Difference between revisions of "User:C7X/Some ordinals"

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Revision as of 19:49, 4 December 2021

Arranged in order of their size in a "reasonable" model. Brackets contain citations

ω₁^CK - admissible - Π₂-rfl. [RichterAczel74]

ω\_ω^CK - limit of the admissibles below it, not admissible [Zoo of ordinals]

CK fixed point

Least rec. inacc. - admissible and limit of admissibles

Least rec. 1-inacc. (in Taranovsky's name, Madore calls this rec. hyper-inacc.)

Least rec. ω-inacc. - rec. n-inacc. for all n<ω

Least rec. hyper-inacc. (α that is rec. α-inacc.) - Hyp\_cos says C(Z+*d*^C(Z\*2,0),0) in Taranovsky's notation behaves like this in collapse

Least rec. Mahlo - Π₂-rfl. on the set of Π₂-rfl. ordinals below it - Taranovsky says C(Z+*d*^2,0) behaves like this in collapse

Least rec. 1-Mahlo - Π₂-rfl. on the set of rec. Mahlos - Taranovsky says C(Z+*d*^3,0) behaves like this in collapse

Least rec. ω-Mahlo - rec. n-Mahlo for all n<ω - Taranovsky says C(Z+*d*^(ω+1),0)

Least Π₃-rfl. - Taranovsky says C(Z+*d*^*d*,0) behaves like this in collapse

Least Π₂-rfl. on the set of Π₃-rfl. ordinals below - Taranovsky says C(Z+*d*^(*d*+1),0) - Madore says to watch out for this type of reflection and not miss its existence [1]

Least Π₃-rfl. that's Π₂-rfl. on the Π₃-rfl.s - Taranovsky says C(Z+*d*^(*d*\*2),0)

Least Π₃-rfl. on Π₃-rfl.s - Taranovsky says C(Z+*d*^(*d*^2),0) - Arai has studied this case, it relates to iterating thinning operators for reflection along lexicographic orderings [Arai2010]

Least "Π₃-rfl. on Π₃-rfl. on Π₃-rfl. on ..." (length ω) - Richter and Aczel proved we can iterate Π\_n-reflection quite far before reaching Π\_(n+1)-reflection [RichterAczel74] - Taranovsky says C(Z+*d*^*d*^(ω+1))

Least Π₄-rfl. - Taranovsky says C(Z+*d*^*d*^*d*,0) - Duchhardt analyzed KP+Π₄-rfl. in 2008 and has an OCF collapsing this ordinal, IDK how it works

Least Π₅-rfl. - Taranovsky says C(Z+*d*^*d*^*d*^*d*,0)

Limit of "least Π\_n-rfl." for n<ω - Some important structure from here to the next ordinal

Least (+1)-stb. - Π\_n-rfl. for all n<ω

Insert important ordinals from User:C7X/Stability list here

Least α that's Π₂-rfl. on {β∈α|β is α-stable} - for some results about this see User:C7X/Stability

Least Σ₂-admissible - least α that's Π₃-rfl. on {β∈α|β is α-stable} [2]

Least Σ₃-admissible - least α that's Π₄-rfl. on {β∈α|β is (α,2)-stable} [above source]

Least Σ₄-admissible - least α that's Π₅-rfl. on {β∈α|β is (α,3)-stable} [above]

Least gap ordinal - Σ\_n-admissible for all n<ω - least height of β-model of Z₂ - least height of model of ZFC⁻+"V=HC" [Gaps in the constructible universe]

/!\ I know less about the remaining structure /!\

Least gap of length 2 - this is a limit of gap ordinals [3]

β that starts a gap of length β - exists according to https://core.ac.uk/download/pdf/81133582.pdf

β that starts gap of length β^β - exists and is mentioned in a corollary of https://core.ac.uk/download/pdf/81133582.pdf

β that starts gap of length β⁺ - least height of model of KP+"ω₁ exists", this model satisfies "β is uncountable" [4]

Least start of third-order gap - least height of model of ZFC⁻+"beth₁ exists"+"V=H\_beth₁" (we take image of P(ω) under bijection from P(ω) to beth₁ that exists by choice) [5]. If α is the previous ordinal this is >α⁺ [6]

Least start of fourth-order gap - least height of model of ZFC⁻+"beth₂ exists"+"V=H\_beth₂" (same trick)? [Alluded to in above source]

Least height of model of ZFC⁻+"beth\_ω exists"

Least height of model of ZFC⁻+"beth\_ω₁ exists"

Least height of model of ZFC⁻+"beth fixed point exists"

Least height of model of ZFC

Least height of model of ZFC+"inaccessible exists"

Least height of model of ZFC+"subtle exists" [Size informally mentioned by Taranovsky]

Least stable [Size informally mentioned by Taranovsky], this is a limit of gap ordinals [7]

Least stable that's also during a gap - height of least β₂-model of Z₂ [8], if this ordinal is α then it's the αth stable ordinal [same paper]

Past here is some stuff that Rathjen wrote a bit about, the ordinals α such that L\_α is Σ\_n-elementary-substructure of L when 1<n<ω. I hazard a guess that these are related to heights of β\_(n-1)-models of Z₂ [maybe http://matwbn.icm.edu.pl/ksiazki/fm/fm82/fm82112.pdf, by extending theorem 2.2?].

The least α such that L\_α is power-admissible should also be in this list at least as far down as Σ₂-admissible [9], but IDK where it is exactly