Utm theorem

In computability theory the utm theorem, or Universal Turing machine theorem, is a basic result about Gödel numberings of the set of computable functions. It affirms the existence of a computable universal function, which is capable of calculating any other computable function (Rogers 1967:22). The universal function is an abstract version of the universal turing machine, thus the name of the theorem.

Roger's equivalence theorem provides a characterization of the Gödel numbering of the computable functions in terms of the smn theorem and the utm theorem.

The utm theorem states that that there is a partial computable function u of two variables such that, for every computable function f of one variable, the there is an e such that ${\displaystyle f(x)\simeq u(e,x)}$ for all x. This means that, for each x, either f(x) and u(e,x) are both defined and are equal, or are both undefined (Soare 1987: 15).

The theorem thus shows that, defining φ_e(x) as u(e,x), the sequence φ₁, φ₂, … is an enumeration of the partial computable functions. The function ${\displaystyle u}$ in the statement of the theorem is called a universal function.

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