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FLORIAN POP’S MATH 702 FALL 2005 Topics in Algebra: Seminar in Model Theory and Forms 1. AN INTRODUCTION TO MODEL THEORY Asher Auel 1.1. First Order Predicate Calculus. Definition 1. Given index sets I and J, families of natural numbers µ = (µi)i∈I and ν = (νj)j∈J , and a set K, a first order language or language L = (µ, ν, K) is the free semigroup generated by the following elements • variable symbols X1,X2,... • constant symbols, i.e. the elements of K • for each i ∈ I, a µi-ary relation symbol Ri • the binary relation symbol = called equals • for each j ∈ J, a νj-ary function symbol Fj • the logical symbols ∀, ∃, ¬, ∧, ∨, →, ↔, called the universal quantifier, existential quantifier, negation, and (or disjunction), or (or conjunction), implication, and dou- ble implication symbols, respectively • the parentheses (, ) and bracket [, ] symbols. Words of L are called strings. The natural numbers µi and νj are called the arity of Ri and Fj, respectively. Remark 2. One need not include all the above logical symbols as independent symbols of the language L, since for the purposes of first order logic, some of them can be defined in terms of the others. For example, the convention in [3] is to include only ¬, ∨ and ∃, since for any strings ϕ, ψ ∈ L we may define, • ϕ ∧ ψ as ¬[¬ϕ ∨ ¬ψ] • ϕ → ψ as ¬ϕ ∨ ψ • ϕ ↔ ψ as [ϕ → ψ] ∧ [ψ → ϕ] • (∀Xl)[φ] as ¬(∃Xl)[¬φ]. Being even more clever, one needs in fact only define the logical symbol | called alternative denial and then define • ¬ϕ as ϕ|ϕ • ϕ ∨ ψ as ¬ϕ|¬ψ. Intuitively, ϕ|ψ means “¬[ϕ ∧ ψ].” Similarly, one need only take the “xor” symbol. Thus it’s sufficient to take | and ∃ as the only logical symbols for our language, though we’ll stick with the convention of [3]. We now define several distinguished subsets of L: terms: The subset terms ⊂ L of terms is the smallest subset containing the vari- able and constant symbols and closed under function symbols, i.e. terms contains {Xn}n∈N ∪ K and satisfies Fj(t1, . , tνj ) ∈ terms, for all t1, . , tνj ∈ terms, j ∈ J. 1 2 atomic formulae: The subset atoms ⊂ L of atomic formulae is the set of all relations among terms, i.e. the set 0 0 {t = t ,Ri(t1, . , tµi ): t, t , t1, . , tµi ∈ terms, i ∈ I}. formulae: Finally, the subset formulas ⊂ L of formula is the smallest subset contain- ing atoms and satisfying f1: ¬φ ∈ formulas for every φ ∈ formulas f2: ϕ ∨ ϕ0 ∈ formulas for every ϕ, ϕ0 ∈ formulas f3: (∃Xl)[φ] ∈ formulas for every φ ∈ formulas Example 3. The first order language of groups Lgrp is generated by a single binary function symbol ·, where we usually write ·(t, t0) as (t · t0) for terms t, t0, and constant symbol e. We also define the language of rings Lring, which is generated by two binary operations usually denoted by + and ·, and two constant symbols 0 and 1. We might as well also define the language of ordered rings Lordring by adding a single additional binary relation ≤ to Lrign. The following are example of strings, terms, atomic formula, and formula in Lordring: • )¬((1 ∨ X10 ≤ [0 · 111]∃0X3 → 111( • (1 · (1 + 1)) · (0 · (1 + (0 + 1))) • 1 ≤ (0 · (1 + 0)) • (∃X1)[X1 · (1 + 1) = X2] → (∃X3)[X3 · (1 + 1 + 1) = X2]. A languages is just a set of strings of symbols. In fact, we can write any formula ϕ(X1,...,Xm) of a language L in prenex disjunctive normal form (Q1Y1) ... (QnYn)[∨i ∧j Aij] where each Qi is a quantifier (∃ or ∀), and each Aij is an atomic formula, and where “write” we mean the two formulae are “logically equivalent,” where we’ll find out later on what that means. Remark 4. By the recursive construction of the set of formulas, we may prove properties of formulas by an induction on structure or induction of formulas. Given a property P of strings, we first prove P holds for all atomic formula and then supposing it holds for formula 0 0 ϕ and ϕ we show that P holds for ¬ϕ, ϕ ∨ ϕ and (∃Xl)[ϕ] for all l ∈ N. Definition 5. Let L be a language and X a variable symbol. Then we’ll define the notion of a free occurrence of a variable X in a formula by induction on structure. We’ll say