All Questions
7 questions with no upvoted or accepted answers
9
votes
0
answers
529
views
How many steps does it take to "Tarski-Vaughtify" second-order logic?
Given a regular logic $\mathcal{L}$, let $\preccurlyeq_\mathcal{L}$ be the usual elementary submodelhood relation for $\mathcal{L}$. There is also a separate submodelhood relation coming from the ...
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6
votes
0
answers
251
views
Whence compactness of automorphism quantifiers?
The following question arose while trying to read Shelah's papers Models with second-order properties I-V. For simplicity, I'm assuming a much stronger theory here than Shelah: throughout, we work in $...
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6
votes
0
answers
189
views
Reference requestion: theorem guaranteeing self-embeddings of expansions of $\mathit{Ord}$
This is an attempt to locate a theorem I vaguely remember but cannot find (which arose in the context of Consistency of non-trivial elementary embedding $j : \mathit{Ord} \to \mathit{Ord}$ and ...
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6
votes
0
answers
249
views
Number of models vs. complexity for SOL theories
This was previously asked at MSE without success.
Suppose $T$ is a complete first-order theory with continuum-many countable models up to isomorphism. We define two sets of Turing degrees associated ...
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4
votes
0
answers
151
views
How big a "scaffold" does second-order logic need to detect its own equivalence notion?
(Previously asked and bountied at MSE:)
Let $\Sigma$ be the language consisting of a single binary relation symbol. Second-order logic can "detect" second-order-elementary-equivalence of $\...
- 20.5k
4
votes
0
answers
171
views
Can SOL characterize its own equivalence notion, without "scaffolding," for graphs?
Consider the following property $(*)_\mathcal{L}$ of a logic $\mathcal{L}$:
$(*)_\mathcal{L}:\quad$ There is no $\mathcal{L}$-sentence $\varphi$ such that for all graphs $\mathcal{A},\mathcal{B}$ we ...
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3
votes
0
answers
133
views
Comparing two fragments of SOL with the downward Lowenheim-Skolem property
For $S$ a set of (parameter-free) second-order formulas and $\mathfrak{A},\mathfrak{B}$ structures, write $\mathfrak{A}\trianglelefteq^S\mathfrak{B}$ iff $\mathfrak{A}$ is a substructure of $\mathfrak{...
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