The increasing complexity of mathematical models, and the related need to introduce simplifying assumptions and numerical approximations, has led to the need to consider approximate solutions. When dealing with any mathematical model, some of the basic questions to be asked are whether the solution is stable to perturbations, what the approximate solutions are, and if the set of approximate solutions is close to the original solution set. The interrelationships between these aspects are also of theoretical interest. Such concepts are described in the present volume, which emphasizes the concepts of approximate solution, well-posedness and stability in optimization, calculus of variations, optimal control, and the mathematics of conflict (e.g. game theory and vector optimization). The most recent developments are covered.

This volume contains several surveys focused on the ideas of approximate solutions, well-posedness and stability of problems in scalar and vector optimization, game theory and calculus of variations. These concepts are of particular interest in many fields of mathematics. The idea of stability goes back at least to J. Hadamard who introduced it in the setting of differential equations; the concept of well-posedness for minimum problems is more recent (the mid-sixties) and originates with A.N. Tykhonov. It turns out that there are connections between the two properties in the sense that a well-posed problem which, at least in principle, is "easy to solve", has a solution set that does not vary too much under perturbation of the data of the problem, i.e. it is "stable". These themes have been studied in depth for minimum problems and now we have a general picture of the related phenomena in this case. But, of course, the same concepts can be studied in other more complicated situations as, e.g. vector optimization, game theory and variational inequalities. Let us mention that in several of these new areas there is not even a unique idea of what should be called approximate solution, and the latter is at the basis of the definition of well­ posed problem.

The increasing complexity of mathematical models, and the related need to introduce simplifying assumptions and numerical approximations, has led to the need to consider approximate solutions. When dealing with any mathematical model, some of the basic questions to be asked are whether the solution is stable to perturbations, what the approximate solutions are, and if the set of approximate solutions is close to the original solution set. The interrelationships between these aspects are also of theoretical interest. Such concepts are described in the present volume, which emphasizes the concepts of approximate solution, well-posedness and stability in optimization, calculus of variations, optimal control, and the mathematics of conflict (e.g. game theory and vector optimization). The most recent developments are covered.

This volume contains several surveys focused on the ideas of approximate solutions, well-posedness and stability of problems in scalar and vector optimization, game theory and calculus of variations. These concepts are of particular interest in many fields of mathematics. The idea of stability goes back at least to J. Hadamard who introduced it in the setting of differential equations; the concept of well-posedness for minimum problems is more recent (the mid-sixties) and originates with A.N. Tykhonov. It turns out that there are connections between the two properties in the sense that a well-posed problem which, at least in principle, is "easy to solve", has a solution set that does not vary too much under perturbation of the data of the problem, i.e. it is "stable". These themes have been studied in depth for minimum problems and now we have a general picture of the related phenomena in this case. But, of course, the same concepts can be studied in other more complicated situations as, e.g. vector optimization, game theory and variational inequalities. Let us mention that in several of these new areas there is not even a unique idea of what should be called approximate solution, and the latter is at the basis of the definition of well­ posed problem.