Difference between revisions of "Huge"
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[[Category:Large cardinal axioms]] | [[Category:Large cardinal axioms]] | ||
[[Category:Critical points]] | [[Category:Critical points]] | ||
− | Huge cardinals (and their variants) were introduced by Kenneth Kunen in 1978 as a very large cardinal axiom. Each of their variants are [[Vopenka|Vopěnka]] cardinals (that is, Vopěnka's principle holds in their ranks), although they have strictly stronger consistency strength. | + | Huge cardinals (and their variants) were introduced by Kenneth Kunen in 1978 as a very large cardinal axiom. Each of their variants are [[Vopenka|Vopěnka]] cardinals (that is, Vopěnka's principle holds in their ranks), although they have strictly stronger consistency strength. <cite>Kanamori2009:HigherInfinite</cite> |
= Definitions = | = Definitions = | ||
− | Their formulation is similar to that of the formulation of [[superstrong]] cardinals. A huge cardinal is to a [[supercompact]] cardinal as a superstrong cardinal is to a [[strong]] cardinal, more precisely. The definition is part of a generalized phenomenon known as the "double helix", in which for some large cardinal properties $n$-$P_0$ and $n$-$P_1$, $n$-$P_0$ has less consistency strength than $n$-$P_1$, which has less consistency strength than $n+1$-$P_0$, and so on. This phenomenon is seen only around the [[n-fold variants|$n$-fold variants]] as of modern set theoretic concerns. | + | Their formulation is similar to that of the formulation of [[superstrong]] cardinals. A huge cardinal is to a [[supercompact]] cardinal as a superstrong cardinal is to a [[strong]] cardinal, more precisely. The definition is part of a generalized phenomenon known as the "double helix", in which for some large cardinal properties $n$-$P_0$ and $n$-$P_1$, $n$-$P_0$ has less consistency strength than $n$-$P_1$, which has less consistency strength than $n+1$-$P_0$, and so on. This phenomenon is seen only around the [[n-fold variants|$n$-fold variants]] as of modern set theoretic concerns. <cite>Kentaro2007:DoubleHelix</cite> |
Although they are very large, there is a first-order definition which is equivalent to $n$-hugeness, so the $\theta$-th $n$-huge cardinal is first-order definable whenever $\theta$ is first-order definable. This definition can be seen as a (very strong) strengthening of the first-order definition of [[measurable|measurability]]. | Although they are very large, there is a first-order definition which is equivalent to $n$-hugeness, so the $\theta$-th $n$-huge cardinal is first-order definable whenever $\theta$ is first-order definable. This definition can be seen as a (very strong) strengthening of the first-order definition of [[measurable|measurability]]. | ||
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$$\forall i<n\forall x\subseteq\lambda(ot(x\cap\lambda_{i+1})=\lambda_i\rightarrow x\in U)$$ | $$\forall i<n\forall x\subseteq\lambda(ot(x\cap\lambda_{i+1})=\lambda_i\rightarrow x\in U)$$ | ||
− | Where $ot(X)$ is the [[Order-isomorphism|order-type]] of the poset $(X,\in)$. This definition is, more intuitively, making $U$ very large, like most ultrafilter characterizations of large cardinals ([[supercompact]], [[strongly compact]], etc.). | + | Where $ot(X)$ is the [[Order-isomorphism|order-type]] of the poset $(X,\in)$. <cite>Kanamori2009:HigherInfinite</cite> This definition is, more intuitively, making $U$ very large, like most ultrafilter characterizations of large cardinals ([[supercompact]], [[strongly compact]], etc.). |
= Consistency Strength and Size = | = Consistency Strength and Size = | ||
− | Hugeness exhibits a phenomenon associated with similarly defined large cardinals (the [[n-fold variants|$n$-fold variants]]) known as the ''double helix''. This phenomenon is when for one $n$-fold variant, letting a cardinal be called $n$-$P_0$ iff it has the property, and another variant, $n$-$P_1$, $n$-$P_0$ is weaker than $n$-$P_1$, which is weaker than $n+1$-$P_0$. In the consistency strength hierarchy, here is where these lay (top being weakest): | + | Hugeness exhibits a phenomenon associated with similarly defined large cardinals (the [[n-fold variants|$n$-fold variants]]) known as the ''double helix''. This phenomenon is when for one $n$-fold variant, letting a cardinal be called $n$-$P_0$ iff it has the property, and another variant, $n$-$P_1$, $n$-$P_0$ is weaker than $n$-$P_1$, which is weaker than $n+1$-$P_0$. <cite>Kentaro2007:DoubleHelix</cite> In the consistency strength hierarchy, here is where these lay (top being weakest): |
