23 pages; v1 split into 3 docs: this (v4), 2006.01103, 2006.10574. See v2 comments. Added citations to 1106.1951: in introduction, regarding generalizations of Suzuki 99; in Footnote 3 (p.2) and Remark 3.20 (Theorem 35 of 1106.1951 already establishes a fact discussed at those points); and in the introduction to section 2 (Corollary 34 of 1106.1951 is related). Added Remark 4.7. Other minor edits
Assume ZF($j$) and there is a Reinhardt cardinal, as witnessed by the elementary embedding $j:V\to V$. We investigate the linear iterates $(N_α,j_α)$ of $(V,j)$, and their relationship to $(V,j)$, forcing and definability, including that for each infinite ordinal $α$, every set is set-generic over $N_α$, but $N_α$ is not a set-ground.
Assume second order ZF. We prove that the existence of super Reinhardt cardinals and total Reinhardt cardinals is not affected by small forcing. And if $V[G]$ has a set of ordinals which is not in $V$, then $V[G]$ has no elementary embedding $j:V[G]\to M\subseteq V$ (even allowing $M$ to be illfounded).
We prove a weakened version of the reflection of Reinhardt cardinals by super Reinhardt cardinals: Let $M=(V^M,P)$ be a countable model of second order set theory $\mathsf{ZF}_2$ (with universe $V^M$ and classes $P$) which models "$κ$ is super Reinhardt". We show that there are unboundedly many $μ<κ$ such that there is $j$ such that $(V^M,j)$ models $\mathsf{ZF}(j)+$"$μ$ is Reinhardt, as witnessed by $j$". In particular, $j\upharpoonright X\in V^M$ for all $X\in V^M$ (but we allow $j\notin P$).
A cardinal is weakly Reinhardt if it is the critical point of an elementary embedding from the universe of sets into a model that contains the double powerset of every ordinal. This note establishes the equiconsistency of a proper class of weakly Reinhardt cardinals with a proper class of Reinhardt cardinals in the context of second-order set theory without the Axiom of Choice.
We investigate the lower bound of the consistency strength of $\mathsf{CZF}$ with Full Separation $\mathsf{Sep}$ and a Reinhardt set, a constructive analogue of Reinhardt cardinals. We show that $\mathsf{CZF+Sep}$ with a Reinhardt set interprets $\mathsf{ZF^-}$ with a cofinal elementary embedding $j\colon V\prec V$. We also see that $\mathsf{CZF+Sep}$ with a Reinhardt set interprets $\mathsf{ZF^-}$ with a model of $\mathsf{ZF+WA_0}$, the Wholeness axiom for bounded formulas.
We investigate the problem of when $\leqλ$--support iterations of $<λ$--complete notions of forcing preserve $λ^+$. We isolate a property -- {\em properness over diamonds} -- that implies $λ^+$ is preserved and show that this property is preserved by $λ$--support iterations. We close with an application of our technology by presenting a consistency result on uniformizing colorings of ladder systems on $\{δ<λ^+:\cf(δ)=λ\}$ that complements a theorem of Shelah.
It is shown that if there is a measurable cardinal above n Woodin cardinals and M_{n+1}^# doesn't exist then K exists. K is not fully iterable, though, but only iterable with respect to stacks of certain trees living between the Woodin cardinals. However, it is still true that if M is an omega-closed iterate of V then K^M is an iterate of K.
We prove that a large class of presaturated ideals at inaccessible cardinals can be de-saturated while preserving their presaturation, answering both a question of Foreman and of Cox and Eskew. We do so by iterating a generalized version of Baumgartner and Taylor's forcing to add a club with finite conditions along an inaccessible cardinal, and invoking Foreman's Duality Theorem.
Kunen's proof of the non-existence of Reinhardt cardinals opened up the research on very large cardinals, i.e., hypotheses at the limit of inconsistency. One of these large cardinals, I0, proved to have descriptive-set-theoretical characteristics, similar to those implied by the Axiom of Determinacy: if $λ$ witnesses I0, then there is a topology for $V_{λ+1}$ that is completely metrizable and with weight $λ$ (i.e., it is a $λ$-Polish space), and it turns out that all the subsets of $V_{λ+1}$ in $L(V_{λ+1})$ have the $λ$-Perfect Set Property in such topology. In this paper, we find another gene…
We deal with (< kappa)-supported iterated forcing notions which are (E_0,E_1)-complete, have in mind problems on Whitehead groups, uniformizations and the general problem. We deal mainly with the successor of a singular case. This continues [Sh:587]. We also deal with complimentary combinatorial results.
