Forcing (computability): Difference between revisions

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'''Forcing''' in [[recursion theory]] is a modification of [[Paul Cohen (mathematician)|Paul Cohen's]] original [[set theory|set theoretic]] technique of [[forcing (set theory)|forcing]] to deal with the effective concerns in [[recursion theory]]. Conceptually the two techniques are quite similar, in both one attempts to build [[generic set|generic]] objects (intuitively objects that are somehow 'typical') by meeting dense sets. Also both techniques are elegantly described as a relation (customarily denoted <math>\Vdash</math>) between 'conditions' and sentences. However, where set theoretic forcing is usually interested in creating objects that meet every dense set of conditions in the ground model, recursion theoretic forcing only aims to meet dense sets that are arithmetically or hyperarithmetically definable. Therefore some of the more difficult machinery used in set theoretic forcing can be eliminate or substantially simplified when defining forcing in recursion theory. But while the machinery may be somewhat different recursion theoretic and set theoretic forcing are properly regarded as an application of the same technique to different classes of formulas.

==Motivation==

Stuff goes here about why we want to use forcing.

==Terminology==

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Note that for cohen forcing <math>\succ_{C}</math> is the '''reverse''' of the containment relation. This leads to an unfortunate notational confusion where some recursion theorists reverse the direction of the forcing partial order (exchanging <math>\succ_P</math> with <math>\prec_P</math> which is more natural for Cohen forcing but is at odds with the notation used in set theory.

== Generic Objectsobjects ==

== Generic Objects ==

The intuition behind forcing is that our conditions are finite approximations to some object we wish to build and that <math>\sigma</math> is stronger than <math>\tau</math> when <math>\sigma</math> agrees with everything <math>\tau</math> says about the object we are building and adds some information of it's own. For instance in Cohen forcing the conditions can be viewed as finite approximations to a real and if <math>\tau \succ_C \sigma</math> then <math>\sigma</math> tells us the value of the real on more places.

Theoretically one could view any forcing argument as the construction of an [[ultrafilter]] with special properties, that is a maximal compatible subset of

In a moment we will define a relation <math>\sigma \Vdash_P \psi</math> (read <math>\sigma</math> forces <math>\psi</math>) that holds between conditions (elements of <math>P</math>) and sentences but first we need to explain the [[language (mathematics)]] that <math>\psi</math> is a sentence for. However, forcing is a technique not a definition and the language for <math>\psi</math> will depend on the application one has in mind and the choice of <math>P</math>.

The idea is that our language should express facts about the object we wish to build with our forcing construction.

{{mathlogic-stub}}