S^m_n theorem

In computability theory the S m
n  theorem, (also called the translation lemma, parameter theorem, and the parameterization theorem) is a basic result about programming languages (and, more generally, Gödel numberings of the computable functions) (Soare 1987, Rogers 1967). It was first proved by Stephen Cole Kleene (1943). The name S m
n  comes from the occurrence of an S with subscript n and superscript m in the original formulation of the theorem (see below).

In practical terms, the theorem says that for a given programming language and positive integers m and n, there exists a particular algorithm that accepts as input the source code of a program with m + n free variables, together with m values. This algorithm generates source code that effectively substitutes the values for the first m free variables, leaving the rest of the variables free.

Details

The basic form of the theorem applies to functions of two arguments (Nies 2009, p. 6). Given a Gödel numbering ${\displaystyle \varphi }$ of recursive functions, there is a primitive recursive function s of two arguments with the following property: for every Gödel number p of a partial computable function f with two arguments, the expressions ${\displaystyle \varphi _{s(p,x)}(y)}$ and ${\displaystyle f(x,y)}$ are defined for the same combinations of natural numbers x and y, and their values are equal for any such combination. In other words, the following extensional equality of functions holds for every x:

${\displaystyle \varphi _{s(p,x)}\simeq \lambda y.\varphi _{p}(x,y).}$

More generally, for any m, n > 0, there exists a primitive recursive function ${\displaystyle S_{n}^{m}}$ of m + 1 arguments that behaves as follows: for every Gödel number p of a partial computable function with m + n arguments, and all values of x₁, …, x_m:

${\displaystyle \varphi _{S_{n}^{m}(p,x_{1},\dots ,x_{m})}\simeq \lambda y_{1},\dots ,y_{n}.\varphi _{p}(x_{1},\dots ,x_{m},y_{1},\dots ,y_{n}).}$

The function s described above can be taken to be ${\displaystyle S_{1}^{1}}$.

Formal statement

Given arities ${\displaystyle m}$ and ${\displaystyle n}$, for every Turing Machine ${\displaystyle {\text{TM}}_{x}}$ of arity ${\displaystyle m+n}$ and for all possible values of inputs ${\displaystyle y_{1},\dots ,y_{m}}$, there exists a Turing machine ${\displaystyle {\text{TM}}_{k}}$ of arity ${\displaystyle n}$, such that

${\displaystyle \forall z_{1},\dots ,z_{n}:{\text{TM}}_{x}(y_{1},\dots ,y_{m},z_{1},\dots ,z_{n})={\text{TM}}_{k}(z_{1},\dots ,z_{n}).}$

Furthermore, there is a Turing machine ${\displaystyle S}$ that allows ${\displaystyle k}$ to be calculated from ${\displaystyle x}$ and ${\displaystyle y}$; it is denoted ${\displaystyle k=S_{n}^{m}(x,y_{1},\dots ,y_{m})}$.

Informally, ${\displaystyle S}$ finds the Turing Machine ${\displaystyle {\text{TM}}_{k}}$ that is the result of hardcoding the values of ${\displaystyle y}$ into ${\displaystyle {\text{TM}}_{x}}$. The result generalizes to any Turing-complete computing model.

Example

The following Lisp code implements s₁₁ for Lisp.

 (defun s11 (f x)
   (let ((y (gensym)))
     (list 'lambda (list y) (list f x y))))

For example, (s11 '(lambda (x y) (+ x y)) 3) evaluates to (lambda (g42) ((lambda (x y) (+ x y)) 3 g42)).

See also

• Currying
• Kleene's recursion theorem
• Partial evaluation

References

• Kleene, S. C. (1936). "General recursive functions of natural numbers". Mathematische Annalen. 112 (1): 727–742. doi:10.1007/BF01565439.
• Kleene, S. C. (1938). "On Notations for Ordinal Numbers" (PDF). The Journal of Symbolic Logic. 3: 150–155. (This is the reference that the 1989 edition of Odifreddi's "Classical Recursion Theory" gives on p. 131 for the ${\displaystyle S_{n}^{m}}$ theorem.)
• Nies, A. (2009). Computability and randomness. Oxford Logic Guides. 51. Oxford: Oxford University Press. ISBN 978-0-19-923076-1. Zbl 1169.03034.
• Odifreddi, P. (1999). Classical Recursion Theory. North-Holland. ISBN 0-444-87295-7.
• Rogers, H. (1987) [1967]. The Theory of Recursive Functions and Effective Computability. First MIT press paperback edition. ISBN 0-262-68052-1.
• Soare, R. (1987). Recursively enumerable sets and degrees. Perspectives in Mathematical Logic. Springer-Verlag. ISBN 3-540-15299-7.

External links

• Weisstein, Eric W. "Kleene's s-m-n Theorem". MathWorld.