Finite Model Theory/Logics and Structures
FMT studies logics on finite structures. An outline of the most important of these objects of study is given here.
- 1 Logics
- 2 Structures
The logics defined here and used throughout the book are always relational, i.e. without function symbols, and finite, i.e. have a finite universe, without further notice.
Fragments of FO
The subsequent restrictions can analogously be found in other logics like SO.
- MFO ...
- ESO and USO ...
- FOn ...
Second Order Logic (SO)
Second-order logic extends first-order logic by adding variables and quantifiers that range over sets of individuals. For example, the second-order sentence says that for every set S of individuals and every individual x, either x is in S or it is not. That is, the rules are extended by:
- If X is a n-ary relation variable and t1 ... tn are terms then X t1 ... tn is a formula
- If φ is a formula and X a relation variable then is a formula
Fragments of Second Order Logic
- The fragment monadic second-order logic (MSO) only unary relation variables ("set variables")are allowed.
- The existential fragment (ESO) is second-order logic without universal second-order quantifiers, and without negative occurrences of existential second-order quantifiers.
- USO ...
The intention is to extend FO by an infinite disjunction element over a set ψ of formulas (of infinitary logic)
So the following infinitary logics can be defined
- where ψ is an arbitrary set of formulas, e.g. uncountable
- where ψ is a countable set of formulas