∆-Definability of Sequence-Coding Operations
th Let p1 = 2, p2 = 3,..., pi = i prime number.
N If a = ha1, . . . , aki, then |a| = k and( a)i = ai. All of the sequence-coding operations are ∆-definable (hence, representable). We have∆-formulas: Divides(y, x): ≡ (∃z ≤ x)[x = y · z], Prime(x): ≡ S0 < x ∧ (∀y ≤ x)[Divides(y, x) → (y = 1 ∨ y = x)] We have∆-formulas: Divides(y, x): ≡ (∃z ≤ x)[x = y · z], Prime(x): ≡ S0 < x ∧ (∀y ≤ x)[Divides(y, x) → (y = 1 ∨ y = x)] | {z } ¬Divides(y, x) ∨ (y = 1 ∨ y = x) Remark: Technically, ¬Divides(y, x) is not a∆-formula. However, it is equivalent to a∆-formula. The set 2 PrimePair := {(pi, pi+1): i ≥ 1} ⊆ N is defined by the∆-formula PrimePair(x, y):≡ Prime(x) ∧ Prime(y) ∧ x < y ∧ (∀z < y)[Prime(z) → z ≤ x]. The set 2 PrimePair := {(pi, pi+1): i ≥ 1} ⊆ N is defined by the∆-formula PrimePair(x, y):≡ Prime(x) ∧ Prime(y) ∧ x < y ∧ (∀z < y)[Prime(z) → z ≤ x]. The set