F= = F + “ = ” + 2 Axiom Schemata Axiom Schema 6 x = x. Axiom Schema 7 x = y ⊃ (SzxA ⊃ SzyA) where A is an atomic wff. A first order theory is a first-order theory with equality if it has a binary predicate = such that the wffs above are theorem of the theory
The primitive symbols of E are those of F, plus the symbol ∃. The formation Rules of E are those of F, plus the following If B is a wff of E and x is an individual variable, then ∃xB is a wff of E. The axiom schemata of E are those of F plus
Some Properties 1. ∆n is consistent. 2. Γ ⊆ ∆n ⊆ ∆n+1 ⊆ ∆Γ 3. ∆Γ is complete. 4. If ∆Γ ` A then there exists n ∈ N such that ∆n ` A. 5. A ∈ ∆Γ iff ∆Γ ` A 6. ∆Γ is consistent