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Search options not deleted user 26705
7 votes
2 answers
373 views

A Weakening of the Tree Property

If $f$ and $g$ are two functions, define $f \sim g$ if they differ only finitely often on their common domain. The following property of a large cardinal arose from a problem in model theory. I am in …
Danielle Ulrich's user avatar
6 votes
2 answers
636 views

A "Completion" of $ZFC^-$

Let $T_0$ be the set theory axiomatized by $ZFC^-$ (that is $ZFC$ without powerset) + every set is countable + $\mathbb{V}=\mathbb{L}$. Question 1: Suppose $\phi$ is a sentence of set theory. Must t …
Danielle Ulrich's user avatar
1 vote

A "Completion" of $ZFC^-$

Overnight the following occurred to me... The answer to Question 2 is negative (with an asterisk), and so the same is true of Question 1. Namely, let $T$ be a set of $\Pi_2$ sentence with $ZFC^- \cup …
Danielle Ulrich's user avatar
8 votes
1 answer
333 views

Consistency Strength of "HC is elementary in V[G]"

Let $P$ be the Levy-collapse of the ordinals, so $P$ is a class forcing notion that makes every ordinal countable. Note that since $P$ is weakly homogeneous, for any formula $\phi(\overline{a})$ wit …
Danielle Ulrich's user avatar
16 votes
Accepted

Can an ultrapower be undone by forcing?

For set-forcing, the answer is no, see the following article Joel David Hamkins, Greg Kirmayer, and Norman Lewis Perlmutter, Generalizations of the Kunen inconsistency, Ann. Pure Appl. Logic 163 (201 …
Danielle Ulrich's user avatar