In mathematics, in set theory, the constructible universe (or Gödel's constructible universe), denoted by L, is a particular class of sets that can be described entirely in terms of simpler sets. L is the union of the constructible hierarchy L_\alpha. It was introduced by Kurt Gödel in his 1938 paper "The Consistency of the Axiom of Choice and of the Generalized Continuum-Hypothesis". In this paper, he proved that the constructible universe is an inner model of ZF set theory (that is, of Zermelo–Fraenkel set theory with the axiom of choice excluded), and also that the axiom of choice and the generalized continuum hypothesis are true in the constructible universe. This shows that both propositions are consistent with the basic axioms of set theory, if ZF itself is consistent. Since many other theorems only hold in systems in which one or both of the propositions is true, their consistency is an important result.

What L is

L can be thought of as being built in "stages" resembling the construction of von Neumann universe, V. The stages are indexed by ordinals. In von Neumann's universe, at a successor stage, one takes to be the set of all subsets of the previous stage, V_\alpha. By contrast, in Gödel's constructible universe L, one uses only those subsets of the previous stage that are: By limiting oneself to sets defined only in terms of what has already been constructed, one ensures that the resulting sets will be constructed in a way that is independent of the peculiarities of the surrounding model of set theory and contained in any such model. Define the Def operator: L is defined by transfinite recursion as follows: If z is an element of L_\alpha, then. So L_\alpha is a subset of, which is a subset of the power set of L_\alpha. Consequently, this is a tower of nested transitive sets. But L itself is a proper class. The elements of L are called "constructible" sets; and L itself is the "constructible universe". The "axiom of constructibility", aka "V = L", says that every set (of V) is constructible, i.e. in L.

Additional facts about the sets Lα

An equivalent definition for L_\alpha is: For any finite ordinal n, the sets L_n and V_n are the same (whether V equals L or not), and thus L_\omega = V_\omega: their elements are exactly the hereditarily finite sets. Equality beyond this point does not hold. Even in models of ZFC in which V equals L, is a proper subset of, and thereafter is a proper subset of the power set of L_\alpha for all. On the other hand, V = L does imply that V_\alpha equals L_\alpha if, for example if \alpha is inaccessible. More generally, V = L implies H \alpha = L_\alpha for all infinite cardinals \alpha. If \alpha is an infinite ordinal then there is a bijection between L_\alpha and \alpha, and the bijection is constructible. So these sets are equinumerous in any model of set theory that includes them. As defined above, is the set of subsets of X defined by \Delta_0 formulas (with respect to the Levy hierarchy, i.e., formulas of set theory containing only bounded quantifiers) that use as parameters only X and its elements. Another definition, due to Gödel, characterizes each as the intersection of the power set of L_\alpha with the closure of under a collection of nine explicit functions, similar to Gödel operations. This definition makes no reference to definability. All arithmetical subsets of \omega and relations on \omega belong to (because the arithmetic definition gives one in ). Conversely, any subset of \omega belonging to is arithmetical (because elements of L_\omega can be coded by natural numbers in such a way that \in is definable, i.e., arithmetic). On the other hand, already contains certain non-arithmetical subsets of \omega, such as the set of (natural numbers coding) true arithmetical statements (this can be defined from so it is in ). All hyperarithmetical subsets of \omega and relations on \omega belong to (where stands for the Church–Kleene ordinal), and conversely any subset of \omega that belongs to is hyperarithmetical.

L is a standard inner model of ZFC

(L,\in) is a standard model, i.e. L is a transitive class and the interpretation uses the real element relationship, so it is well-founded. L is an inner model, i.e. it contains all the ordinal numbers of V and it has no "extra" sets beyond those in V. However L might be strictly a subclass of V. L is a model of ZFC, which means that it satisfies the following axioms: Notice that the proof that L is a model of ZFC only requires that V be a model of ZF, i.e. we do not assume that the axiom of choice holds in V.

