1. Clark, D. M., Davey, B. A.: Natural Dualities for the Working Algebraist. Cambridge studies in advanced mathematics, vol. 57, Cambridge University Press, Cambridge (1998)
2. Couceiro, M., Lehtonen, E., Waldhauser, T.: A Survey on the Arity Gap. In: Multiple-Valued Logic (ISMVL), 2011 41st IEEE International Symposium on, pp. 277–281 (2011)
3. Csákány B.: Completeness in coalgebras. Acta Sci. Math. 48, 75–84 (1985)
4. Davey B. A.: Natural dualities for structures. Acta Univ. M. Belii Ser. Math. 13, 3–28 (2006)
5. Grätzer, G., Kisielewicz, A.: A survey of some open problems on p n -sequences and free spectra of algebras and varieties. In: Universal algebra and quasigroup theory (Jadwisin, 1989), Res. Exp. Math., vol. 19, pp. 57–88, Heldermann, Berlin (1992)