1. S. Abramsky, A. Jung, Domain theory, in: Handbook of Logic in Computer Science, vol. 3, Oxford, 1994, 1–168.
2. B.A. Davey, H.A. Pristley, Introduction to Lattices and Order, Cambridge, 1994.
3. Rigid Borel sets and better quasiorder theory;van Engelen;Contem. Math.,1987
4. Hierarchies of function classes defined by the first-value operator;Hemmerling;RAIRO – Theor. Inform. Appl.,2008
5. P. Hertling, Topologische Komplexitätsgrade von Funktionen mit endlichem Bild. Informatik-Berichte 152, Fernuniversität Hagen, December 1993.