Integrable Nonlocal Nonlinear Equations : Integrable Nonlocal Nonlinear Equations

The condition of self-adjointness ensures that the eigenvalues of a Hamiltonian are real and bounded below. Replacing this condition by the weaker condition of \({\cal PT}\) symmetry, one obtains new infinite classes of complex Hamiltonians whose spectra are also real and positive. These \({\cal PT}\) symmetric theories may be viewed as analytic continuations of conventional theories from real to complex phase space. This paper describes the unusual classical and quantum properties of these theories.