# George J. Klir (1969). Approach to General Systems Theory, 1969.
# George J. Klir (1969). Approach to General Systems Theory, 1969.
==External links==
==External links==
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* [http://www.physicalgeography.net/fundamentals/4b.html ''Definitions of Systems and Models''] by Michael Pidwirny, 1999-2007.
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* [https://www.physicalgeography.net/fundamentals/4b.html ''Definitions of Systems and Models''] by Michael Pidwirny, 1999-2007.
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* [http://www.muellerscience.com/SPEZIALITAETEN/System/System_Definitionen.htm ''Definitionen von "System" (1572-2002)''] by Roland Müller, 2001-2007 (most in German).
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* [https://www.muellerscience.com/SPEZIALITAETEN/System/System_Definitionen.htm ''Definitionen von "System" (1572-2002)''] by Roland Müller, 2001-2007 (most in German).