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| In [[mathematical logic]], the '''compactness theorem''' states that a set of [[first-order predicate calculus|first-order]] sentences has a [[model theory|model]] if and only if every finite [[subset]] of it has a model. This theorem is an important tool in [[model theory]], as it provides a useful method for constructing models of any set of sentences that is finitely [[consistency|consistent]].
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| The compactness theorem for the [[propositional calculus]] is a consequence of [[Tychonoff's theorem]] (which says that the product of [[compact space]]s is compact) applied to compact [[Stone space]]s;<ref>See Truss (1997).</ref> hence, the theorem's name. Likewise, it is analogous to the [[finite intersection property]] characterization of compactness in topological spaces: a collection of closed sets in a compact space has a non-empty intersection if every finite subcollection has a non-empty intersection.
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| The compactness theorem is one of the two key properties, along with the downward [[Löwenheim–Skolem theorem]], that is used in [[Lindström's theorem]] to characterize first-order logic. Although there are some generalizations of the compactness theorem to non-first-order logics, the compactness theorem itself does not hold in them.
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| ==History==
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| [[Kurt Gödel]] proved the countable compactness theorem in 1930. [[Anatoly Maltsev]] proved the uncountable case in 1936.<ref>[[Robert Lawson Vaught|Vaught, Robert L.]]: Alfred Tarski's work in model theory. J. Symbolic Logic 51 (1986), no. 4, 869–882</ref>
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| <ref>[[Abraham Robinson|Robinson, A.]]: Non-standard analysis. North-Holland Publishing Co., Amsterdam 1966. page 48.</ref>
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| == Applications ==
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| The compactness theorem has many applications in model theory; a few typical results are sketched here.
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| The compactness theorem implies [[Robinson's principle]]: If a first-order sentence holds in every [[field (mathematics)|field]] of [[characteristic (algebra)|characteristic]] zero, then there exists a constant ''p'' such that the sentence holds for every field of characteristic larger than ''p''. This can be seen as follows: suppose φ is a sentence that holds in every field of characteristic zero. Then its negation ¬φ, together with the field axioms and the infinite sequence of sentences 1+1 ≠ 0, 1+1+1 ≠ 0, …, is not satisfiable (because there is no field of characteristic 0 in which ¬φ holds, and the infinite sequence of sentences ensures any model would be a field of characteristic 0). Therefore, there is a finite subset ''A'' of these sentences that is not satisfiable. We can assume that ''A'' contains ¬φ, the field axioms, and, for some ''k'', the first ''k'' sentences of the form 1+1+...+1 ≠ 0 (because adding more sentences doesn't change unsatisfiability). Let ''B'' contains all the sentences of ''A'' except ¬φ. Then any model of ''B'' is a field of characteristic greater than ''k'', and ¬φ together with ''B'' is not satisfiable. This means that φ must hold in every model of ''B'', which means precisely that φ holds in every field of characteristic greater than ''k''.
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| A second application of the compactness theorem shows that any theory that has arbitrarily large finite models, or a single infinite model, has models of arbitrary large [[cardinality]] (this is the [[Upward Löwenheim–Skolem theorem]]). So, for instance, there are nonstandard models of [[Peano arithmetic]] with uncountably many 'natural numbers'. To achieve this, let ''T'' be the initial theory and let κ be any [[cardinal number]]. Add to the language of ''T'' one constant symbol for every element of κ. Then add to ''T'' a collection of sentences that say that the objects denoted by any two distinct constant symbols from the new collection are distinct (this is a collection of κ<sup>2</sup> sentences). Since every ''finite'' subset of this new theory is satisfiable by a sufficiently large finite model of ''T'', or by any infinite model, the entire extended theory is satisfiable. But any model of the extended theory has cardinality at least κ
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| A third application of the compactness theorem is the construction of [[non-standard analysis|nonstandard models]] of the real numbers, that is, consistent extensions of the theory of the real numbers that contain "infinitesimal" numbers. To see this, let Σ be a first-order axiomatization of the theory of the real numbers. Consider the theory obtained by adding a new constant symbol ε to the language and adjoining to Σ the axiom ε > 0 and the axioms ε < 1/''n'' for all positive integers ''n''. Clearly, the standard real numbers '''R''' are a model for every finite subset of these axioms, because the real numbers satisfy everything in Σ and, by suitable choice of ε, can be made to satisfy any finite subset of the axioms about ε. By the compactness theorem, there is a model *'''R''' that satisfies Σ and also contains an infinitesimal element ε. A similar argument, adjoining axioms ω > 0, ω > 1, etc., shows that the existence of infinitely large integers cannot be ruled out by any axiomatization Σ of the reals.<ref name="Goldblatt">{{cite book |title=Lectures on the Hyperreals |last=Goldblatt |first=Robert |authorlink=Robert Goldblatt |year=1998 |publisher=[[Springer Verlag|Springer]] |location=New York |isbn=0-387-98464-X |pages=10–11 }}</ref>
