On the Well-Posedness Concept in the Sense of Tykhonov

We introduce a general concept of well-posedness in the sense of Tykhonov for abstract problems formulated on metric spaces and characterize it in terms of properties for a family of approximating sets. Then, we illustrate these results in the study of some relevant particular problems with history-dependent operators: a fixed point problem, a nonlinear operator equation, a variational inequality and a hemivariational inequality, both formulated in the framework of real normed spaces. For each problem, we clearly indicate the approximating sets, characterize its well-posedness by using our abstract results, then we state and prove specific results which guarantee the well-posedness under appropriate assumptions on the data. For part of the problems, we provide the continuous dependence of the solution with respect to the data and/or present specific examples.