L-theories
An
L-theory is a
set of
sentences over the
language L
Given an
L-
structure M, the
theory of
M, denoted
Th(M), is the
set of
sentences that
M satisfies; ie, the
set {L-sentence ϕ:M⊨ϕ}
A theory T is called:
satisfiable if there exists an
L-
structure M where
M⊨ϕ for all
ϕ∈T. Then we say that
M is a
model for
T
finitely satisfiable if for every
finite subset
T0⊆T we have that
T0 is satisfiable.
maximal if for every
L-
sentence ϕ, either
ϕ∈T or
¬ϕ∈T.
Note that all theories
Th(M) are maximal.
complete if for every
L-
sentence ϕ, either
T⊨ϕ or
T⊨¬ϕ, where by
T⊨ψ we mean that for every
M modelling T we have
M⊨ψ