There always is a variational source condition for nonlinear problems in Banach spaces

Variational source conditions proved useful for deriving convergence rates for Tikhonov’s regularization method and also for other methods. Up to now such conditions have been verified only for few examples or for situations which can be handled by classical techniques, too. Here we show that for almost every ill-posed inverse problem variational source conditions are satisfied. Whether linear or nonlinear, whether Hilbert or Banach spaces, whether one or multiple solutions, variational source conditions are a universal tool for proving convergence rates.