1. [1] Andréka, H., J. van Benthem, and I. Németi, "Back and forth between modal logic and classical logic", Logic Journal of the IGPL, vol. 3 (1995), pp. 685--720.
2. [2] Bagheri, S. M., and M. Moniri, "Some results on Kripke models over an arbitrary fixed frame", Mathematical Logic Quarterly, vol. 49 (2003), pp. 479--84.
3. [3] Burr, W., "Fragments of Heyting arithmetic", The Journal of Symbolic Logic, vol. 65 (2000), pp. 1223--40.
4. [4] Chang, C. C., and H. J. Keisler, Model Theory, 3d edition, vol. 73 of Studies in Logic and the Foundations of Mathematics, North-Holland Publishing Co., Amsterdam, 1990.
5. [5] van Dalen, D., Logic and Structure, 4th edition, Universitext. Springer-Verlag, Berlin, 2004.