*[[measurable]] = [[superstrong|$0$-superstrong]] = almost $0$-huge = super almost $0$-huge = $0$-huge = super $0$-huge | *[[measurable]] = [[superstrong|$0$-superstrong]] = almost $0$-huge = super almost $0$-huge = $0$-huge = super $0$-huge | ||
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*$n+1$-superstrong | *$n+1$-superstrong | ||
− | All huge variants lay at the top of the double helix restricted to some [[Omega|natural number]] $n$, although each are bested by [[rank-into-rank|I3]] cardinals (the [[elementary embedding|critical points]] of the I3 elementary embeddings). In fact, letting $\kappa$ be I3, there is a normal [[filter|ultrafilter]] $U$ over $\kappa$ which contains every cardinal $\lambda<\kappa$ such that for each $n$, $\lambda$ is $n$-huge. | + | All huge variants lay at the top of the double helix restricted to some [[Omega|natural number]] $n$, although each are bested by [[rank-into-rank|I3]] cardinals (the [[elementary embedding|critical points]] of the I3 elementary embeddings). In fact, letting $\kappa$ be I3, there is a normal [[filter|ultrafilter]] $U$ over $\kappa$ which contains every cardinal $\lambda<\kappa$ such that for each $n$, $\lambda$ is $n$-huge. <cite>Kanamori2009:HigherInfinite</cite> |
− | Similarly, every huge cardinal $\kappa$ is almost huge, and there is a normal ultrafilter over $\kappa$ which contains every almost huge cardinal $\lambda<\kappa$. Every superhuge cardinal $\kappa$ is [[extendible]] and there is a normal ultrafilter over $\kappa$ which contains every extendible cardinal $\lambda<\kappa$. Every $2$-huge cardinal $\kappa$ has a normal ultrafilter which contains every cardinal $\lambda$ such that $V_\kappa\models\lambda\;\mathrm{is}\;\mathrm{superhuge}$. | + | Similarly, every huge cardinal $\kappa$ is almost huge, and there is a normal ultrafilter over $\kappa$ which contains every almost huge cardinal $\lambda<\kappa$. Every superhuge cardinal $\kappa$ is [[extendible]] and there is a normal ultrafilter over $\kappa$ which contains every extendible cardinal $\lambda<\kappa$. Every $2$-huge cardinal $\kappa$ has a normal ultrafilter which contains every cardinal $\lambda$ such that $V_\kappa\models\lambda\;\mathrm{is}\;\mathrm{superhuge}$. <cite>Kanamori2009:HigherInfinite</cite> |
− | In terms of size however, the least $n$-huge cardinal is smaller than the least [[supercompact]] cardinal. Assuming both exist, for any $\kappa$ which is supercompact and has an $n$-huge cardinal above it, there are $\kappa$ many $n$-huge cardinals less than $\kappa$. | + | In terms of size however, the least $n$-huge cardinal is smaller than the least [[supercompact]] cardinal. Assuming both exist, for any $\kappa$ which is supercompact and has an $n$-huge cardinal above it, there are $\kappa$ many $n$-huge cardinals less than $\kappa$. <cite>Kanamori2009:HigherInfinite</cite> |
− | Every $n$-huge cardinal is $m$-huge for every $m\leq n$. Similarly with almost $n$-hugeness, super $n$-hugeness, and super almost $n$-hugeness. Every almost huge cardinal is [[Vopenka|Vopěnka]] (therefore $\mathrm{Con}(\mathrm{ZFC}+VP)$ where $VP$ is Vopěnka's principle). | + | Every $n$-huge cardinal is $m$-huge for every $m\leq n$<cite>Kanamori2009:HigherInfinite</cite>. Similarly with almost $n$-hugeness, super $n$-hugeness, and super almost $n$-hugeness. Every almost huge cardinal is [[Vopenka|Vopěnka]] (therefore $\mathrm{Con}(\mathrm{ZFC}+VP)$ where $VP$ is Vopěnka's principle). <cite>Kanamori2009:HigherInfinite</cite> |
− | + | {{References}} | |
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Revision as of 21:57, 4 November 2017
Huge cardinals (and their variants) were introduced by Kenneth Kunen in 1978 as a very large cardinal axiom. Each of their variants are Vopěnka cardinals (that is, Vopěnka's principle holds in their ranks), although they have strictly stronger consistency strength. [1]
Contents
Definitions
Their formulation is similar to that of the formulation of superstrong cardinals. A huge cardinal is to a supercompact cardinal as a superstrong cardinal is to a strong cardinal, more precisely. The definition is part of a generalized phenomenon known as the "double helix", in which for some large cardinal properties $n$-$P_0$ and $n$-$P_1$, $n$-$P_0$ has less consistency strength than $n$-$P_1$, which has less consistency strength than $n+1$-$P_0$, and so on. This phenomenon is seen only around the $n$-fold variants as of modern set theoretic concerns. [2]
Although they are very large, there is a first-order definition which is equivalent to $n$-hugeness, so the $\theta$-th $n$-huge cardinal is first-order definable whenever $\theta$ is first-order definable. This definition can be seen as a (very strong) strengthening of the first-order definition of measurability.
