J.de ROSNAY, thus resumes the basic concepts of functions in FORRESTER's Systems Dynamics:
"The basic functions of any system are as follows:
"Flow (of energy, information or elements are moving from stores to stores. These flows are meeted in quantity per time unit (figuring outputs)… as for example: money flows (wages…per month), …products (cars produced per day)… or people (travellers per hour), or information (bits… per micro second in a computer).
"Energy and matter's flows rise or lower the levels in the stores. They move through communication channels and are represented by thick black arrows.
"The information flows are represented by dotted arrows. The informations are the base for decisions which allow for action on the flows, in order to maintain, increase or lower levels in the stores.
"Gates (or rates)… control the turnover of the different flows. Each gate may be seen as a decision center receiving informations and transforming them in actions: for example a manager, an institution, some transformation agent or an enzyme as catalyser.
Such actions result in an increase or decrease of the flows intensity.
Their symbolic representation is a gate or tap on a flow line.
"Delays result of the different velocities of circulation of the flows, of storing periods in the stores, or of frictions between the elements of the system. Delays playa very important role in the amplification or inhibition phenomena, which are typical of the complex systems.
"Finally, there are information loops, called feedbacks. They play a determinating role in the systems behavior by combining the effects of stores (levels), delays, gates and flows.
"There are two types of feedbacks: positive and negative feedbacks. The whole dynamics of change in a system (Growth and evolution for example) depend on positive feedback. Regulation and stability depend on negative feedback (Restoring of equilibria and self-maintenance)" (1975, p.99-100).
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Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]
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