We discuss Robinson's model theoretic proof of Tarski's theorem on undefinability of truth. We present two other "diagonal-free" proofs of Tarski's theorem, and we compare undefinability of truth to other forms of undefinability in nonstandard models of arithmetic. © 2003 Elsevier B.V. All rights reserved.
Kossak, R. (2004). Undefinability of truth and nonstandard models. Annals of Pure and Applied Logic, 126(1-3 SPEC. ISS.), 115–123. https://doi.org/10.1016/j.apal.2003.10.011