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the study of any complex system

chaos theory

Computational complexity theory, a field in theoretical computer science and mathematics dealing with the resources required during computation to solve a given problem

the theoretical treatment of Kolmogorov complexity of a string studied in algorithmic information theory by identifying the length of the shortest binary program which can output that string

the application of complexity theory to organizations, which has been influential in strategic management and organizational studies; this area is sometimes referred to as complexity strategy or the study of complex adaptive organizations.

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Systems theory (or systemics or general systems theory), an interdisciplinary field including engineering, biology, and philosophy that incorporates science to study large systems of trees