# Von Neumann universe

(Redirected from Cumulative hierarchy)

In set theory and related branches of mathematics, the von Neumann universe, or von Neumann hierarchy of sets, denoted V, is the class of hereditary well-founded sets. This collection, which is formalized by Zermelo–Fraenkel set theory (ZFC), is often used to provide an interpretation or motivation of the axioms of ZFC.

The rank of a well-founded set is defined inductively as the smallest ordinal number greater than the ranks of all members of the set.[1] In particular, the rank of the empty set is zero, and every ordinal has a rank equal to itself. The sets in V are divided into a transfinite hierarchy, called the cumulative hierarchy, based on their rank.

## History

The cumulative type hierarchy, also known as the von Neumann universe, is claimed by Gregory H. Moore (1982) to be inaccurately attributed to von Neumann.[2] The first publication of the von Neumann universe was by Ernst Zermelo in 1930.[3]

Existence and uniqueness of the general transfinite recursive definition of sets was demonstrated in 1928 by von Neumann for both Zermelo-Fraenkel set theory[4] and Neumann's own set theory (which later developed into NBG set theory).[5] In neither of these papers did he apply his transfinite recursive method to construct the universe of all sets. The presentations of the von Neumann universe by Bernays[6] and Mendelson[7] both give credit to von Neumann for the transfinite induction construction method, although not for its application to the construction of the universe of ordinary sets.

The notation V is not a tribute to the name of von Neumann. It was used for the universe of sets in 1889 by Peano, the letter V signifying "Verum", which he used both as a logical symbol and to denote the class of all individuals.[8] Peano's notation V was adopted also by Whitehead and Russell for the class of all sets in 1910.[9] The V notation (for the class of all sets) was not used by von Neumann in his 1920s papers about ordinal numbers and transfinite induction. Paul Cohen[10] explicitly attributes his use of the letter V (for the class of all sets) to a 1940 paper by Gödel,[11] although Gödel most likely obtained the notation from earlier sources such as Whitehead and Russell.[9]

The formula V = ⋃αVα is often considered to be a theorem, not a definition.[6][7] Roitman[12] states (without references) that the realization that the axiom of regularity is equivalent to the equality of the universe of ZF sets to the cumulative hierarchy is due to von Neumann.

## Definition

The 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:

• Let V0 be the empty set, {}:
$V_0 := \{\} .$
• For any ordinal number β, let Vβ+1 be the power set of Vβ:
$V_{\beta+1} := \mathcal{P} (V_\beta) .$
• For any limit ordinal λ, let Vλ be the union of all the V-stages so far:
$V_\lambda := \bigcup_{\beta < \lambda} V_\beta .$

A crucial fact about this definition is that there is a single formula φ(α,x) in the language of ZFC that defines "the set x is in Vα".

The class V is defined to be the union of all the V-stages:

$V := \bigcup_{\alpha} V_\alpha.$

An equivalent definition sets

$V_\alpha := \bigcup_{\beta < \alpha} \mathcal{P} (V_\beta)$

for each ordinal α, where $\mathcal{P} (X) \!$ is the powerset of $X$.

The rank of a set S is the smallest α such that $S \subseteq V_\alpha \,.$

The first five von Neumann stages V0 to V4 may be visualized as follows. (An empty box represents the empty set. A box containing only an empty box represents the set containing only the empty set, and so forth.)

The set V5 contains 65536 elements. The set V6 contains 265536 elements, which very substantially exceeds the number of atoms in the known universe. So the finite stages of the cumulative hierarchy cannot be written down explicitly after stage 5. The set Vω has the same cardinality as ω. The set Vω+1 has the same cardinality as the set of real numbers.

## V and set theory

If ω is the set of natural numbers, then Vω is the set of hereditarily finite sets, which is a model of set theory without the axiom of infinity. Vω+ω is the universe of "ordinary mathematics", and is a model of Zermelo set theory. If κ is an inaccessible cardinal, then Vκ is a model of Zermelo-Fraenkel set theory (ZFC) itself, and Vκ+1 is a model of Morse–Kelley set theory.

