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The Lumenary

Practice · First-year graduate

Forcing and independence

Forcing and independence

Warm-upHuman

In Cohen forcing Fn(ω,2)\mathrm{Fn}(\omega, 2) (finite partial functions, stronger = larger), which of these sets are dense? Choose all that apply.

Choose all that apply

Forcing and independence

Warm-upHuman

How many conditions does Fn(3,2)\mathrm{Fn}(3, 2) have, i.e. how many partial functions from {0,1,2}\{0, 1, 2\} to {0,1}\{0, 1\} are there (including the empty function 1\mathbb 1)?

Forcing and independence

CoreHuman

Why does the classical presentation of forcing start from a countable transitive model MM?

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Forcing and independence

CoreHuman

In the proof that an MM-generic filter GG (for a forcing in which every condition has two incompatible extensions) is not in MM, which dense set does GG fail to meet if G∈MG \in M?

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Forcing and independence

CoreHuman

The proof that Fn(I,2)\mathrm{Fn}(I, 2) has the countable chain condition takes uncountably many conditions pξp_\xi. What is the key step?

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Forcing and independence

StretchHuman

Assuming ZFC is consistent, which of these statements are independent of ZFC? Choose all that apply.

Choose all that apply

Forcing and independence

CoreFormal

The Lean file orders Cohen conditions by extension: p ≤ q means q extends p. In this order, what does Mathlib's Order.Ideal Cohen correspond to in forcing?

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Forcing and independence

CoreFormal

The theorem no_filter_meets_every_dense (G : Order.Ideal Cohen) : ∃ D : Set Cohen, IsCofinal D ∧ ∀ p ∈ D, p ∉ G is proved with which witness D?

Choose one

Forcing and independence

StretchComputational

Write escape(reals, n): given a list of reals (functions from naturals to {0,1}\{0, 1\}) and nn, return a tuple of bits pp of length at least nn that disagrees with every listed real at some position. This is the Rasiowa–Sikorski construction for the dense sets 'decide bit kk' (k<nk \lt n) and 'disagree with xx'.

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