Sharp growth conditions for boundedness of maximal function in generalized Orlicz space

We study sharp growth conditions for the boundedness of the Hardy-Littlewood maximal function in the generalized Orlicz spaces. We assume that the generalized Orlicz function satisfies the standard continuity properties (A0), (A1) and (A2). We show that if the Hardy-Littlewood maximal function is bounded from the generalized Orlicz space to itself then is almost increasing for large for some . Moreover we show that the Hardy-Littlewood maximal function is bounded from the generalized Orlicz space to itself if and only if is weakly equivalent to a generalized Orlicz function satisfying (A0), (A1) and (A2) for which is almost increasing for all and some .