invariant or symmetric if (1) |x(t)| ≤ |y(t)| and y ∈ E implies x ∈ E and x E ≤ y E ;(2) equimeasurability of x(t) and y(t) with y ∈ E implies x ∈ E and x E = y E . We suppose henceforth that E is separable or dual to a separable space. Denote the characteristic function of a measurable set e ⊂ [0, 1] by κ e (t). For every τ > 0 the operatoris bounded in a rearrangement invariant space E and σ τ E ≤ max(1, τ ). The numberslog σ τ E log τ , β E = lim τ →∞ log σ τ E log τ Tol yatti; Voronezh.