Definability of Initial Segments

In any nonstandard model of Peano arithmetic, the standard part is not first order definable. But we show that in some model the standard part is definable as the unique solution of a formula (P), where P is a unary predicate variable.

• Type: Journal
• Year: 2002
• Where: NDJFL
• Authors: Saharon Shelah, Akito Tsuboi
• Keywords: NDJFL 2002, Nonstandard Model, Peano Arithmetic, Unary Predicate