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Questions designed to generate a "big list" of certain results, examples, conjectures, etc. via many individual answers, each contributing one or a few instances. Such a question should typically be in Community Wiki mode (CW); after asking, please, flag for moderators attention requesting the question to be made CW.

22 votes

What notable theorems cannot be automatically proven without choice using Shoenfield absolut...

$\text{ZF}+ \text{AC}_{\omega}$ is not $\Sigma^1_4$-conservative over ZF and ZF + DC is not $\Sigma^1_4$-conservative over $\text{ZF}+ \text{AC}_{\omega}.$ An example of the former: the sentence $\ome …
Elliot Glazer's user avatar
13 votes

What are some reasonable-sounding statements that are independent of ZFC?

Is there a vector space with three non-isomorphic Hilbert space structures? A negative answer is equivalent to the conjunction CH + SCH. Pf: An infinite-dimensional Hilbert space with Hilbert dimensio …
Elliot Glazer's user avatar