This is an interesting expository article about the approximation of operators on a complex infinite-dimensional Hilbert space. Although the article does not include research published during the past twenty years or so, it provides a nice account of the stability of the spectrum under approximation (or perturbation). The reader interested in pursuing this area of research might refer to the bibliography in [D. A. Herrero, Approximation of Hilbert space operators. Vol. I, Pitman, Boston, MA, 1982; MR0676127] and the subsequent publications of those authors.
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Based on conjugate duality we construct several gap functions for general variational inequalities and equilibrium problems, in the formulation of which a so-called perturbation function is used. These functions are written with the help of the Fenchel-Moreau conjugate of the functions involved. In case we are working in the convex setting and a regularity condition is fulfilled, these functions become gap functions. The techniques used are the ones considered in [Altangerel L., Boţ R.I., Wanka G., On gap functions for equilibrium problems via Fenchel duality, Pac. J. Optim., 2006, 2(3), 667–678] and [Altangerel L., Boţ R.I., Wanka G., On the construction of gap functions for variational inequalities via conjugate duality, Asia-Pac. J. Oper. Res., 2007, 24(3), 353–371]. By particularizing the perturbation function we rediscover several gap functions from the literature. We also characterize the solutions of various variational inequalities and equilibrium problems by means of the properties of the convex subdifferential. In case no regularity condition is fulfilled, we deliver also necessary and sufficient sequential characterizations for these solutions. Several examples are illustrating the theoretical aspects.
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