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Wikipedia / Bar induction

Bar induction is a reasoning principle used in intuitionistic mathematics, introduced by L. E. J. Brouwer. Bar induction's main use is the intuitionistic derivation of the fan theorem, a key result used in the derivation of the uniform continuity theorem.

It is also useful in giving constructive alternatives to other classical results.

The goal of the principle is to prove properties for all infinite sequences of natural numbers (called choice sequences in intuitionistic terminology), by inductively reducing them to properties of finite lists. Bar induction can also be used to prove properties about all choice sequences in a spread (a special kind of set).

Definition

of this sequence is called an initial segment of this choice sequence.

There are three forms of bar induction currently in the literature, each one places certain restrictions on a pair of predicates and the key differences are highlighted using bold font.

Decidable bar induction (BID)

on finite sequences of natural numbers such that all of the following conditions hold:

  • is a bar);

  • is decidable (i.e. our bar is decidable);

  • holds for every choice sequence beginning with the aforementioned finite sequence);

  • being upward hereditary);

holds for the empty sequence (i.e. A holds for all choice sequences starting with the empty sequence).

This principle of bar induction is favoured in the works of, A. S. Troelstra, S. C. Kleene and Albert Dragalin.

Thin bar induction (BIT)

on finite sequences of natural numbers such that all of the following conditions hold:

  • every choice sequence contains a unique at some point (i.e. our bar is thin);

  • ;

  • ;

holds for the empty sequence.

This principle of bar induction is favoured in the works of Joan Moschovakis and is (intuitionistically) provably equivalent to decidable bar induction.

Monotonic bar induction (BIM)

on finite sequences of natural numbers such that all of the following conditions hold:

  • at some point;

  • (i.e. our bar is monotonic);

  • ;

  • ;

holds for the empty sequence.

This principle of bar induction is used in the works of A. S. Troelstra, S. C. Kleene, Dragalin and Joan Moschovakis.

Relationships between these schemata and other information

The following results about these schemata can be intuitionistically proved:

" is a "turnstile".)

Unrestricted bar induction

An additional schema of bar induction was originally given as a theorem by Brouwer under the name The Bar Theorem. However, the proof for this theorem was erroneous, and unrestricted bar induction is not considered to be intuitionistically valid (see Dummett 1977 pp 94–104 for a summary of why this is so). The schema of unrestricted bar induction is given below for completeness.

on finite sequences of natural numbers such that all of the following conditions hold:

  • at some point;

  • ;

  • ;

holds for the empty sequence.

Relations to other fields

In classical reverse mathematics is a well-order, then we have the schema of transfinite induction for arbitrary formulas.

See also

References

  • L. E. J. Brouwer Brouwer, L. E. J. Collected Works, Vol. I, Amsterdam: North-Holland (1975).

  • Michael Dummett, Elements of intuitionism, Clarendon Press (1977)

  • S. C. Kleene, R. E. Vesley, The foundations of intuitionistic mathematics: especially in relation to recursive functions, North-Holland (1965)

  • Michael Rathjen, The role of parameters in bar rule and bar induction, Journal of Symbolic Logic 56 (1991), no.2, pp.715–730.

  • A. S. Troelstra, Choice sequences, Clarendon Press (1977)

  • A. S. Troelstra and Dirk van Dalen, Constructivism in Mathematics, Studies in Logic and the Foundations of Mathematics, Elsevier (1988)