Combining Texts

Ideas for 'On the Question of Absolute Undecidability', 'First-order Logic, 2nd-order, Completeness' and 'First-Order Logic'

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4 ideas

5. Theory of Logic / J. Model Theory in Logic / 1. Logical Models
10756 A model is a domain, and an interpretation assigning objects, predicates, relations etc. [Rossberg]
     Full Idea: A standard model is a set of objects called the 'domain', and an interpretation function, assigning objects in the domain to names, subsets to predicate letters, subsets of the Cartesian product of the domain with itself to binary relation symbols etc.
     From: Marcus Rossberg (First-order Logic, 2nd-order, Completeness [2004], §3)
     A reaction: The model actually specifies which objects have which predicates, and which objects are in which relations. Tarski's account of truth in terms of 'satisfaction' seems to be just a description of those pre-decided facts.
5. Theory of Logic / J. Model Theory in Logic / 2. Isomorphisms
10758 If models of a mathematical theory are all isomorphic, it is 'categorical', with essentially one model [Rossberg]
     Full Idea: A mathematical theory is 'categorical' if, and only if, all of its models are isomorphic. Such a theory then essentially has just one model, the standard one.
     From: Marcus Rossberg (First-order Logic, 2nd-order, Completeness [2004], §3)
     A reaction: So the term 'categorical' is gradually replacing the much-used phrase 'up to isomorphism'.
5. Theory of Logic / J. Model Theory in Logic / 3. Löwenheim-Skolem Theorems
10288 Down Löwenheim-Skolem: if a countable language has a consistent theory, that has a countable model [Hodges,W]
     Full Idea: Downward Löwenheim-Skolem (the weakest form): If L is a first-order language with at most countably many formulas, and T is a consistent theory in L. Then T has a model with at most countably many elements.
     From: Wilfrid Hodges (First-Order Logic [2001], 1.10)
10289 Up Löwenheim-Skolem: if infinite models, then arbitrarily large models [Hodges,W]
     Full Idea: Upward Löwenheim-Skolem: every first-order theory with infinite models has arbitrarily large models.
     From: Wilfrid Hodges (First-Order Logic [2001], 1.10)