[1] K. KURATOWSKI A. MOSTOWSKI: Teoria mnogości. Warszawa - Wroclaw, 1952. MR 0054684

[2] P. VOPĚNKA: General theory of $\nabla $ -models. Comment. Math. Univ. Carolinae 8, 1 (1967), 145-170. MR 0214460

[3] P. VOPĚNKA: Properties of $\nabla $ -models. Bull. Acad. Polon. Sci., Sér. Sci. Math., Astr. et Phys., Vol. XIII, No 7 (1965).

[4] P. VOPĚNKA K. HRBÁČEK: The consistency of some theorems on real numbers. Bull. Acad. Polon. Sci. (to appear). MR 0225662