# cantors-attic

Climb into Cantor’s Attic, where you will find infinities large and small. We aim to provide a comprehensive resource of information about all notions of mathematical infinity.

View the Project on GitHub neugierde/cantors-attic

# Constructible universe

The Constructible universe (denoted $L$) was invented by Kurt Gödel as a transitive inner model of $\text{ZFC+}$$\text{GCH}$ (assuming the consistency of $\text{ZFC}$) showing that $\text{ZFC}$ cannot disprove $\text{GCH}$. It was then shown to be an important model of $\text{ZFC}$ for its satisfying of other axioms, thus making them consistent with $\text{ZFC}$. The idea is that $L$ is built up by ranks like $V$. $L_0$ is the empty set, and $L_{\alpha+1}$ is the set of all easily definable subsets of $L_\alpha$. The assumption that $V=L$ (also known as the Axiom of constructibility) is undecidable from $\text{ZFC}$, and implies many axioms which are consistent with $\text{ZFC}$. A set $X$ is constructible iff $X\in L$. $V=L$ iff every set is constructible.

## Definition

$\mathrm{def}(X)$ is the set of all “easily definable” subsets of $X$ (specifically the $\Delta_0$ definable subsets). More specifically, a subset $x$ of $X$ is in $\mathrm{def}(X)$ iff there is a first-order formula $\varphi$ and $v_0,v_1…v_n\in X$ such that $x=\{y\in X:\varphi^X[y,v_0,v_1…v_n]\}$. Then, $L_\alpha$ and $L$ are defined as follows:

• $L_0=\emptyset$
• $L_{\alpha+1}=\mathrm{def}(L_\alpha)$
• $L_\beta=\bigcup_{\alpha<\beta} L_\alpha$ if $\beta$ is a limit ordinal
• $L=\bigcup_{\alpha\in\mathrm{Ord}} L_\alpha$

### The Relativized constructible universes $L_\alpha(W)$ and $L_\alpha[W]$

$L_\alpha(W)$ for a class $W$ is defined the same way except $L_0(W)=\text{TC}(\{W\})$ (the transitive closure of $\{W\}$). $L_\alpha[W]$ for a class $W$ is defined in the same way as $L$ except using $\mathrm{def}_W(X)$, where $\mathrm{def}_W(X)$ is the set of all $x\subseteq X$ such that there is a first-order formula $\varphi$ and $v_0,v_1…v_n\in X$ such that $x=\{y\in X:\varphi^X[y,W,v_0,v_1…v_n]\}$ (because the relativization of $\varphi$ to $X$ is used and $\langle X,\in\rangle$ is not used, this definition makes sense even when $W$ is not in $X$).

$L[W]=\bigcup_{\alpha\in\mathrm{Ord}}L_\alpha[W]$ is always a model of $\text{ZFC}$, and always satisfies $\text{GCH}$ past a certain cardinality. $L(W)=\bigcup_{\alpha\in\mathrm{Ord}}L_\alpha(W)$ is always a model of $\text{ZF}$ but need not satisfy $\text{AC}$ (the axiom of choice). In particular, $L(\mathbb{R})$ is, under large cardinal assumptions, a model of the axiom of determinacy. However, Shelah proved that if $\lambda$ is a strong limit cardinal of uncountable cofinality then $L(\mathcal{P}(\lambda))$ is a model of $\text{AC}$.

## The difference between $L_\alpha$ and $V_\alpha$

For $\alpha\leq\omega$, $L_\alpha=V_\alpha$. However, $|L_{\omega+\alpha}|=\aleph_0 + |\alpha|$ whilst $|V_{\omega+\alpha}|=\beth_\alpha$. Unless $\alpha$ is a $\beth$-fixed point, $|L_{\omega+\alpha}|<|V_{\omega+\alpha}|$. Although $L_\alpha$ is quite small compared to $V_\alpha$, $L$ is a tall model, meaning $L$ contains every ordinal. In fact, $V_\alpha\cap\mathrm{Ord}=L_\alpha\cap\mathrm{Ord}=\alpha$, so the ordinals in $V_\alpha$ are precisely those in $L_\alpha$.

If $0^{\#}$ exists (see below), then every uncountable cardinal $\kappa$ has $L\models$”$\kappa$is totally ineffable (and therefore the smallest actually totally ineffable cardinal $\lambda$ has many more large cardinal properties in $L$).

However, if $\kappa$ is inaccessible and $V=L$, then $V_\kappa=L_\kappa$. Furthermore, $V_\kappa\models (V=L)$. In the case where $V\neq L$, it is still true that $V_\kappa^L=L_\kappa$, although $V_\kappa^L$ will not be $V_\kappa$. In fact, $\mathcal{P}(\omega)\not\in V_\kappa^L$ if $0^{\#}$ exists.

