A **system** is an organized (and at least theoretically delimited) universe, whose components are interconnected thus giving up their independence from the whole (in contrast to an aggregate).

According to Frerichs, a system is defined as an autonomic structure consisting of parts that organize according to their own rules.

The interconnections may be quantified by the degree of cross-linking or clustering coefficient, i.e. the quotient of actual and possible links among the system's components.

Open systems interact (via matter, energy and/or information) with their environment. They usually show a higher degree of stability compared to closed systems.

Additional classification criteria cover (according to Varjú^{[1]}):

System | |
---|---|

analog | digital |

without memory | with memory (with energy storage) |

linear | nonlinear |

passive | active |

with concentrated parameters | with distributed parameters |

time invariant | not time invariant |

## Complexity[]

A system is complex if it is characterised by many components that interact dynamically giving rise to a number of hierarchical levels. The interactions in turn, give rise to common kinds of behaviours across different types of systems and scales. Therefore, the interaction of the many parts at high levels of complexity produces emergent behaviour, one that cannot be predicted from the behaviour of the different components in isolation.^{[2]}

## Boundaries of a system[]

In reality, nearly every system is open with respect to at least one of the entities matter, energy and/or information. Therefore, for modelling purposes it will be difficult to determine where the system ends. A viable strategy may consist in constructing a model that contains as few elements as possible while still allowing correct decisions about the further evolution of the system or the actions to be taken.^{[3]}

## References[]

- ↑ Varjú D. Systemtheorie für Biologen und Mediziner. Heidelberger Taschenbücher. Vol. 182. Berlin, Heidelberg, New York: Springer, 1977. ISBN 3540080864
- ↑ Costa J. Systems pathology: a critical review. Mol Oncol. 2012 Feb;6(1):27-32. doi: 10.1016/j.molonc.2011.11.007. PMID 22178234.
- ↑ Juan Martín García (2011),
*Theory and Practical Exercises of System Dynamics*, Barcelona, Spain: Juan Martín García, ISBN 8460998045, 8460998045, http://openlibrary.org/books/OL25175096M/Theory_and_Practical_Exercises_of_System_Dynamics