Under "collapse of the wave function" (or "state vector reduction") one understands the 'sudden' change of the system's state in a measurement. This change is not reducible to classical "information gain", but is a genuine quantum mechanical concept, directly related to the concept of quantum state. It is especially relevant if we consider that quantum mechanics describes the behaviour of individual systems. In the following we shall first describe the role of the collapse as a formal concept

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... this context, then we shall discuss some variants of physical approaches to collapse. We shall comment on the notion of "individual systems" in quantum mechanics at the end of this article.