The boundedness of operators in Muckenhoupt weighted Morrey spaces via   extrapolation techniques and duality
  The bidual of the closure of smooth functions with respect to the Morrey norm coincides with the Morrey space. This assertion is generalized to some Muckenhoupt weighted Morrey spaces. We combine this fact with basic extrapolation techniques due to Rubio de Francia adapted to weighted Morrey spaces. This leads to new results on the boundedness of operators even for the unweighted case. 
Marcel Rosenthal and Hans-Juergen Schmeisser
math.FA