1. Andréka, H., Madarász, J.X., Németi, I.: Mutual definability does not imply definitional equivalence, a simple example. Math. Log. Q. 51(6), 591–597 (2005)
2. Andréka, H., Németi, I.: Definability Theory Course Notes (2014).
https://old.renyi.hu/pub/algebraic-logic/DefThNotes0828.pdf
3. Boolos, G., Jeffrey, R.C.: Computability and Logic, 3rd edn. Cambridge University Press, Cambridge (1989)
4. de Bouvère, K.L.: Logical synonymity. Indagat. Math. 27, 622–9 (1965)
5. Enderton, H.: A Mathematical Introduction to Logic, 2nd edn. Harcourt/Academic Press, San Diego (2001)