Let us now consider a system of many nucleons combined together within one composite object. We know that such composite particles (called nuclei or nuclides, as you know) exist in Nature. There exist exactly 253 species of stable nuclei.1About 2500 other ones have been synthesized in laboratories - they decay by different processes, like electron, positron, proton, or neutron emission, or by fission, i.e., by splitting into two lighter nuclei (including the case when one of the lighter nuclei is the He nucleus, called the particle). According to theoretical predictions, there probably exist another 3000 nuclei, not synthesized yet, that are stable with respect to nucleon emission. At present, their synthesis, investigation, and description is at the centre of interest of nuclear structure physicists, and most of the lectures presented during this Summer School were devoted precisely to this subject.
Nuclei are fascinating objects. They are fermionic systems that exhibit single-particle (s.p.) and collective features at the same scale. Apart from very light ones, they contain too many constituents for an application of exact methods, and too few constituents for an application of statistical methods. Their elementary modes of excitation can, nevertheless, be very well defined based on using quasi-constituents and/or effective interactions.