Chaitin's construction
Introduction
Wikipedia article on Chaitin's construction, referring to e.g.
- Computing a Glimpse of Randomness (written by Cristian S. Calude, Michael J. Dinneen, and Chi-Kou Shu)
- Omega and why math has no TOEs (Gregory Chaitin).
Basing it on combinatory logic
Some more direct relatedness to functional programming: we can base on combinatory logic (instead of a Turing machine), see the prefix coding system described in Binary Lambda Calculus and Combinatory Logic (page 20) written by John Tromp:
of course, , are metavariables, and also some other notations are changed slightly.
Now, Chaitin's construction will be here
where
- should denote an unary predicate “has normal form” (“terminates”)
- should mean an operator “decode” (a function from finite bit sequences to combinatory logic terms)
- should denote the set of all finite bit sequences
- should denote the range of decoding function, e.g. the syntactically correct bit sequences (semantically, they may either terminate or diverge),
- “Absolut value”
- should mean the length of a bit sequence (not combinatory logic term evaluation!)
Here, is a partial function (from finite bit sequences). If this is confusing or annoying, then we can choose a more Haskell-like approach, making a total function:
dc :: [Bit] -> Maybe CL
then, Chaitin's construction will be
where should denote false truth value.