# Bar recursion

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+nR to O is defined by bar recursion from the functions Ln : RO and B with Bn : ((Vi+nR) x (VnR)) → 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+nR.

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 ViR, 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

1. 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. Recursive Function Theory: Proc. Symposia in Pure Mathematics. 5. American Mathematical Society. pp. 1–27.
2. Martín Escardó; Paulo Oliva. "Selection functions, Bar recursion, and Backwards Induction". Math. Struct. in Comp.Science.
3. Jeremy Avigad; Solomon Feferman (1999). "VI: Gödel's functional ("Dialectica") interpretation". in S. R. Buss. Handbook of Proof Theory.