L is absolute and minimal

If W is any standard model of ZF sharing the same ordinals as V, then the L defined in W is the same as the L defined in V. In particular, L_\alpha is the same in W and V, for any ordinal \alpha. And the same formulas and parameters in produce the same constructible sets in. Furthermore, since L is a subclass of V and, similarly, L is a subclass of W, L is the smallest class containing all the ordinals that is a standard model of ZF. Indeed, L is the intersection of all such classes. If there is a set W in V that is a standard model of ZF, and the ordinal \kappa is the set of ordinals that occur in W, then L_\kappa is the L of W. If there is a set that is a standard model of ZF, then the smallest such set is such a L_\kappa. This set is called the minimal model of ZFC. Using the downward Löwenheim–Skolem theorem, one can show that the minimal model (if it exists) is a countable set. Of course, any consistent theory must have a model, so even within the minimal model of set theory there are sets that are models of ZF (assuming ZF is consistent). However, those set models are non-standard. In particular, they do not use the normal element relation and they are not well founded. Because both "L constructed within L" and "V constructed within L" result in the real L, and both the L of L_\kappa and the V of L_\kappa are the real L_\kappa, we get that V=L is true in L and in any L_\kappa that is a model of ZF. However, V=L does not hold in any other standard model of ZF.

L and large cardinals

Since, properties of ordinals that depend on the absence of a function or other structure (i.e. formulas) are preserved when going down from V to L. Hence initial ordinals of cardinals remain initial in L. Regular ordinals remain regular in L. Weak limit cardinals become strong limit cardinals in L because the generalized continuum hypothesis holds in L. Weakly inaccessible cardinals become strongly inaccessible. Weakly Mahlo cardinals become strongly Mahlo. And more generally, any large cardinal property weaker than 0# (see the list of large cardinal properties) will be retained in L. However, 0^\sharp is false in L even if true in V. So all the large cardinals whose existence implies 0^\sharp cease to have those large cardinal properties, but retain the properties weaker than 0^\sharp which they also possess. For example, measurable cardinals cease to be measurable but remain Mahlo in L. If 0^\sharp holds in V, then there is a closed unbounded class of ordinals that are indiscernible in L. While some of these are not even initial ordinals in V, they have all the large cardinal properties weaker than 0^\sharp in L. Furthermore, any strictly increasing class function from the class of indiscernibles to itself can be extended in a unique way to an elementary embedding of L into L. This gives L a nice structure of repeating segments.

L can be well-ordered

There are various ways of well-ordering L. Some of these involve the "fine structure" of L, which was first described by Ronald Bjorn Jensen in his 1972 paper entitled "The fine structure of the constructible hierarchy". Instead of explaining the fine structure, we will give an outline of how L could be well-ordered using only the definition given above. Suppose x and y are two different sets in L and we wish to determine whether x y. If x first appears in and y first appears in L_{\beta+1} and \beta is different from \alpha, then let x < y if and only if. Henceforth, we suppose that. The stage uses formulas with parameters from L_\alpha to define the sets x and y. If one discounts (for the moment) the parameters, the formulas can be given a standard Gödel numbering by the natural numbers. If \Phi is the formula with the smallest Gödel number that can be used to define x, and \Psi is the formula with the smallest Gödel number that can be used to define y, and \Psi is different from \Phi, then let x < y if and only if \Phi<\Psi in the Gödel numbering. Henceforth, we suppose that \Psi=\Phi. Suppose that \Phi uses n parameters from L_\alpha. Suppose is the sequence of parameters that can be used with \Phi to define x, and does the same for y. Then let x < y if and only if either z_n < w_n or (z_n = w_n and ) or (z_n=w_n and and ), etc. This is called the reverse lexicographic ordering; if there are multiple sequences of parameters that define one of the sets, we choose the least one under this ordering. It being understood that each parameter's possible values are ordered according to the restriction of the ordering of L to L_\alpha, so this definition involves transfinite recursion on \alpha. The well-ordering of the values of single parameters is provided by the inductive hypothesis of the transfinite induction. The values of n-tuples of parameters are well-ordered by the product ordering. The formulas with parameters are well-ordered by the ordered sum (by Gödel numbers) of well-orderings. And L is well-ordered by the ordered sum (indexed by \alpha) of the orderings on. Notice that this well-ordering can be defined within L itself by a formula of set theory with no parameters, only the free-variables x and y. And this formula gives the same truth value regardless of whether it is evaluated in L, V, or W (some other standard model of ZF with the same ordinals) and we will suppose that the formula is false if either x or y is not in L. It is well known that the axiom of choice is equivalent to the ability to well-order every set. Being able to well-order the proper class V (as we have done here with L) is equivalent to the axiom of global choice, which is more powerful than the ordinary axiom of choice because it also covers proper classes of non-empty sets.

