Chapter 11: The Notion of Computation

Section 12: Universality in Turing Machines and Other Systems

Testing universality [in symbolic systems]

One can tell that a symbolic system is universal if one can find expressions that act like the s and κ combinators, so that, for example, for some expression e, e[x][y][z] evolves to x[z][y[z]].

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From Stephen Wolfram: A New Kind of Science [citation]