Weakly compact cardinal
Weakly compact cardinals lie at the focal point of a number of diverse concepts in infinite combinatorics, admitting various characterizations in terms of these concepts. If $\kappa^{{<}\kappa} = \kappa$, then the following are equivalent:
- Weak compactness
- A cardinal $\kappa$ is weakly compact if and only if it is uncountable and every $\kappa$-satisfiable theory in an $\mathcal{L}_{\kappa,\kappa}$ language of size at most $\kappa$ is satisfiable.
- Extension property
- A cardinal $\kappa$ is weakly compact if and only if for every $A\subset V_\kappa$, there is a transitive structure $W$ properly extending $V_\kappa$ and $A^*\subset W$ such that $\langle V_\kappa,{\in},A\rangle\prec\langle W,{\in},A^*\rangle$.
- Tree property
- A cardinal $\kappa$ is weakly compact if and only if it is inaccessible and has the tree property.
- Filter property
- A cardinal $\kappa$ is weakly compact if and only if whenever $M$ is a set containing at most $\kappa$-many subsets of $\kappa$, then there is a $\kappa$-complete nonprincipal filter $F$ measuring every set in $M$.
- Weak embedding property
- A cardinal $\kappa$ is weakly compact if and only if for every $A\subset\kappa$ there is a transitive set $M$ of size $\kappa$ with $\kappa\in M$ and a transitive set $N$ with an embedding $j:M\to N$ with critical point $\kappa$.
- Embedding characterization
- A cardinal $\kappa$ is weakly compact if and only if for every transitive set $M$ of size $\kappa$ with $\kappa\in M$ there is a transitive set $N$ and an embedding $j:M\to N$ with critical point $\kappa$.
- Normal embedding characterization
- A cardinal $\kappa$ is weakly compact if and only if for every $\kappa$-model $M$ there is a $\kappa$-model $N$ and an embedding $j:M\to N$ with critical point $\kappa$, such that $N=\{\ j(f)(\kappa)\mid f\in M\ \}$.
- Hauser embedding characterization
- A cardinal $\kappa$ is weakly compact if and only if for every $\kappa$-model $M$ there is a $\kappa$-model $N$ and an embedding $j:M\to N$ with critical point $\kappa$ such that $j,M\in N$.
- Partition property
- A cardinal $\kappa$ is weakly compact if and only if the partition property $\kappa\to(\kappa)^2_2$ holds.
- Indescribability property
- A cardinal $\kappa$ is weakly compact if and only if it is $\Pi_1^1$-indescribable.
- Skolem Property
- A cardinal $\kappa$ is weakly compact if and only if $\kappa$ is inaccessible and every $\kappa$-unboundedly satisfiable $\mathcal{L}_{\kappa,\kappa}$-theory $T$ of size at most $\kappa$ has a model of size at least $\kappa$. A theory $T$ is $\kappa$-unboundedly satisfiable if and only if for any $\lambda<\kappa$, there exists a model $\mathcal{M}\models T$ with $\lambda\leq|M|<\kappa$. For more info see here.
Weakly compact cardinals first arose in connection with (and were named for) the question of whether certain infinitary logics satisfy the compactness theorem of first order logic. Specifically, in a language with a signature consisting, as in the first order context, of a set of constant, finitary function and relation symbols, we build up the language of $\mathcal{L}_{\kappa,\lambda}$ formulas by closing the collection of formulas under infinitary conjunctions $\wedge_{\alpha<\delta}\varphi_\alpha$ and disjunctions $\vee_{\alpha<\delta}\varphi_\alpha$ of any size $\delta<\kappa$, as well as infinitary quantification $\exists\vec x$ and $\forall\vec x$ over blocks of variables $\vec x=\langle x_\alpha\mid\alpha<\delta\rangle$ of size less than $\kappa$. A theory in such a language is satisfiable if it has a model under the natural semantics. A theory is $\theta$-satisfiable if every subtheory consisting of fewer than $\theta$ many sentences of it is satisfiable. First order logic is precisely $L_{\omega,\omega}$, and the classical Compactness theorem asserts that every $\omega$-satisfiable $\mathcal{L}_{\omega,\omega}$ theory is satisfiable. A uncountable cardinal $\kappa$ is strongly compact if every $\kappa$-satisfiable $\mathcal{L}_{\kappa,\kappa}$ theory is satisfiable. The cardinal $\kappa$ is weakly compact if every $\kappa$-satisfiable $\mathcal{L}_{\kappa,\kappa}$ theory, in a language having at most $\kappa$ many constant, function and relation symbols, is satisfiable.
