This article, and the two companion articles it mentions, can be found in the book "Metamagical Themas" [0] in chapters 17-19, as well as all of his other articles that appeared in this series of Scientific American.
[0]: https://www.goodreads.com/book/show/181239.Metamagical_Thema...
(the book's title is the article series, which originated as an anagram of the article series that Martin Gardner authored, "Mathematical Games," also published in Scientific American and which Hofstadter then took over)
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