scholarly journals THE EXACT STRENGTH OF THE CLASS FORCING THEOREM

2020 ◽  
pp. 1-37
Author(s):  
VICTORIA GITMAN ◽  
JOEL DAVID HAMKINS ◽  
PETER HOLY ◽  
PHILIPP SCHLICHT ◽  
KAMERYN J. WILLIAMS
Keyword(s):  
Class Forcing

Generic Σ31 absoluteness

2004 ◽  
Vol 69 (1) ◽  
pp. 73-80 ◽  
Author(s):  
Sy D. Friedman
Keyword(s):  
Regular Cardinal ◽  
Forcing Axioms ◽  
Proper Forcing ◽  
Class Forcing ◽  
The Universe ◽  
Generic Extensions

In this article we study the strength of absoluteness (with real parameters) in various types of generic extensions, correcting and improving some results from [3]. (In particular, see Theorem 3 below.) We shall also make some comments relating this work to the bounded forcing axioms BMM, BPFA and BSPFA.The statement “ absoluteness holds for ccc forcing” means that if a formula with real parameters has a solution in a ccc set-forcing extension of the universe V, then it already has a solution in V. The analogous definition applies when ccc is replaced by other set-forcing notions, or by class-forcing.Theorem 1. [1] absoluteness for ccc has no strength; i.e., if ZFC is consistent then so is ZFC + absoluteness for ccc.The following results concerning (arbitrary) set-forcing and class-forcing can be found in [3].Theorem 2 (Feng-Magidor-Woodin). (a) absoluteness for arbitrary set-forcing is equiconsistent with the existence of a reflecting cardinal, i.e., a regular cardinal κ such that H(κ) is ∑2-elementary in V.(b) absoluteness for class-forcing is inconsistent.We consider next the following set-forcing notions, which lie strictly between ccc and arbitrary set-forcing: proper, semiproper, stationary-preserving and ω1-preserving. We refer the reader to [8] for the definitions of these forcing notions.Using a variant of an argument due to Goldstern-Shelah (see [6]), we show the following. This result corrects Theorem 2 of [3] (whose proof only shows that if absoluteness holds in a certain proper forcing extension, then in L either ω1 is Mahlo or ω2 is inaccessible).

scholarly journals HYPERCLASS FORCING IN MORSE-KELLEY CLASS THEORY

2017 ◽  
Vol 82 (2) ◽  
pp. 549-575 ◽  
Author(s):  
CAROLIN ANTOS ◽  
SY-DAVID FRIEDMAN
Keyword(s):  
Second Order ◽  
Inaccessible Cardinal ◽  
Second Order Arithmetic ◽  
Class Theory ◽  
Order Arithmetic ◽  
Image Position ◽  
Class Forcing

AbstractIn this article we introduce and study hyperclass-forcing (where the conditions of the forcing notion are themselves classes) in the context of an extension of Morse-Kelley class theory, called MK**. We define this forcing by using a symmetry between MK** models and models of ZFC− plus there exists a strongly inaccessible cardinal (called SetMK**). We develop a coding between β-models ${\cal M}$ of MK** and transitive models M+ of SetMK** which will allow us to go from ${\cal M}$ to M+ and vice versa. So instead of forcing with a hyperclass in MK** we can force over the corresponding SetMK** model with a class of conditions. For class-forcing to work in the context of ZFC− we show that the SetMK** model M+ can be forced to look like LK*[X], where κ* is the height of M+, κ strongly inaccessible in M+ and $X \subseteq \kappa$. Over such a model we can apply definable class forcing and we arrive at an extension of M+ from which we can go back to the corresponding β-model of MK**, which will in turn be an extension of the original ${\cal M}$. Our main result combines hyperclass forcing with coding methods of [3] and [4] to show that every β-model of MK** can be extended to a minimal such model of MK** with the same ordinals. A simpler version of the proof also provides a new and analogous minimality result for models of second-order arithmetic.

