Using the BMO-H1 duality (among other things), D. R. Adams proved in [1] the strong type inequality
∫Mf(x)dHα(x)[les ]C∫[mid ]f(x)[mid ]dHα(x), 0<α<n,(1)
where C is some positive constant independent of f. Here M is the Hardy–Littlewood maximal operator in ℝn, Hα is the α-dimensional Hausdorff content, and the integrals are taken in the Choquet sense. The Choquet integral of ϕ[ges ]0 with respect to a set function C is defined by
formula here
Precise definitions of M and Hα will be given below. For an application of (1) to the Sobolev space W1, 1 (ℝn), see [1, p. 114].
The purpose of this note is to provide a self-contained, direct proof of a result more general than (1).