1. Åqvist, L. (1984). Deontic logic In Gabbay, D., & Günthner, F. (Eds.), Handbook of philosophical logic: Extensions of classical logic (Vol. 2, pp. 605–714). Dordrecht: Reidel.
2. Borceux, F. (1994). In Handbook of categorical algebra 3: Categories of sheaves. Cambridge: Cambridge.
3. Božić, M. (1984). Positive logic with double negation. Publications de l’Institut Mathématique (Beograd), 35, 21–30.
4. Došen, K. (1984). Intuitionistic double negation as a necessity operator. Publications de l’Institut Mathématique (Beograd), 35, 15–20.
5. Esakia, L., & Löwe, B. (2012). Fatal Heyting algebras and forcing persistent sentences. Studia Logica, 100, 163–173.