L has a reflection principle

Proving that the axiom of separation, axiom of replacement, and axiom of choice hold in L requires (at least as shown above) the use of a reflection principle for L. Here we describe such a principle. By induction on n<\omega, we can use ZF in V to prove that for any ordinal \alpha, there is an ordinal such that for any sentence with in L_\beta and containing fewer than n symbols (counting a constant symbol for an element of L_\beta as one symbol) we get that holds in L_\beta if and only if it holds in L.

The generalized continuum hypothesis holds in L

Let, and let T be any constructible subset of S. Then there is some \beta with , so , for some formula \Phi and some z_i drawn from L_\beta. By the downward Löwenheim–Skolem theorem and Mostowski collapse, there must be some transitive set K containing L_\alpha and some w_i, and having the same first-order theory as L_\beta with the w_i substituted for the z_i; and this K will have the same cardinal as L_\alpha. Since V = L is true in L_\beta, it is also true in K, so for some \gamma having the same cardinal as \alpha. And because L_\beta and L_\gamma have the same theory. So T is in fact in. So all the constructible subsets of an infinite set S have ranks with (at most) the same cardinal \kappa as the rank of S; it follows that if \delta is the initial ordinal for \kappa^+, then serves as the "power set" of S within L Thus this "power set". And this in turn means that the "power set" of S has cardinal at most. Assuming S itself has cardinal \kappa, the "power set" must then have cardinal exactly \kappa^+. But this is precisely the generalized continuum hypothesis relativized to L.

Constructible sets are definable from the ordinals

There is a formula of set theory that expresses the idea that. It has only free variables for X and \alpha. Using this we can expand the definition of each constructible set. If, then for some formula \Phi and some in L_\alpha. This is equivalent to saying that: for all y, y \in S if and only if [there exists X such that X=L_\alpha and y \in X and ] where is the result of restricting each quantifier in to X. Notice that each for some. Combine formulas for the z's with the formula for S and apply existential quantifiers over the z's outside and one gets a formula that defines the constructible set S using only the ordinals \alpha that appear in expressions like as parameters. Example: The set is constructible. It is the unique set s that satisfies the formula: where Ord (a) is short for: Actually, even this complex formula has been simplified from what the instructions given in the first paragraph would yield. But the point remains, there is a formula of set theory that is true only for the desired constructible set S and that contains parameters only for ordinals.

Relative constructibility

Sometimes it is desirable to find a model of set theory that is narrow like L, but that includes or is influenced by a set that is not constructible. This gives rise to the concept of relative constructibility, of which there are two flavors, denoted by L(A) and L[A]. The class L(A) for a non-constructible set A is the intersection of all classes that are standard models of set theory and contain A and all the ordinals. L(A) is defined by transfinite recursion as follows: If L(A) contains a well-ordering of the transitive closure of {A}, then this can be extended to a well-ordering of L(A). Otherwise, the axiom of choice will fail in L(A). A common example is, the smallest model that contains all the real numbers, which is used extensively in modern descriptive set theory. The class L[A] is the class of sets whose construction is influenced by A, where A may be a (presumably non-constructible) set or a proper class. The definition of this class uses, which is the same as except instead of evaluating the truth of formulas \Phi in the model (X,\in), one uses the model (X,\in,A) where A is a unary predicate. The intended interpretation of A(y) is y \in A. Then the definition of L[A] is exactly that of L only with replaced by. L[A] is always a model of the axiom of choice. Even if A is a set, A is not necessarily itself a member of L[A], although it always is if A is a set of ordinals. The sets in L(A) or L[A] are usually not actually constructible, and the properties of these models may be quite different from the properties of L itself.