We consider a cardinal invariant closely related to Hindman's theorem. We prove that this cardinal invariant is small in the iterated Sacks perfect set forcing model, and that its corresponding parametrized diamond principle implies the existence of union ultrafilters. As a corollary, this establishes the existence of union ultrafilters in the iterated Sacks model of Set Theory.
Let GCH hold and let $j:V\longrightarrow M$ be a definable elementary embedding such that $crit(j)=κ$, $^κM\subseteq M$ and $κ^{++}=κ_{M}^{++}$. H. Woodin proved that there is a cofinality preserving generic extension in which $κ$ is measurable and GCH fails at it. This is done by using an Easton support iteration of Cohen forcings for blowing the power of every inaccessible $α\leqκ$ to $α^{++}$, and then adding another forcing on top of that. We show that it is enough to use the iterated forcing, and that the latter forcing is not needed. We will show this not only for the case where $κ^{++}=…
We present a version with non-definable forcing notions of Shelah's theory of iterated forcing along a template. Our main result, as an application, is that, if $κ$ is a measurable cardinal and $θ<κ<μ<λ$ are uncountable regular cardinals, then there is a ccc poset forcing $\mathfrak{s}=θ<\mathfrak{b}=μ<\mathfrak{a}=λ$. Another application is to get models with large continuum where the groupwise-density number $\mathfrak{g}$ assumes an arbitrary regular value.
We continue investigations of reasonable ultrafilters on uncountable cardinals defined in math.LO/0407498. We introduce stronger properties of ultrafilters and we show that those properties may be handled in lambda-support iterations of reasonably bounding forcing notions. We use this to show that consistently there are reasonable ultrafilters on an inaccessible cardinal lambda with generating system of size less than 2^lambda . We also show how reasonable ultrafilters can be killed by forcing notions which have enough reasonable completeness to be iterated with lambda-supports (and we show th…
The resurrection axioms are forcing axioms introduced recently by Hamkins and Johnstone, developing on ideas of Chalons and Velicković. We introduce a stronger form of resurrection axioms (the \emph{iterated} resurrection axioms $\textrm{RA}_α(Γ)$ for a class of forcings $Γ$ and a given ordinal $α$), and show that $\textrm{RA}_ω(Γ)$ implies generic absoluteness for the first-order theory of $H_{γ^+}$ with respect to forcings in $Γ$ preserving the axiom, where $γ=γ_Γ$ is a cardinal which depends on $Γ$ ($γ_Γ=ω_1$ if $Γ$ is any among the classes of countably closed, proper, semiproper, stationar…
This dissertation surveys several topics in the general areas of iterated forcing, infinite combinatorics and set theory of the reals. There are two parts. In the first half I consider alternative versions of the Cichoń diagram. First I show that for a wide variety of reduction concepts there is a Cichoń diagram for effective cardinal characteristics relativized to that reduction. As an application I investigate in detail the Cichoń diagram for degrees of constructibility relative to a fixed inner model of ZFC. Then I study generalizations of cardinal characteristics to the space of functions …
We study the nonstationary-support iteration of Prikry forcings below a measurable cardinal κ, characterizing all the normal measures it carries in the generic extension. We then analyze the restriction of ultrapower embeddings, taken with such a normal measure in the generic extension, to the ground model. We prove that every such restriction is an iterated ultrapwer of the ground model, and provide a sufficient condition for its definability there. This is done without core-model theoretic arguments: the assumption that GCH holds in the ground model up to κsuffices.
We investigate the structure of rank-to-rank elementary embeddings, working in ZF set theory without the Axiom of Choice. Recall that the levels $V_α$ of the cumulative hierarchy are defined via iterated application of the power set operation, starting from $V_0=\emptyset$, and taking unions at limit stages. Assuming that $j:V_{α+1}\to V_{α+1}$ is a (non-trivial) elementary embedding, we show that the structure of $V_α$ is fundamentally different to that of $V_{α+1}$. We show that $j$ is definable from parameters over $V_{α+1}$ iff $α+1$ is an odd ordinal. Moreover, if $α+1$ is odd then $j$ is…