## Statements True in $L$

Here is a list of statements true in $L$ of any model of $\text{ZF}$:

## Determinacy of $L(\R)$

Main article: axiom of determinacy

## Using other logic systems than first-order logic

When using second order logic in the definition of $\mathrm{def}$, the new hierarchy is called $L_\alpha^{II}$. Interestingly, $L^{II}=\text{HOD}$. When using $\mathcal{L}_{\kappa,\kappa}$, the hierarchy is called $L_\alpha^{\mathcal{L}_{\kappa,\kappa}}$, and $L\subseteq L^{\mathcal{L}_{\kappa,\kappa}}\subseteq L(V_\kappa)$. Finally, when using $\mathcal{L}_{\infty,\infty}$, it turns out that the result is $V$.

Chang’s Model is $L^{\mathcal{L}_{\omega_1,\omega_1}}$. Chang proved that $L^{\mathcal{L}_{\kappa,\kappa}}$ is the smallest inner model of $\text{ZFC}$ closed under sequences of length $<\kappa$.

To be expanded.

## Silver cardinals

A cardinal $κ$ is Silver if in a set-forcing extension there is a club in $κ$ of generating indiscernibles for $V_κ$ of order-type $κ$. This is a very strong property downwards absolute to $L$, e.g.:(Gitman & Shindler, n.d.)

• Every element of a club $C$ witnessing that $κ$ is a Silver cardinal is virtually rank-into-rank.
• If $C ∈ V[H]$, a forcing extension by $\mathrm{Coll}(ω, V_κ)$, is a club in $κ$ of generating indiscernibles for $V_κ$ of order-type $κ$, then each $ξ ∈ C$ is $< ω_1$-iterable.

## Sharps

$0^{\#}$ (zero sharp) is a $\Sigma_3^1$ real number which, under the existence of many Silver indiscernibles (a statement independent of $\text{ZFC}$), has a certain number of properties that contredicts the axiom of constructibility and implies that, in short, $L$ and $V$ are “very different”. Technically, under the standard definition of $0^\#$ as a (real number encoding a) set of formulas, $0^\#$ provably exists in $\text{ZFC}$, but lacks all its important properties. Thus the expression “$0^\#$ exists” is to be understood as “$0^\#$ exists and there are uncountably many Silver indiscernibles”.

### Definition of $0^{\#}$

Assume there is an uncountable set of Silver indiscernibles. Then $0^{\#}$ is defined as the set of all Gödel numberings of first-order formula $\varphi$ such that $L_{\aleph_{\omega}}\models\varphi(\aleph_0,\aleph_1…\aleph_n)$ for some $n$.

“$0^{\#}$ exists” is used as a shorthand for “there is an uncountable set of Silver indiscernibles”; since $L_{\aleph_\omega}$ is a set, $\text{ZFC}$ can define a truth predicate for it, and so the existence of $0^{\#}$ as a mere set of formulas would be trivial. It is interesting only when there are many (in fact proper class many) Silver indiscernibles. Similarly, we say that “$0^{\#}$ does not exist” if there are no Silver indiscernibles.

### Implications, equivalences, and consequences of $0^♯$’s existence

If $0^♯$ exists then:

• $L_{\aleph_\omega}\prec L$ and so $0^♯$ also corresponds to the set of the Gödel numberings of first-order formulas $\varphi$ such that $L\models\varphi(\aleph_0,\aleph_1…\aleph_n)$
• In fact, $L_\kappa\prec L$ for every Silver indiscernible, and thus for every uncountable cardinal.
• Given any set $X\in L$ which is first-order definable in $L$, $X\in L_{\omega_1}$. This of course implies that $\aleph_1$ is not first-order definable in $L$, because $\aleph_1\not\in L_{\omega_1}$. This is already a disproof of $V=L$ (because $\aleph_1$ is first-order definable).
• For every $\alpha\in\omega_1^L$, every Silver indiscernible (and in particular every uncountable cardinal) in $L$ is a Silver cardinal, $\alpha$-iterable, $\geq$ an $\alpha$-Erdős, totally ineffable and completely remarkable and has most other virtual large cardinal properties and other large cardinal properties consistent with $V=L$.(Gitman & Shindler, n.d.; Bagaria et al., 2017)
• There are only countably many reals in $L$, i.e. $|\R\cap L|=\aleph_0$ in $V$.
• By elementary-embedding absoluteness results (The hypothesis can be weakened, because one can chop at off the universe at any Silver indiscernible and use reflection.):(Gitman & Hamkins, 2018)
• $L$, equipped with only its definable classes, is a model of the generic Vopěnka principle.
• In $L$ there are numerous virtual rank-into-rank embeddings $j : V_θ^L → V_θ^L$, where $θ$ is far above the supremum of the critical sequence.
• Therefore every Silver indiscernible
• is virtually $A$-extendible in $L$ for every definable class $A$
• and is the critical point of virtual rank-into-rank embeddings with targets as high as desired and fixed points as high above the critical sequence as desired.
• There is a class-forcing notion $\mathbb{P}$ definable in $L$, such that in any $L$-generic extension $L[C]$ by this forcing, $\text{GBC}$ and the generic Vopěnka principle hold, yet $\text{Ord}$ is not Mahlo.(Gitman & Hamkins, 2018)
• Proof includes a lemma stating: For any ordinal $δ$ and any natural number (of the meta-theory — this lemma is a scheme) $n$, if $D_{δ,n} ⊂ \mathbb{P}$ is the collection of conditions $c$ for which there is an ordinal $θ$ such that
• $L_θ ≺_{Σ_n} L$,
• $c ∩ θ$ is $L_θ$-generic for $\mathbb{P}^{L_θ}$ and
• in some forcing extension of $L$, there is an elementary embedding $j : ⟨ L_θ , ∈, c ∩ θ ⟩ → ⟨ L_θ , ∈, c ∩ θ ⟩$ with critical point above $δ$,

