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Gödel and Formalism freenessPowerPoint Presentation

Gödel and Formalism freeness

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### Gödel and Formalism freeness

Juliette Kennedy

Gödel’s two notions of definability

- Two canonical inner models:
- Constructible sets
- Model of ZFC
- Model of GCH

- Hereditarily ordinal definable sets
- Model of ZFC
- CH? – independent of ZFC

- Constructible sets

Constructibility

- Constructible sets (L):

Ordinal definability

- Hereditarily ordinal definable sets (HOD):
- A set is ordinal definable if it is of the form
{a : φ(a,α1,…, αn)}

where φ(x,y1,…, yn) is a first order formula of set theory.

- A set is hereditarily ordinal definable if it and all elements of its transitive closure are ordinal definable.

- A set is ordinal definable if it is of the form

- Myhill-Scott result: Hereditarily ordinal definable sets (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):
- Chang considered a similar construction with the infinitary logic Lω1ω1 in place of first order logic.

- If V=L, then V=HOD=Chang’s model=L. (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):
- If there are uncountably many measurable cardinals then AC fails in the Chang model. (Kunen.)

C( (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic): L*)

- L* any logic. We define C(L*):
- C(L*) = the union of all L´α

Looking ahead: (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- For a variety of logics C(L*)=L
- Gödel’s L is robust, not limited to first order logic

- For a variety of logics C(L*)=HOD
- Gödel’s HOD is robust, not limited to second order logic

- For some logics C(L*) is a potentially interesting new inner model.

Robustness of L (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- Q1xφ(x) {a : φ(a)} is uncountable
- C(L(Q1)) = L.
- In fact: C(L(Qα)) = L, where
- Qαxφ(x) |{a : φ(a)}| ≥ אα

- Other logics, e.g. weak second order logic, ``absolute” logics, etc.

Observations: avoiding L (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- C(Lω1ω) = L(R)
- C(L∞ω) = V

Quantifiers (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- Q1MMxyφ(x,y) there is an uncountable X such that φ(a,b) for all a,b in X
- Can express Suslinity.

- Q0cfxyφ(x,y) {(a,b) : φ(a,b)} is a linear order of cofinality ω
- Fully compact extension of first order logic.

Theorems (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- C(L(Q1MM)) = L, assuming 0#
- C(L(Q0cf)) ≠ L, assuming 0#
- Lµ⊆C(L(Q0cf)), if Lµ exists

What Myhill-Scott really prove (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- In second order logic L2 one can quantify over arbitrary subsets of the domain.
- A more general logic L2,F: in domain M can quantifier only over subsets of cardinality κ with F(κ) ≤ |M|.
- F any function, e.g. F(κ)=κ, κ+, 2κ, בκ, etc
- L2 = L2,F with F(κ)≡0
- Note that if one wants to quantify over subsets of cardinality κ one just has to work in a domain of cardinality at least F(κ).

Theorem (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- For all F: C(L2,F)=HOD
- Third, fourth order, etc logics give HOD.

Bernays (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

“It seems in no way appropriate that Cantor’s Absolute be identified with set theory formalized in standardized logic, which is considered from a more comprehensive model theory.”

-Letter to Gödel, 1961. (Collected Works, vol. 4, Oxford)

Thank you! (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

Robustness of L (contd.) (HOD) can be seen as the constructible hierarchy with second order logic (in place of first order logic):

- A logic L* is absolute if ``φ∈L*” is Σ1 in φ and ``M⊨φ” is Δ1 inM and φ in ZFC.
- First order logic
- Weak second order logic
- L(Q0): ``there exists infinitely many”
- Lω1ω, L∞ω: infinitary logic
- Lω1G,L∞G: game quantifier logic
- Fragments of the above

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