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| == Proofs ==
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| One can prove the compactness theorem using [[Gödel's completeness theorem]], which establishes that a set of sentences is satisfiable if and only if no contradiction can be proven from it. Since proofs are always finite and therefore involve only finitely many of the given sentences, the compactness theorem follows. In fact, the compactness theorem is equivalent to Gödel's completeness theorem, and both are equivalent to the [[Boolean prime ideal theorem]], a weak form of the [[axiom of choice]].<ref>See Hodges (1993).</ref>
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| Gödel originally proved the compactness theorem in just this way, but later some "purely semantic" proofs of the compactness theorem were found, i.e., proofs that refer to ''truth'' but not to ''provability''. One of those proofs relies on [[ultraproduct]]s hinging on the axiom of choice as follows:
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| Proof: Fix a first-order language L, and let Σ be a collection of L-sentences such that every finite subcollection of L-sentences, ''i'' ⊆ Σ of it has a model <math>\mathcal{M}_i</math>. Also let <math>\prod_{i \subseteq \Sigma}\mathcal{M}_i</math> be the direct product of the structures and ''I'' be the collection of finite subsets of Σ. For each ''i'' in ''I'' let
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| A<sub>''i''</sub> := { ''j'' ∈ ''I'' : ''j'' ⊇ ''i''}.
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| The family of all of these sets A<sub>''i''</sub> generates a proper [[Filter (mathematics)|filter]], so there is an [[ultrafilter]] ''U'' containing all sets of the form A<sub>''i''</sub>.
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| Now for any formula φ in Σ we have:
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| * the set A<sub>{φ}</sub> is in ''U''
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| * whenever ''j'' ∈ A<sub>{φ}</sub>, then φ ∈ ''j'', hence φ holds in <math>\mathcal M_j</math>
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| * the set of all ''j'' with the property that φ holds in <math>\mathcal M_j</math> is a superset of A<sub>{φ}</sub>, hence also in ''U''
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| Using [[ultraproduct#Łoś's theorem|Łoś's theorem]] we see that φ holds in the [[ultraproduct]] <math>\prod_{i \subseteq \Sigma}\mathcal{M}_i/U</math>. So this ultraproduct satisfies all formulas in Σ.
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| ==See also==
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| * [[List of Boolean algebra topics]]
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| * [[Löwenheim-Skolem theorem]]
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| * [[Herbrand's theorem]]
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| * [[Barwise compactness theorem]]
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| * {{cite book | last=Boolos | first=George | coauthors=Jeffrey, Richard; Burgess, John | title=''Computability and Logic'' | edition=fourth | year=2004 | publisher="Cambridge University Press }}
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| * {{cite book | last=Chang | first=C.C. | coauthors=[[Howard Jerome Keisler|Keisler, H. Jerome]] | publisher=[[Elsevier]] | title=Model Theory | year=1989 | edition=third | isbn=0-7204-0692-7 }}
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| * {{cite journal | last=Dawson | first=John W. junior | title=The compactness of first-order logic: From Gödel to Lindström | journal=History and Philosophy of Logic | year=1993 | volume=14 | pages=15–37 | doi=10.1080/01445349308837208 }}
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| * {{cite book | last=Hodges | first=Wilfrid | authorlink=Wilfrid Hodges | publisher=[[Cambridge University Press]] | title=Model theory | year=1993 | isbn=0-521-30442-3 }}
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| * {{cite book | last=Marker | first=David | title= Model Theory: An Introduction | publisher=Springer | year=2002 | isbn=0-387-98760-6| series=[[Graduate Texts in Mathematics]] 217}}
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| * {{cite book | last=Truss | first=John K. | title=Foundations of Mathematical Analysis | year=1997 | publisher=[[Oxford University Press]] | isbn=0-19-853375-6 }}
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| == Further reading ==
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| * {{cite book |author=Hummel, Christoph |title=Gromov's compactness theorem for pseudo-holomorphic curves |publisher=Birkhäuser |location=Basel, Switzerland |year=1997 |pages= |isbn=3-7643-5735-5 |oclc= |doi=}}
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| [[Category:Model theory]]
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| [[Category:Theorems in the foundations of mathematics]]
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| [[Category:Metatheorems]]
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