then $D_{δ,n}$ is a definable dense subclass of $\mathbb{P}$ in $L$.

• There is a definable class-forcing notion in $L$, such that in the corresponding $L$-generic extension, $\text{GBC}$ holds, the generic Vopěnka scheme holds, but $\text{Ord}$ is not definably Mahlo, because there is a $∆_2$-definable club class avoiding the regular cardinals.
• There is a class-forcing extension $L[G]$ of the constructible universe in which the generic Vopěnka principle holds (so $gVP(κ, \mathbf{Σ_{n+1}})$ and $gVP(Π_n)$ hold for any $κ$ and $n$), but there are no $Σ_2$-reflecting cardinals and hence no remarkable cardinals (or $n$-remarkable cardinals).(Gitman & Hamkins, 2018)

The following statements are equivalent:

• There is an uncountable set of Silver indiscernibles (i.e. “$0^♯$ exists”)
• There is a proper class of Silver indiscernibles (unboundedly many of them).
• There is a unique well-founded remarkable E.M. set (see below).
• Jensen’s Covering Theorem fails (see below).
• $L$ is thin, i.e. $|L\cap V_\alpha|=|\alpha|$ for all $\alpha\geq\omega$.
• $\Sigma^1_1$-determinacy (lightface form).
• $\aleph_\omega$ is regular (hence weakly inaccessible) in $L$.
• There is a nontrivial elementary embedding $j:L\to L$.
• There is a proper class of nontrivial elementary embeddings $j:L\to L$.
• There is a nontrivial elementary embedding $j:L_\alpha\to L_\beta$ with $\text{crit}(j)<|\alpha|$.

The existence of $0^♯$ is implied by:

• Chang’s conjecture
• Both $\omega_1$ and $\omega_2$ being singular (requires $\neg\text{AC}$).
• The negation of the singular cardinal hypothesis ($\text{SCH}$).
• The existence of an $\omega_1$-iterable cardinal or of a $\omega_1$-Erdős cardinal.
• The existence of a weakly compact cardinal $\kappa$ such that $|(\kappa^+)^L|=\kappa$.
• The existence of some uncountable regular cardinal $\kappa$ such that every constructible $X\subseteq\kappa$ either contains or is disjoint from a closed unbounded set.

Note that if $0^♯$ exists then for every Silver indiscernible (in particular for every uncountable cardinal) there is a nontrivial elementary embedding $j:L\rightarrow L$ with that indiscernible as its critical point. Thus if any such embedding exists, then a proper class of those embeddings exists.

### EM blueprints and alternative characterizations of $0^\#$

An EM blueprint (Ehrenfeucht-Mostowski blueprint) $T$ is any theory of the form $\{\varphi:(L_\delta;\in,\alpha_0,\alpha_1…)\models\varphi\}$ for some ordinal $\delta>\omega$ and $\alpha_0<\alpha_1<\alpha_2…$ are indiscernible in the structure $L_\delta$. Roughly speaking, it’s the set of all true statements about $\alpha_0,\alpha_1,\alpha_2…$ in $L_\delta$.

