Sufficient (almost necessary) conditions are given on the weight functions u(·), v(·) for Φ−1 2 [ ∫ Rn Φ2 ( C2(Msf)(x) ) u(x)dx ] ≤ Φ−1 1 [ C1 ∫ Rn Φ1(|f(x)|)v(x)dx ] to hold when Φ1, Φ2 are φ-functions with subadditive Φ1Φ 2 , and Ms (0 ≤ s < n), is the usual fractional maximal operator. §
