Abstract
In this paper we develop tools to study within a family of non-selfadjoint operators L(φ) depending on a parameter φ in a real Hilbert space, those with (partially) simple spectrum. As a case study we consider the Zakharov-Shabat operators L(φ) appearing in the Lax pair of the focusing NLS on the circle. In particular, the main result implies that the set of potentials φ of Sobolev class HN, N≥ 0, so that all non real eigenvalues of L(φ) are simple, is path connected and dense.