“…holds for all p > 2n n−1 , where 1/p+1/p ′ = 1. This was proved by Fefferman and Stein in two dimensions [10], but remains open in higher dimensions despite extensive study; see, for example, [1,2,6,8,9,11,12,22,26,31,33,34,35,36,37,38,42] and the references therein.…”