Computable self-reference and recursion theorem
For any Turing machine T taking a machine description and ordinary input, a machine R can behave on input x as T does on (⟨R⟩,x).
Key knowledge
For any Turing machine T taking a machine description and ordinary input, a machine R can behave on input x as T does on (⟨R⟩,x). Consequently each total computable transformation F mapping valid machine descriptions to valid descriptions has a semantic fixed point R with L(R)=L(F(⟨R⟩)).
Conditions and scope
- Use a standard effective machine encoding with computable construction, quoting and universal simulation; T itself may fail to halt.
- The fixed point concerns behavior or recognized language, not equality of source text, efficiency, or a noncomputable transformation. Totality of F ensures that a valid transformed machine can be obtained.
Sources · 1
References
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Evidence and review
Lecture 11 slides 4 and 6 machine-description construction and recursion theorem; slide 8 fixed-point theorem and simulation proof. Input-pair order is a declared equivalent encoding convention.
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