Remarks on a Nonlinear Quantum Theory
  Motivated by recent proposals (Bialynicki-Birula, Mycielski; Haag, Bannier; Weinberg; Doebner, Goldin) for nonlinear quantum mechanical evolution equations for pure states some principal difficulties in the framework of usual quantum theory, which is based on its inherent linear structure, are discussed. A generic construction of nonlinear evolution equations through nonlinear gauge transformations is indicated. 
H. D. Doebner