that • any occurrence of X in any atomic formula is free • if an occurrence of X in a formula ϕ is free then this occurrence is – also free in ¬ϕ – also free in ϕ ∨ ϕ0 for any formula ϕ0 – also free in (∃X0)[ϕ] for any variable symbol X0 distinct from X and not free in (∃X)[ϕ]. Any occurrence of X in ϕ that is not free is called bound. If X has a free occurrence in a formula ϕ, then X is called a free variable of ϕ, and we’ll write ϕ as ϕ(X1,...,Xm) to indicate that X1,...,Xm include all free variables of ϕ. We’ll call a formula without free variables a sentence of L, and denote by sents the set of sentences of L. 3 1.2. Structures. Though a language is a completely formal object, we usually “interpret” a language through a structure, and usually don’t even realize there is an ambient language. Unless otherwise stated, let L = (µ, ν, K) be a fixed language. A A Definition 6. A structure A for L (or an L-structure) is a 4-tuple A = (A, φ, (Ri )i∈I , (Fj )j∈J ), where • φ : K → A is a map of sets A µi • Ri ⊂ A is a µi-ary relation on A A νi • Fj : A → A is a µi-ary operation on A and where the elements of φ(K) ⊂ A are called constants and the set A is called the domain of A.. We “see” or “interpret” languages through their structures via substitutions. Definition 7. Let A be an L-structure, then a substitution from L into A is a function f : terms → A defined by images • f : Xi 7→ xi ∈ A for all variables Xi, i ∈ N • f : k 7→ φ(k) ∈ A for all constant symbols k ∈ K ⊂ terms such that for each j ∈ J the following diagram commute Fj termsνj −→ terms f ↓ ↓ f A Fj Aνj −→ A A i.e. f(Fj(t1, . , tνj )) = Fj (f(t1), . , f(tνj )) for all terms t1, . , tνj of L. Definition 8. Let A be an L-structure. Then we’ll define the truth value of a formula ϕ under a substitution f or the f-truth of ϕ by induction on structure: • t = t0 is true if f(t) = f(t0) ∈ A A • Ri(t1, . , tµi ) is true if (f(t1), . , f(tµi )) ∈ Ri • and supposing that formulas ϕ, ϕ0 have a true value under every substitution then – ¬ϕ is true if ϕ is false – ϕ ∨ ϕ0 is true if ϕ is true and/or if ϕ0 is true – (∃X)[ϕ] is true if there exists x ∈ A such that ϕ is true under the modified 0 0 0 substitution fX7→x defined by X 7→ x and X 7→ f(X ) for all variables X distinct from X. It’s clear that the f-truth of a formula ϕ = ϕ(X1,...,Xm) only depends on the images of f : Xi 7→ xi, and so if ϕ is f-true then we’ll write A |=f ϕ or A |= ϕ(x1, . , xm). In particular, if ϕ is a sentence, then the truth value of ϕ is independent of the substitution f and so we’ll write simply A |= ϕ if ϕ is f-true for any substitution f of L into A. Remark 9. The standard truth tables hold for the composite logical symbols ∧, →, ↔, ∀, as expected. Example 10. How could you tell the structures N, Z, and Z/3 apart as structures of the language Lgrp? 4 1.3. Models. Models are structures for a language satisfying axioms. They are what we’re used to calling groups, rings, fields, etc. Definition 11. • A theory of L or an L-theory is a subset T ⊂ sents of sentences of L. • An L-structure A is called a model of a theory T if A |= ϕ for every ϕ ∈ T, and in this case we’ll write A |= T . • If T 0 is another L-theory, then we’ll write T 0 |= T if A |= T 0 ⇒ A |= T. • If Π is an L-theory and T = {ϕ ∈ sents :Π |= ϕ}, then we’ll say that Π is a set of axioms for T . • Denote by Mod(T ) the class of all models of a theory T . If A is an L-structure, denote by Th(A, L) the set of all sentences of L that are true in A, called the theory of A. For an L-structure A, we’ll denote by L(A) the language L(µ, ν, K tA) gotten by adding a constant symbol for each element of A. Example 12. We write the standard axioms for the algebraic objects we know and love. • Axioms for a group: in Lgrp we have the following set Π of axioms for the theory T of groups, (∀X)(∀Y )(∀Z)[(X · Y ) · Z = X · (Y · Z)] (∀X)[e · X = X · e = X] (∀X)(∃Y )[X · Y = Y · X = e] Then any model G |= T is a group. Now for a given group H, in the language Lgrp(H) we can extend Π by the positive diagram of H, i.e.
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