Strongly unfoldable cardinals made indestructible

2008 ◽  
Vol 73 (4) ◽  
pp. 1215-1248 ◽  
Author(s):  
Thomas A. Johnstone
Keyword(s):  
Large Cardinals ◽  
Large Cardinal ◽  
Weakly Compact ◽  
Proper Forcing ◽  
Class Forcing

AbstractI provide indestructibility results for large cardinals consistent with V = L, such as weakly compact, indescribable and strongly unfoldable cardinals. The Main Theorem shows that any strongly unfoldable cardinal κ can be made indestructible by <κ-closed, κ-proper forcing. This class of posets includes for instance all <κ-closed posets that are either κ−-c.c. or <κ-strategically closed as well as finite iterations of such posets. Since strongly unfoldable cardinals strengthen both indescribable and weakly compact cardinals, the Main Theorem therefore makes these two large cardinal notions similarly indestructible. Finally. I apply the Main Theorem to obtain a class forcing extension preserving all strongly unfoldable cardinals in which every strongly unfoldable cardinal κ is indestructible by <κ-closed, κ-proper forcing.

scholarly journals Outer models and genericity

2003 ◽  
Vol 68 (2) ◽  
pp. 389-418 ◽  
Author(s):  
M. C. Stanley
Keyword(s):  
Set Theory ◽  
Generic Extension ◽  
Transitive Model ◽  
Class Forcing ◽  
The Universe ◽  
Models Of Set Theory

Why is forcing the only known method for constructing outer models of set theory?If V is a standard transitive model of ZFC, then a standard transitive model W of ZFC is an outer model of V if V ⊆ W and V ∩ OR = W ∩ OR.Is every outer model of a given model a generic extension? At one point Solovay conjectured that if 0# exists, then every real that does not construct 0# lies in L[G], for some G that is generic for some forcing ℙ ∈ L. Famously, this was refuted by Jensen's coding theorem. He produced a real that is generic for an L-definable class forcing property, but does not lie in any set forcing extension of L.Beller, Jensen, and Welch in Coding the universe [BJW] revived Solovay's conjecture by asking the following question: Let a ⊆ ω be such that L[a] ⊨ “0# does not exist”. Is there ab∈ L[a] such that a ∉ L[b] and a is set generic over L[b].

scholarly journals CLASS FORCING, THE FORCING THEOREM AND BOOLEAN COMPLETIONS

2016 ◽  
Vol 81 (4) ◽  
pp. 1500-1530 ◽  
Author(s):  
PETER HOLY ◽  
REGULA KRAPF ◽  
PHILIPP LÜCKE ◽  
ANA NJEGOMIR ◽  
PHILIPP SCHLICHT
Keyword(s):  
Combinatorial Property ◽  
Fundamental Result ◽  
Generic Extension ◽  
Negative Results ◽  
Generic Filter ◽  
Class Forcing

AbstractThe forcing theorem is the most fundamental result about set forcing, stating that the forcing relation for any set forcing is definable and that the truth lemma holds, that is everything that holds in a generic extension is forced by a condition in the relevant generic filter. We show that both the definability (and, in fact, even the amenability) of the forcing relation and the truth lemma can fail for class forcing.In addition to these negative results, we show that the forcing theorem is equivalent to the existence of a (certain kind of) Boolean completion, and we introduce a weak combinatorial property (approachability by projections) that implies the forcing theorem to hold. Finally, we show that unlike for set forcing, Boolean completions need not be unique for class forcing.

The independence of

1999 ◽  
Vol 64 (1) ◽  
pp. 350-362
Author(s):  
Amir Leshem ◽  
Menachem Magidor
Keyword(s):  
Class Forcing

AbstractIn this paper we prove the independence of for n ≥ 3. We show that can be forced to be above any ordinal of L using set forcing. For we prove that it can be forced, using set forcing, to be above any L cardinal κ such that κ is Π1 definable without parameters in L. We then show that cannot be forced by a set forcing to be above every cardinal of L Finally we present a class forcing construction to make greater than any given L cardinal.

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