Elementary Embedding Definitions
The elementary embedding definitions are somewhat standard. Let $j:V\rightarrow M$ be an elementary embedding with critical point $\kappa$ such that $M$ is a standard inner model of ZFC. Then:
- $\kappa$ is almost $n$-huge with target $\lambda$ iff $\lambda=j^n(\kappa)$ and $M$ is closed under all of its sequences of length less than $\lambda$ (that is, $M^{<\lambda}\subset M$).
- $\kappa$ is $n$-huge with target $\lambda$ iff $\lambda=j^n(\kappa)$ and $M$ is closed under all of its sequences of length $\lambda$ ($M^\lambda\subset M$).
- $\kappa$ is almost $n$-huge iff it is almost $n$-huge with target $\lambda$ for some $\lambda$.
- $\kappa$ is $n$-huge iff it is $n$-huge with target $\lambda$ for some $\lambda$.
- $\kappa$ is super almost $n$-huge iff for every $\gamma$, there is some $\lambda>\gamma$ for which $\kappa$ is almost $n$-huge with target $\lambda$ (that is, the target can be made arbitrarily large).
- $\kappa$ is super $n$-huge iff for every $\gamma$, there is some $\lambda>\gamma$ for which $\kappa$ is $n$-huge with target $\lambda$.
- $\kappa$ is almost huge, huge, super almost huge, and superhuge iff it is almost $1$-huge, $1$-huge, etc. respectively.
First-order Definition
The first-order definition of $n$-huge is somewhat similar to measurability. Specifically, $\kappa$ is measurable iff there is a nonprinciple $\kappa$-complete ultrafilter, $U$, over $\kappa$. A cardinal $\kappa$ is $n$-huge iff there is some cardinal $\lambda$, a nonprinciple $\kappa$-complete ultrafilter, $U$, over $\mathcal{P}(\lambda)$, and cardinals $\kappa=\lambda_0<\lambda_1<\lambda_2...<\lambda_{n-1}<\lambda_n=\lambda$ such that:
$$\forall i<n\forall x\subseteq\lambda(ot(x\cap\lambda_{i+1})=\lambda_i\rightarrow x\in U)$$
Where $ot(X)$ is the order-type of the poset $(X,\in)$. [1] This definition is, more intuitively, making $U$ very large, like most ultrafilter characterizations of large cardinals (supercompact, strongly compact, etc.).
Consistency Strength and Size
Hugeness exhibits a phenomenon associated with similarly defined large cardinals (the $n$-fold variants) known as the double helix. This phenomenon is when for one $n$-fold variant, letting a cardinal be called $n$-$P_0$ iff it has the property, and another variant, $n$-$P_1$, $n$-$P_0$ is weaker than $n$-$P_1$, which is weaker than $n+1$-$P_0$. [2] In the consistency strength hierarchy, here is where these lay (top being weakest):
- measurable = $0$-superstrong = almost $0$-huge = super almost $0$-huge = $0$-huge = super $0$-huge
- $n$-superstrong
- $n$-fold supercompact
- $n+1$-fold strong, $n$-fold extendible
- $n+1$-fold Woodin, $n$-fold Vopěnka
- $n+1$-fold Shelah
- almost $n$-huge
- super almost $n$-huge
- $n$-huge
- super $n$-huge
- $n+1$-superstrong
All huge variants lay at the top of the double helix restricted to some natural number $n$, although each are bested by I3 cardinals (the critical points of the I3 elementary embeddings). In fact, letting $\kappa$ be I3, there is a normal ultrafilter $U$ over $\kappa$ which contains every cardinal $\lambda<\kappa$ such that for each $n$, $\lambda$ is $n$-huge. [1]
Similarly, every huge cardinal $\kappa$ is almost huge, and there is a normal ultrafilter over $\kappa$ which contains every almost huge cardinal $\lambda<\kappa$. Every superhuge cardinal $\kappa$ is extendible and there is a normal ultrafilter over $\kappa$ which contains every extendible cardinal $\lambda<\kappa$. Every $2$-huge cardinal $\kappa$ has a normal ultrafilter which contains every cardinal $\lambda$ such that $V_\kappa\models\lambda\;\mathrm{is}\;\mathrm{superhuge}$. [1]
In terms of size however, the least $n$-huge cardinal is smaller than the least supercompact cardinal. Assuming both exist, for any $\kappa$ which is supercompact and has an $n$-huge cardinal above it, there are $\kappa$ many $n$-huge cardinals less than $\kappa$. [1]
Every $n$-huge cardinal is $m$-huge for every $m\leq n$[1]. Similarly with almost $n$-hugeness, super $n$-hugeness, and super almost $n$-hugeness. Every almost huge cardinal is Vopěnka (therefore $\mathrm{Con}(\mathrm{ZFC}+VP)$ where $VP$ is Vopěnka's principle). [1]
References
- Kanamori, Akihiro. The higher infinite. Second, Springer-Verlag, Berlin, 2009. (Large cardinals in set theory from their beginnings, Paperback reprint of the 2003 edition) www bibtex
- Kentaro, Sato. Double helix in large large cardinals and iteration ofelementary embeddings. , 2007. www bibtex