Complex System
A complex system is a system composed of interconnected parts that as a whole exhibit one or more properties (behavior among the possible properties) not obvious from the properties of the individual parts
A system’s complexity may be of one of two forms: disorganized complexity and organized complexity. In essence, disorganized complexity is a matter of a very large number of parts, and organized complexity is a matter of the subject system (quite possibly with only a limited number of parts) exhibiting emergent properties.
Examples of complex systems include ant colonies, human economies and social structures, climate, nervous systems, cells and living things, including human beings, as well as modern energy or telecommunication infrastructures. Indeed, many systems of interest to humans are complex systems.
Complex systems are studied by many areas of natural science, mathematics, and social science. Fields that specialize in the interdisciplinary study of complex systems include systems theory, complexity theory, systems ecology, and cybernetics

Overview
A complex system is any system featuring a large number of interacting components, whose aggregate activity is non-linear and typically exhibits self-organization under selective pressures. Now the term complex systems has multiple meaning:

  * A specific kind of systems, that are complex
  * A field of science studying these systems, see further complex systems
  * A paradigm, that complex systems have to be studied with non-linear dynamics, see further complexity

Various informal descriptions of complex systems have been put forward, and these may give some insight into their properties. A special edition of Science about complex systems highlighted several of these:

  * A complex system is a highly structured system, which shows structure with variations (N. Goldenfeld and Kadanoff)
  * A complex system is one whose evolution is very sensitive to initial conditions or to small perturbations, one in which the number of independent interacting components is large, or one in which there are multiple pathways by which the system can evolve (Whitesides and Ismagilov)
  * A complex system is one that by design or function or both is difficult to understand and verify (Weng, Bhalla and Iyengar)
  * A complex system is one in which there are multiple interactions between many different components (D. Rind)
  * Complex systems are systems in process that constantly evolve and unfold over time (W. Brian Arthur)

Features of complex systems:

Complex systems may have the following features:
Difficult to determine boundaries

It can be difficult to determine the boundaries of a complex system[citation needed]. The decision is ultimately made by the observer.
Complex systems may be open

Complex systems are usually open systems — that is, they exist in a thermodynamic gradient and dissipate energy. In other words, complex systems are frequently far from energetic equilibrium: but despite this flux, there may be pattern stability, see synergetics.
Complex systems may have a memory

The history of a complex system may be important. Because complex systems are dynamical systems they change over time, and prior states may have an influence on present states. More formally, complex systems often exhibit hysteresis.
Complex systems may be nested

The components of a complex system may themselves be complex systems. For example, an economy is made up of organisations, which are made up of people, which are made up of cells - all of which are complex systems.
Dynamic network of multiplicity

As well as coupling rules, the dynamic network of a complex system is important. Small-world or scale-free networks which have many local interactions and a smaller number of inter-area connections are often employed. Natural complex systems often exhibit such topologies. In the human cortex for example, we see dense local connectivity and a few very long axon projections between regions inside the cortex and to other brain regions.
May produce emergent phenomena

Complex systems may exhibit behaviors that are emergent, which is to say that while the results may be deterministic, they may have properties that can only be studied at a higher level. For example, the termites in a mound have physiology, biochemistry and biological development that are at one level of analysis, but their social behavior and mound building is a property that emerges from the collection of termites and needs to be analysed at a different level.
Relationships are non-linear

In practical terms, this means a small perturbation may cause a large effect (see butterfly effect), a proportional effect, or even no effect at all. In linear systems, effect is always directly proportional to cause. See nonlinearity.
Relationships contain feedback loops

Both negative (damping) and positive (amplifying) feedback are often found in complex systems. The effects of an element’s behaviour are fed back to in such a way that the element itself is altered.

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