Bar recursion is a generalized form of recursion developed by C. Spector in his 1962 paper.[1] It is related to bar induction in the same fashion that primitive recursion is related to ordinary induction, or transfinite recursion is related to transfinite induction.
Technical definition
Let V, R, and O be types, and i be any natural number, representing a sequence of parameters taken from V. Then the function sequence f of functions fn from Vi+n → R to O is defined by bar recursion from the functions Ln : R → O and B with Bn : ((Vi+n → R) x (Vn → R)) → O if:
- fn((λα:Vi+n)r) = Ln(r) for any r long enough that Ln+k on any extension of r equals Ln. Assuming L is a continuous sequence, there must be such r, because a continuous function can use only finitely much data.
- fn(p) = Bn(p, (λx:V)fn+1(cat(p, x))) for any p in Vi+n → R.
Here "cat" is the concatenation function, sending p, x to the sequence which starts with p, and has x as its last term.
(This definition is based on the one by Escardó and Oliva.[2])
Provided that for every sufficiently long function (λα)r of type Vi → R, there is some n with Ln(r) = Bn((λα)r, (λx:V)Ln+1(r)), the bar induction rule ensures that f is well-defined.
The idea is that one extends the sequence arbitrarily, using the recursion term B to determine the effect, until a sufficiently long node of the tree of sequences over V is reached; then the base term L determines the final value of f. The well-definedness condition corresponds to the requirement that every infinite path must eventually pass through a sufficiently long node: the same requirement that is needed to invoke a bar induction.
The principles of bar induction and bar recursion are the intuitionistic equivalents of the axiom of dependent choices.[3]
References
- ^ C. Spector (1962). "Provably recursive functionals of analysis: a consistency proof of analysis by an extension of principles in current intuitionistic mathematics". In F. D. E. Dekker (ed.). Recursive Function Theory: Proc. Symposia in Pure Mathematics. Vol. 5. American Mathematical Society. pp. 1–27.
- ^ Martín Escardó; Paulo Oliva. "Selection functions, Bar recursion, and Backwards Induction" (PDF). Math. Struct. in Comp.Science.
- ^ Jeremy Avigad; Solomon Feferman (1999). "VI: Gödel's functional ("Dialectica") interpretation". In S. R. Buss (ed.). Handbook of Proof Theory (PDF).
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This page was last edited on 29 December 2018, at 07:15