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Wikipedia / Axiom of constructibility

The axiom of constructibility is a possible axiom for set theory in mathematics that asserts that every set is constructible. The axiom is usually written as V = L represents the von Neumann universe of all well-founded sets represents the constructible sets. In Zermelo–Fraenkel set theory.

The axiom of constructibility, first investigated by Kurt Gödel, is inconsistent with the proposition that zero sharp exists and stronger large cardinal axioms (see list of large cardinal properties). Generalizations (i.e. weaker versions) of this axiom are explored in inner model theory.

Implications

The axiom of constructibility implies the axiom of choice (AC), given Zermelo–Fraenkel set theory without the axiom of choice (ZF). It also settles many natural mathematical questions that are independent of Zermelo–Fraenkel set theory with the axiom of choice (ZFC); for example, the axiom of constructibility implies the generalized continuum hypothesis, the negation of Suslin's hypothesis, and the existence of an analytical) non-measurable set of real numbers, all of which are independent of ZFC.

The axiom of constructibility implies the non-existence of those large cardinals with consistency strength greater or equal to 0#-Erdős does contain the initial ordinals, it excludes the auxiliary structures (e.g. measures) that endow those cardinals with their large cardinal properties.

Although the axiom of constructibility does resolve many set-theoretic questions, it is not typically accepted as an axiom for set theory in the same way as the ZFC axioms. Among set theorists of a realist bent, who believe that the axiom of constructibility is either true or false, most believe that it is false. can't exist), with no clear reason to believe that these are all of them. In part it is because the axiom is contradicted by sufficiently strong large cardinal axioms. This point of view is especially associated with the Cabal, or the "California school" as Saharon Shelah would have it.

In arithmetic

Especially from the 1950s to the 1970s, there have been some investigations into formulating an analogue of the axiom of constructibility for subsystems of second-order arithmetic. A few results stand out in the study of such analogues:

  • is a constructible real.

    . have been given.

Significance

The major significance of the axiom of constructibility is in Kurt Gödel's 1938 proof of the relative consistency of the axiom of choice and the generalized continuum hypothesis to Von Neumann–Bernays–Gödel set theory. (The proof carries over to Zermelo–Fraenkel set theory, which has become more prevalent in recent years.)

thereby establishing that AC and GCH are also relatively consistent.

Gödel's proof was complemented in 1962 by Paul Cohen's result that both AC and GCH are independent) are also relatively consistent to ZF set theory.

Statements true in L

Here is a list of propositions that hold in the constructible universe):

The existence of a definable well-order of all sets (the formula for which can be given explicitly), and as consequences

Jensen's diamond principles , , and (and hence also Ostaszewski's club principle )

The non-existence of 0# and as a consequence

Accepting the axiom of constructibility (which asserts that every set is constructible) these propositions also hold in the von Neumann universe, resolving many propositions in set theory and some interesting questions in analysis.

References