V is not "the set of all sets" for two reasons. First, it is not a set; although each individual stage Vα is a set, their union V is a proper class. Second, the sets in V are only the well-founded sets. The axiom of foundation (or regularity) demands that every set is well founded and hence in V, and thus in ZFC every set is in V. But other axiom systems may omit the axiom of foundation or replace it by a strong negation (for example is Aczel's anti-foundation axiom). These non-well-founded set theories are not commonly employed, but are still possible to study.

A third objection to the "set of all sets" interpretation is that not all sets are necessarily "pure sets" which are constructed from the empty set using power sets and unions. Zermelo proposed in 1908 the inclusion of urelements, from which he constructed a transfinite recursive hierarchy in 1930.[3] Such urelements are used extensively in model theory, particularly in Fraenkel-Mostowski models.[13]

## Philosophical perspectives

There are two approaches to understanding the relationship of the von Neumann universe V to ZFC (along with many variations of each approach, and shadings between them). Roughly, formalists will tend to view V as something that flows from the ZFC axioms (for example, ZFC proves that every set is in V). On the other hand, realists are more likely to see the von Neumann hierarchy as something directly accessible to the intuition, and the axioms of ZFC as propositions for whose truth in V we can give direct intuitive arguments in natural language. A possible middle position is that the mental picture of the von Neumann hierarchy provides the ZFC axioms with a motivation (so that they are not arbitrary), but does not necessarily describe objects with real existence.

## References

1. ^ Mirimanoff 1917; Moore 1982, pp. 261-262; Rubin 1967, p. 214
2. ^ Gregory H. Moore, "Zermelo's axiom of choice: Its origins, development & influence", 1982, 2013, Dover Publications, ISBN 978-0-486-48841-7. (See page 279 for the assertion of the false attribution to von Neumann. See pages 270 and 281 for the attribution to Zermelo.)
3. ^ a b Ernst Zermelo, "Über Grenzzahlen und Mengenbereiche: Neue Untersuchungen über die Grundlagen der Mengenlehre", Fundamenta Mathematicae, 16 (1930) 29–47 (See particularly pages 36–40.)
4. ^ von Neumann, John (1928), "Über die Definition durch transfinite Induktion und verwandte Fragen der allgemeinen Mengenlehre", Mathematische Annalen 99: 373–391
5. ^ von Neumann, John (1928), "Die Axiomatisierung der Mengenlehre", Mathematische Zeitschrift 27: 669–752 (See pages 745–752.)
6. ^ a b Bernays, Paul (1991) [1958]. Axiomatic Set Theory. Dover Publications. ISBN 0-486-66637-9. (See pages 203–209.)
7. ^ a b Mendelson, Elliott (1964). Introduction to Mathematical Logic. Van Nostrand Reinhold. (See page 202.)
8. ^ Peano, Giuseppe (1889). Arithmetices principia, nova methodo exposita. (See pages VIII and XI.)
9. ^ a b Alfred North Whitehead; Bertrand Russell (2009) [1910]. Principia Mathematica. Volume One. Merchant Books. ISBN 978-1-60386-182-3. (See page 229.)
10. ^ Cohen, Paul Joseph (1966). Set theory and the continuum hypothesis. Addison–Wesley. ISBN 0-8053-2327-9. (See page 88)
11. ^ Gödel, Kurt (1940). The consistency of the axiom of choice and of the generalized continuum-hypothesis with the axioms of set theory. Annals of Mathematics Studies 3. Princeton, N. J.: Princeton University Press.
12. ^ Roitman, Judith (2011) [1990]. Introduction to Modern Set Theory. Virginia Commonwealth University. ISBN 978-0-9824062-4-3. (See page 79.)
13. ^ Howard, Paul; Rubin, Jean (1998). Consequences of the axiom of choice. Providence, Rhode Island: American Mathematical Society. pp. 175–221. ISBN 9780821809778.
• Jech, Thomas, 2003. Set Theory: The Third Millennium Edition, Revised and Expanded. Springer. ISBN 3-540-44085-2.
• Kunen, Kenneth, 1980. Set Theory: An Introduction to Independence Proofs. Elsevier. ISBN 0-444-86839-9.