Result of preconditioned operator to deflation space, i.e.,
        :math:`MM_lAM_rU`.

def MAU(self):
        '''Result of preconditioned operator to deflation space, i.e.,
        :math:`MM_lAM_rU`.'''
        if self._MAU is None:
            self._MAU = self.linear_system.M * self.AU
        return self._MAU