en.wikipedia.org/wiki/Von_Neumann_universe Von Neumann universe The sets in V are divided into the transfinite hierarchy Vα , called the cumulative hierarchy, based on their rank. DefinitionThe cumulative hierarchy is a collection of sets Vα indexed by the class of ordinal numbers; in particular, Vα is the set of all sets having ranks less than α. Thus there is one set Vα for each ordinal number α. Vα may be defined by transfinite recursion as follows: 略す