Next, for any cardinal $\kappa$, a $\kappa$-tree is a tree of height $\kappa$, all of whose levels have size less than $\kappa$. More specifically, $T$ is a tree if $T$ is a partial order such that the predecessors of any node in $T$ are well ordered. The $\alpha^{\rm th}$ level of a tree $T$, denoted $T_\alpha$, consists of the nodes whose predecessors have order type exactly $\alpha$, and these nodes are also said to have height $\alpha$. The height of the tree $T$ is the first $\alpha$ for which $T$ has no nodes of height $\alpha$. A ""$\kappa$-branch"" through a tree $T$ is a maximal linearly ordered subset of $T$ of order type $\kappa$. Such a branch selects exactly one node from each level, in a linearly ordered manner. The set of $\kappa$-branches is denoted $[T]$. A $\kappa$-tree is an Aronszajn tree if it has no $\kappa$-branches. A cardinal $\kappa$ has the tree property if every $\kappa$-tree has a $\kappa$-branch.
A transitive set $M$ is a $\kappa$-model of set theory if $|M|=\kappa$, $M^{\lt\kappa}\subset M$ and $M$ satisfies ZFC$^-$, the theory ZFC without the power set axiom (and using collection and separation rather than merely replacement). For any infinite cardinal $\kappa$ we have that $H_{\kappa^+}$ models ZFC$^-$, and further, if $M\prec H_{\kappa^+}$ and $\kappa\subset M$, then $M$ is transitive. Thus, any $A\in H_{\kappa^+}$ can be placed into such an $M$. If $\kappa^{\lt\kappa}=\kappa$, one can use the downward Löwenheim-Skolem theorem to find such $M$ with $M^{\lt\kappa}\subset M$. So in this case there are abundant $\kappa$-models of set theory (and conversely, if there is a $\kappa$-model of set theory, then $2^{\lt\kappa}=\kappa$).
The partition property $\kappa\to(\lambda)^n_\gamma$ asserts that for every function $F:[\kappa]^n\to\gamma$ there is $H\subset\kappa$ with $|H|=\lambda$ such that $F\upharpoonright[H]^n$ is constant. If one thinks of $F$ as coloring the $n$-tuples, the partition property asserts the existence of a monochromatic set $H$, since all tuples from $H$ get the same color. The partition property $\kappa\to(\kappa)^2_2$ asserts that every partition of $[\kappa]^2$ into two sets admits a set $H\subset\kappa$ of size $\kappa$ such that $[H]^2$ lies on one side of the partition. When defining $F:[\kappa]^n\to\gamma$, we define $F(\alpha_1,\ldots,\alpha_n)$ only when $\alpha_1<\cdots<\alpha_n$.
Contents
Weakly compact cardinals and the constructible universe
Every weakly compact cardinal is weakly compact in $L$. [1]
Nevertheless, the weak compactness property is not generally downward absolute between transitive models of set theory.
Weakly compact cardinals and forcing
- Weakly compact cardinals are invariant under small forcing. [1]
- Weakly compact cardinals are preserved by the canonical forcing of the GCH, by fast function forcing and many other forcing notions [ citation needed ].
- If $\kappa$ is weakly compact, there is a forcing extension in which $\kappa$ remains weakly compact and $2^\kappa\gt\kappa$ [ citation needed ].
- If the existence of weakly compact cardinals is consistent with ZFC, then there is a model of ZFC in which $\kappa$ is not weakly compact, but becomes weakly compact in a forcing extension [2].
Indestructibility of a weakly compact cardinal
To expand using [2]
Relations with other large cardinals
- Every weakly compact cardinal is inaccessible, Mahlo, hyper-Mahlo, hyper-hyper-Mahlo and more.
- Measurable cardinals, Ramsey cardinals, and totally indescribable cardinals are all weakly compact and a stationary limit of weakly compact cardinals.
- Assuming the consistency of a strongly unfoldable cardinal with ZFC, it is also consistent for the least weakly compact cardinal to be the least unfoldable cardinal. [3]
- If GCH holds, then the least weakly compact cardinal is not weakly measurable. However, if there is a measurable cardinal, then it is consistent for the least weakly compact cardinal to be weakly measurable. [3]
- If it is consistent for there to be a nearly supercompact, then it is consistent for the least weakly compact cardinal to be nearly supercompact. [3]
References
- Jech, Thomas J. Set Theory. Third, Springer-Verlag, Berlin, 2003. (The third millennium edition, revised and expanded) www bibtex
- Kunen, Kenneth. Saturated Ideals. J Symbolic Logic 43(1):65--76, 1978. www bibtex
- Cody, Brent, Gitik, Moti, Hamkins, Joel David, and Schanker, Jason. The Least Weakly Compact Cardinal Can Be Unfoldable, Weakly Measurable and Nearly θ-Supercompact. , 2013. www bibtex