This lecture covers fundamental concepts in numerical analysis and optimization, focusing on the structure of R^n as a vector space and the interpretation of points within it. The instructor discusses the notion of distance, induced by norms, and how these concepts relate to open and closed subsets. The lecture introduces the definitions of open and closed sets, the interior of a set, and the concept of adherence. Examples are provided to illustrate these definitions, including the interior and boundary of specific sets. The instructor emphasizes the importance of understanding these definitions to avoid misconceptions. The discussion extends to isolated points and accumulation points, clarifying their roles within subsets of R^n. The lecture concludes with a preview of upcoming topics, including sequences and limits, while encouraging student participation and engagement in future sessions.