Christopher Alexander. 2011. “Systems Generating Systems.” In Computational Design Thinking, edited by Achim Mengers and Sean Ahlquist, 58–67. Chichester, England: John Wiley & Sons. preview at Google Books 
. Reproduced by permission of Christopher Alexander, from “Systems Generating Systems”, Architectural Design, volume 38 (December), John Wiley & Sons Ltd (London), 1968, pp. 605-610. Originally published in Systemat, a journal of the Inland Steel Products Company.
See an analysis of the paper at Systems Generating Systems -- Architectural Design Theory by Christopher Alexander (1968)