What is the definition of complex system?

Complex systems generally refer to nonlinear complex systems.

System dynamics defines nonlinear complex system as a system with high-order, multi-loop and nonlinear information feedback structure. No matter in nature or in social and economic fields, nonlinear complex systems abound.

The feedback loop in the nonlinear complex system forms an interconnected and constrained structure. As far as the socio-economic system is concerned, the feedback loop describes the relationship between the key variable "the lever point of decision" and other variables around it. Decision leads to action, which changes the state around the system and produces new information-the basis of new decisions in the future. This circular action forms a feedback loop.

As we all know, the order of a system is determined by the number of state variables in the system. The order of nonlinear complex systems is generally four or more than five. The order of system dynamics model of typical socio-economic system is between ten and hundreds. Painfully speaking, the order of the simplified system dynamics model of a typical western city is 20.

The American national model built by the Center for System Dynamics of the Massachusetts Institute of Technology has an order of more than 200. The order of real system is often very high, but the order of model can be determined according to different modeling purposes and requirements. The model of an enterprise should generally include the main state variables such as employees, inventory, assets, equipment, market and product reputation level.

Employees can also be divided into skilled and unskilled according to their needs; Inventory is divided into two state variables: finished goods inventory and semi-finished goods inventory, and orders increase, a country's population model; As far as population state is concerned, according to the requirements of modeling, it can be one population state, four population states,15,66 or even 80 or 100 population state variables according to age. By analogy, buildings, machines and equipment can also divide state variables according to age groups.

In addition, it should be noted that although the order of a system dynamic model is mainly determined by the number of equations marked with "L", if the order of the model is to be accurately estimated and the behavior of the system is comprehensively analyzed, it is necessary to consider whether there is time delay and smoothness in the system. I won't elaborate on this.