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A formal proof of the independence of the continuum hypothesis

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# Flypitch

### A formal proof of the independence of the continuum hypothesis

In this repository we give a formal proof of the independence of the continuum hypothesis, completing the original objective of the Flypitch project.

The formal statement of the independence of the continuum hypothesis is given in src/summary.lean:

theorem independence_of_CH : independent ZFC CH_f
• The definition of independent states that a sentence is neither provable nor disprovable from a theory:
def independent {L : Language} (T : Theory L) (f : sentence L) : Prop :=
¬ T ⊢' f ∧ ¬ T ⊢' ∼f

The notation T ⊢' f means that there is a proof tree of f with assumptions in T. The proof system is given in src/fol.lean.

• ZFC is the first-order theory of Zermelo-Fraenkel set theory plus choice, defined in src/ZFC.lean.
• We formulate it in a language with five constant/function symbols, plus (membership): for the empty set, pairs, omega, the power set and union.
• For each constant/function symbol, ZFC contains an axiom giving the defining property of that symbol. The axiom of infinity is modified to additionally specify that ω is the least limit ordinal. Addionally, ZFC contains the axiom of extensionality, regularity, Zorn’s lemma and the axiom schema of collection.
• The five constant/function symbols we added are definable from ZFC formulated only in the language with ; these extra symbols make the formulation of Zorn’s lemma and the continuum hypothesis easier.
• CH_f is the continuum hypothesis as a first-order logic sentence. The sentence we use is ∀x, x is an ordinal ⟹ x ≤ ω ∨ P(ω) ≤ x where a ≤ b means that there is a surjection from a subset of b to a.

#### Forcing

Both consistency proofs for CH and ¬CH were done by forcing with Boolean-valued models. To force ¬CH, we used Cohen forcing, and to force CH, we used collapse forcing. Some details of our implementation of Boolean-valued models and Cohen forcing can be found in our ITP paper.

#### Possible future work

• Formalizing the reduction from ZFC to ZFC without function symbols
• Consistency of CH via construction of the constructible universe
• Proof transfer using the completeness theorem
• Forcing over countable transitive models
• Forcing using modal logic
• Forcing using sheaves

### Documentation

A conventional human-readable account of the proof written in type-theoretic foundations, upon which some parts of the formalization are based, is located here.

### Compilation

To compile the Flypitch project, you will need Lean 3.4.2. Installation instructions for Lean can be found here. This will also install a command-line tool called leanproject.

The leanpkg.toml file points to a certain commit of mathlib, which will be cloned into the project directory. After cloning or extracting the repository to flypitch, navigate to flypitch and run

leanproject get-mathlib-cache
leanproject build
lean --trust=0 ./src/summary.lean # check `independence_of_CH` as rigorously as possible

leanpkg configure
leanpkg build # will take much longer
lean --trust=0 ./src/summary.lean # check `independence_of_CH` as rigorously as possible

### Viewing and navigating the project

To view the project, we recommend the use of a Lean-aware editor like Emacs or VSCode. Among other things, like type information, such editors can display the proof state inside a tactic block, making it easier to understand how theorems are being proved.

A summary of the main results can be found in src/summary.lean, containing #print statements of important definitions and duplicated proofs of the main theorems. From there, one may use their editor’s jump-to-definition feature to trace the dependencies of the definitions and proofs.

We also have a formula pretty-printer which prints de Bruijn indexed-formulas as their named representations. Code and examples for the pretty-printer can be found in src/print_formula.lean.

### Papers

Our paper A formal proof of the independence of the continuum hypothesis, describing this release, was accepted to CPP 2020. It is available here.

Our paper A formalization of forcing and the unprovability of the continuum hypothesis, describing this release, was accepted to ITP 2019. It is available here.

#### Citation information:

@inproceedings{DBLP:conf/cpp/HanD20,
author    = {Jesse Michael Han and
Floris van Doorn},
title     = {A formal proof of the independence of the continuum hypothesis},
booktitle = {Proceedings of the 9th {ACM} {SIGPLAN} International Conference on
Certified Programs and Proofs, {CPP} 2020, New Orleans, LA, {USA}, January
20-21, 2020},
year      = {2020},
crossref  = {DBLP:conf/cpp/2020},
biburl    = {https://dblp.org/rec/bib/conf/cpp/HanD20},
bibsource = {dblp computer science bibliography, https://dblp.org}
}
@inproceedings{DBLP:conf/itp/HanD19,
author    = {Jesse Michael Han and
Floris van Doorn},
title     = {A Formalization of Forcing and the Unprovability of the Continuum
Hypothesis},
booktitle = {10th International Conference on Interactive Theorem Proving, {ITP}
2019, September 9-12, 2019, Portland, OR, {USA.}},
pages     = {19:1--19:19},
year      = {2019},
crossref  = {DBLP:conf/itp/2019},
url       = {https://doi.org/10.4230/LIPIcs.ITP.2019.19},
doi       = {10.4230/LIPIcs.ITP.2019.19},
timestamp = {Sat, 07 Sep 2019 02:31:13 +0200},
biburl    = {https://dblp.org/rec/bib/conf/itp/HanD19},
bibsource = {dblp computer science bibliography, https://dblp.org}
}

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A formal proof of the independence of the continuum hypothesis

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