“…Following the fundamental work of Kováčik and Rákosník [24], the theory of function spaces with variable exponent has made an explosive growth over the past 25 years. In particular, after the boundedness of the Hardy-Littlewood maximal operator has been proved in [6,10,28], Lebesgue spaces and various other function spaces arising in analysis and PDE, such as Morrey spaces, Herz spaces and Hardy spaces and so on, have been intensively studied in the variable exponent setting, see [2,9,12,23,29,[35][36][37][38][39].…”