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[2] K. GÖDEL: The consistency of the axiom of choice [ ...]. Princeton Univ. Press 1940.

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[5] T. JECH, P. VOPĚNKA: Generalized Souslin's hypothesis. to appear in Zeitschr. math. Logik.

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[7] P. KUREPA: L'hypothèse de ramification. Comptes Rendus Acad. Sci. Paris 202 (1936), 185-187. Zbl 0014.39402

[8] A. LÉVY: Indépendence conditionnelle de $V=L$ [.. ]. ibid., 245 (1957), 1582-3. MR 0089155

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[15] P. VOPĚNKA: On $\nabla $-model of set theory. ibid., 13 (1965), 267-272. MR 0182571

[16] P. VOPĚNKA: Properties of $\nabla $ -model. ibid., 13 (1965), 441-444. MR 0189984

[17] P. VOPĚNKA: $\nabla $ -models in which GCH does not hold. ibid., 14 (1966), 95-99. MR 0200142

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