Robinson's joint consistency theorem

Robinson's joint consistency theorem is an important theorem of mathematical logic.

It is related to Craig interpolation and Beth definability.

The classical formulation of Robinson's joint consistency theorem is as follows: Let

be first-order theories.

are consistent and the intersection

is complete (in the common language of

), then the union

is called complete if it decides every formula, meaning that for every sentence

the theory contains the sentence or its negation but not both (that is, either

Since the completeness assumption is quite hard to fulfill, there is a variant of the theorem: Let

be first-order theories.

are consistent and if there is no formula

in the common language of

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