given functional has general linear operators as arguments, not necessarily partial differential operators. Functional analysis and eigenvalue theory It is difficult to single out his contributions to functional analysis since, as stated at the beginning of this section, the methods of functional analysis are ubiquitous in his research: however, it is worth to remember paper , where an important existence theorem is proved. His contributions in the field of eigenvalue theory began with the paper , where he formalizes a method developed by Mauro Picone for the approximation of eigenvalues of operators subject only to the condition that their inverse is compact: however, as he admits in , this method does not give any estimate on the approximation error on the value of the calculated (approximated) eigenvalues. He contributed also to the classical eigenvalue problem for symmetric operators, introducing the method of orthogonal invariants. Approximation theory His work in this field is mainly related to the study of systems of functions, possibly being particular solutions of a given partial differential equation or system of such equations, in order to prove their completeness on the boundary of a given domain. The interest of this research is obvious: given such a system of functions, every solution of a boundary value problem can be approximated by an infinite series or Fourier type integral in the topology of a given function space. One of the most famous examples of this kind of theorem is Mergelyan's theorem, which completely solves the problem in the class of holomorphic functions for a compact set in the complex plane. In his paper , Fichera studies this problem for harmonic functions, relaxing the smoothness requirements on the boundary in the already cited work : a survey on his and others' work in this area, including contributions of Mauro Picone, Bernard Malgrange, Felix Browder and a number of other mathematicians, is contained in the paper . Another branch of his studies on approximation theory is strictly tied to complex analysis in one variable, and to the already cited Mergelyan's theorem: he studied the problem of approximating continuous functions on a compact set (and analytic on its interior if this is non-void) of the complex plane by rational functions with prescribed poles, simple or not. The paper surveys the contribution to the solution of this and related problems by Sergey Mergelyan, Lennart Carleson, Gábor Szegő as well as others, including his