EN
Let Ω be a nonatomic probability space, let X be a Banach function space over Ω, and let ℳ be the collection of all martingales on Ω. For $f = (fₙ)_{n∈ℤ₊} ∈ ℳ $, let Mf and Sf denote the maximal function and the square function of f, respectively. We give some necessary and sufficient conditions for X to have the property that if f, g ∈ ℳ and $||Mg||_{X} ≤ ||Mf||_{X}$, then $||Sg||_{X} ≤ C||Sf||_{X}$, where C is a constant independent of f and g.