For an EM blueprint $T=\{\varphi:(L_\delta;\in,\alpha_0,\alpha_1…)\models\varphi\}$, the theory $T^{-}$ is defined as $\{\varphi:L_\delta\models\varphi\}$ (the set of truths about any definable elements of $L_\delta$). Then, the structure $\mathcal{M}(T,\alpha)=(M(T,\alpha);E)\models T^{-}$ has a very technical definition, but it is indeed uniquely (up to isomorphism) the only structure which satisfies the existence of a set $X$ of $\mathcal{M}(T,\alpha)$-ordinals such that:

1. $X$ is a set of indiscernibles for $\mathcal{M}(T,\alpha)$ and $(X;E)\cong\alpha$ ($X$ has order-type $\alpha$ with respect to $\mathcal{M}(T,\alpha)$)
2. For any formula $\varphi$ and any $x<y<z…$ with $x,y,z…\in X$, $\mathcal{M}(T,\alpha)\models\varphi(x,y,z…)$ iff $\mathcal{M}(T,\alpha)\models\varphi(\alpha_0,\alpha_1,\alpha_2…)$ where $\alpha_0,\alpha_1…$ are the indiscernibles used in the EM blueprint.
3. If $<$ is an $\mathcal{M}(T,\alpha)$-definable $\mathcal{M}(T,\alpha)$-well-ordering of $\mathcal{M}(T,\alpha)$, then: $$\\mathcal{M}(T,\\alpha)=\\{\\min{}\_&lt;^{\\mathcal{M}(T,\\alpha)}\\{x:\\mathcal{M}(T,\\alpha)\\models\\varphi$x,a,b,c...$\\}:\\varphi\\in\\mathcal{L}\_\\in\\text{ and } a,b,c...\\in X\\}$$

$0^\#$ is then defined as the unique EM blueprint $T$ such that:

1. $\mathcal{M}(T,\alpha)$ is isomorphic to a transitive model $M(T,\alpha)$ of ZFC for every $\alpha$
2. For any infinite $\alpha$, the set of indiscernibles $X$ associated with $M(T,\alpha)$ can be made cofinal in $\text{Ord}^{M(T,\alpha)}$.
3. The $L_\delta$-indiscernables $\beta_0<\beta_1…$ can be made so that if $<$ is an $M(T,\alpha)$-definable well-ordering of $M(T,\alpha)$, then for any $(m+n+2)$-ary formula $\varphi$ such that $\min_<^{M(T,\alpha)}\{x:\varphi[x,\beta_0,\beta_1…\beta_{m+n}]\}<\beta_m$, then: $$\\min{}\_&lt;^{M(T,\\alpha)}\\{x:\\varphi$x,\\beta\_0,\\beta\_1...\\beta\_{m+n}$\\}=\\min{}\_&lt;^{M(T,\\alpha)}\\{x:\\varphi$x,\\beta\_0,\\beta\_1...\\beta\_{m-1},\\beta\_{m+n+1}...\\beta\_{m+2n+1}$\\}$$

If the EM blueprint meets 1. then it is called well-founded. If it meets 2. and 3. then it is called remarkable.

If $0^\#$ exists (i.e. there is a well-founded remarkable EM blueprint) then it happens to be equivalent to the set of all $\varphi$ such that $L\models\varphi[\kappa_0,\kappa_1…]$ for some uncountable cardinals $\kappa_0,\kappa_1…<\aleph_\omega$. This is because the associated $M(T,\alpha)$ will always have $M(T,\alpha)\prec L$ and furthermore $\kappa_0,\kappa_1…$ would be indiscernibles for $L$.

$0^\#$ exists interestingly iff some $L_\delta$ has an uncountable set of indiscernables. If $0^\#$ exists, then there is some uncountable $\delta$ such that $M(0^\#,\omega_1)=L_\delta$ and $L_\delta$ therefore has an uncountable set of indiscernables. On the other hand, if some $L_\delta$ has an uncountable set of indiscernables, then the EM blueprint of $L_\delta$ is $0^\#$.

### Generalisations

$0^\dagger$ (zero dagger) is a set of integers analogous to $0^\sharp$ and connected with inner models of measurability.(Kanamori & Awerbuch-Friedlander, 1990)

$0^{sword}$ is connected with nontrivial Mitchell rank. $¬ 0 ^{sword}$ (not zero sword) means that there is no mouse with a measure of Mitchell order $> 0$.(Sharpe & Welch, 2011)

$0^\P$ (zero pistol) is connected with strong cardinals. $¬ 0^\P$ (not zero pistol) means that a core model may be built with a strong cardinal, but that there is no class of indiscernibles for it that is closed and unbounded in $\mathrm{Ord}$).(Sharpe & Welch, 2011) $0^¶$ is “the sharp for a strong cardinal”, meaning the minimal sound active mouse $\mathcal{M}$ with $M | \mathrm{crit}(\dot F^{\mathcal{M}}) \models \text{“There exists a strong cardinaly”}$, with $\dot F^{\mathcal{M}}$ being the top extender of $\mathcal{M}$.(Nielsen & Welch, 2018)

8. Chang, C. C. (1971). Sets Constructible Using $$\mathcal {L}_{κ,κ}$$. In Axiomatic set theory (Proc. Sympos. Pure Math., Vol. XIII, Part I, Univ. California, Los Angeles, Calif., 1967) (pp. 1–8). Amer. Math. Soc.