Abstract. We
Introduction"Some problems open doors, some problems close doors, and some remain curiosities, but all sharpen our wits and act as a challenge and a test of our ingenuity and techniques". So said Atiyah in the preface to Mathematics: frontiers and perspectives (2000, by the International Mathematical Union, published by the American Mathematical Society). This statement is, in our opinion, a good introduction to what a collection of problems is or should be. In mathematics, each area or subarea produces its own lists of (more or less celebrated) problems and open questions, sometimes hard to appreciate or just to understand if one does not work in the concerned field, as given evidence by the lists of problems offered in the above-referenced book.Our objective here is more modest: we expose a selected list of questions that all belong to Optimization or Matrix analysis. They are of unequal importance and diverse origins, but all of rather wide interest. Some have a theoretical flavour, some others clearly hinge on calculation, and what it is asked for each of them varies. There are problems about which we know practically nothing, some others have partial answers; and also some are already solved but in rather indirect manners; for them we would like to have more natural, or at least different, proofs.For each of the nine problems described in this paper, we propose a short presentation, the state of the art and a list of appropriate references. As a result, each of the problems listed can be read independently of the others, according to the interest or knowledge of the reader. The reader could thus try to tackle some of them, that is at least our aim in writing down such a paper.Keywords and phrases. Convex sets, positive (semi)definite matrices, variational problems, energy functions, global optimization, permanent function, bistochastic matrices, normal matrices.