We obtain the explicit upper Bellman function for the natural dyadic maximal operator acting from ${ \rm BMO}(\mathbb{ R}^n)$ into ${ \rm BLO}(\mathbb{ R}^n).$ As a consequence, we show that the ${ \rm BMO}\to{ \rm BLO}$ norm of the natural operator equals 1 for all $n,$ and so does the norm of the classical dyadic maximal operator. The main result is a partial corollary of a theorem for the so-called $\alpha$-trees, which generalize dyadic lattices. The Bellman function in this setting exhibits an interesting quasi-periodic structure depending on $\alpha,$ but also allows a majorant independent of $\alpha,$ hence the dimension-free norm constant. We also describe the decay of the norm with respect to the difference between the average of a function on a cube and the infimum of its maximal function on that cube. An explicit norm-optimizing sequence is constructed.
Categories: math.CA