Nonlinear pseudo-bosons

In a series of recent papers, the author has introduced the notion of (regular) pseudo-bosons showing, in particular, that two number-like operators, whose spectra are N0 := N ∪ {0}, can be naturally introduced. Here we extend this construction to operators with rather more general spectra. Of course, this generalization can be applied to many more physical systems. We discuss several examples of our framework.