Forcing and Independence Proofs

Cooordinated Project, January 2023, ILLC

Coordination: Dr. Yurii Khomskii

  1. Arunavo Ganguly
  2. Ruiting Hu
  3. Kirill Kopnev
  4. Ferreol Lavaud
  5. Paul Talma
  6. Vince Velkey
  7. Annica Vieser
  8. Lingyuan Ye

Project Description

The aim of this project is to study the theory of forcing and independence proofs, including basic principles of models of set theory, absoluteness and reflection theorems, the constructible sets, Martin's Axiom (without its consistency proof), the technical aspects of forcing, and up till the original application of forcing which establishes the consistency of ZFC + ¬CH.

The students will study the material independently, assisted by several group meetings. There will be a four assignments to complete and submit. In the last week of January, the students give talks presenting a segment of the material. Successful evaluation of the project is be based on completion of the assignments and presentations.


We will use the following textbooks:

A note about the notation and conventions in Kunen's textbooks.


Below is a detailed list of topics to be covered, with reference to the corresponding textbook sections.

Topic     Reading Material Assignments    
1. Models of Set Theory
  • Class models
  • Relativization
  • Absoluteness
  • Kunen 1980: Chapter IV §2, §3 and §5. (p 112 ff)
  • Jech: Chapter 12, pp. 161-164 (same content, more concise)
  • Kunen 2011: I.16 (p 95 - 102) (same content, new edition)
Assignment 1

Submit your assignment here.

2. Reflection and Collapse
  • Mostowski Collapse
  • Reflection Principles
  • Jech p. 68 - 69 (Mostowski collapse)
  • Jech p. 168 - 170 (reflection)
  • Kunen 1980: Chapter IV §7 (more detailed explanation of Reflection)
  • Kunen 2011: II.5 (p. 129 ff) (another detailed explanation of Reflection)
Extra: The Constructible Universe L
  • The main ideas will be presented in a meeting; you can read the corresponding section for further details, but it is not obligatory and there are no assignments on this section.
  • Kunen 2011: II.6, pp. 134 - 141.
3. Martin's Axiom MA
  • Definition of the axiom
  • Basic properties

  • Remark: the axiom may seem arbitrary, but it is introduced here as a way of getting used to the combinatorics of forcing
  • Kunen 2011: Section III.3 until incl. Lemma III.3.15, pp. 171-175.
  • Kunen 2011: Lemma III.2.6, pp. 166 - 167 (Delta-Systems Lemma)
Assignment 2

Submit your assignment here.

4. Introduction to forcing
  • The general idea
  • Generic extensions
  • Properties of M[G]
  • The semantic forcing relation ⊩
5. The technicalities of forcing
  • The syntactic forcing relation ⊩*
  • The Truth Lemma and Definability Lemma
  • Equivalence of the two forcing relations
Assignment 3

Submit your assignment here.

6. The ZFC Axioms
  • M[G] ⊨ ZFC
  • Kunen 2011: Lemma IV.2.15, Lemma IV.2.26 and Theorem IV.2.27
7. Forcing ¬CH.
  • Adding κ-many new reals by Cohen forcing
  • Preservation of cardinals
  • ccc forcings preserve cardinals
  • Con(ZFC + ¬CH)
  • Kunen 2011: pp. 263 - 265
Assignment 4

Submit your assignment here.

Links to online forms

Preparatory Meetings

Online meeting link:

Date     What Notes    
1. Monday 2 January, 19h (online) Introductory meeting. Questions about practicalities.
Relativization, issues about formal language vs. meta-langauge, etc.
2. Friday 6 January, 17:30 (online) Discussion/questions. Mini-lecture on Reflection, Gödel's Constructible Universe L, and the relative
consistancy of AC and GCH.
3. Tuesday 17 January, 15:00 (online) Discussing forcing and Martin's Axiom Notes
4. Friday 27 January, 17:00 (online) Intuition about forcing-names, other application of forcing Notes

Student Presentations

Date     Who     Topic Room Notes    
1. Wednesday 1 February, 13:00 Vince Velkey Introduction to Forcing F 1.15   (Seminar Room)
2. Wednesday 1 February, 14:30 Arunavo Ganguly Technicalities of Forcing F 1.15   (Seminar Room) Notes
3. Thursday 2 February, 13:00 Kirill Kopnev ZFC in M[G] L.012   (LAB 42) Notes
4. Thursday 2 February, 14:30 Ruiting Hu Forcing non-CH L.012   (LAB 42) Notes
5. Thursday 2 February, 16:00 Lingyuan Ye Boolean-Valued Sets and Forcing L.012   (LAB 42)
6. Friday 1 February, 16:00 Annica Vieser Martin's Axiom L.012   (LAB 42) Notes
7. Friday 1 February, 17:30 Paul Talma Forcing in Recursion Theory L.012   (LAB 42)
8. Thursday 16 February, 15:00 Ferreol Lavaud Topos-theoretic approach to Forcing A1.04