The proving of the four colour theorem required the paper to be peer reviewed but in 1976 mathematicians saw the computer program much like a calculator, a computational tool which was assumed to give the right results.Hmm .. I reckon I might have some of my original punch-cards which were sucked into an IBM 360/370 around that same timeframe somewhere amongst my cardboard box archives! From my dim recollections, I don't think it was assember though ..
Seems very careless of the original programmers to loose it!?
However proving the four colour theorem underwent an overhaul starting in the early 2000s.
Certainly — the 2017 formalization of the Four Color Theorem (4CT) in the Coq proof assistant is a landmark in the development of formal mathematics. It addressed longstanding concerns about the original 1976 computer-assisted proof by providing a fully machine-checked, formal verification of every step — including the logic, the algorithms, and the computation.
Let’s break this down thoroughly:
Background: The Problem with the 1976 Proof
The original proof by Appel and Haken (1976) showed that:
Their approach:Every planar map can be colored with no more than four colors such that no two adjacent regions share a color.
The issue was:
- Reduced the infinite problem to checking 1,936 "unavoidable" configurations.
- Used a computer to verify that each configuration is reducible (i.e., can be handled with 4 colors).
- This required:
- A program to perform reductions.
- A large computation to verify all cases.
- No one could feasibly check the machine code by hand.
- This challenged mathematical norms: if a proof requires blind trust in a program, is it still a proof?
Enter the Coq Proof Assistant
Coq is a formal proof assistant. It lets mathematicians:
- Define mathematical objects rigorously.
- Write logical proofs in a structured, formal language.
- Check the proofs mechanically, ensuring that every inference step is valid.
This eliminates ambiguity, oversight, or informal reasoning.
Georges Gonthier’s Formalization Project
Georges Gonthier, a computer scientist at Microsoft Research, led the effort to fully formalize the Four Color Theorem in Coq.
This work began in the early 2000s and culminated in a complete formal proof by 2008, with documentation and completion published around 2017.
Goals:
- Avoid reliance on trust in programs or authors.
- Convert the entire argument — human and machine — into verifiable, formal mathematics.
How it was done:
1. Formalization of Graph Theory
- They used combinatorial representations of planar maps as graphs.
- Defined what it means for a graph to be planar, colored, reducible, and so on.
- Built a robust formal library of graph theory within Coq.
2. Formal Specification of the Algorithm
- The core of the original proof involves checking reducibility for thousands of configurations.
- These were implemented as pure functional programs inside Coq.
- Every program and data structure used was defined in Coq’s logic, so the correctness of the algorithm itself could be proved.
3. Formal Checking of the Computations
- Gonthier’s team created a verified checker in Coq.
- They encoded all 633 "configurations" (a simplified and optimized version of the original 1,936) as data in Coq.
- Ran the reduction-checking program inside Coq — meaning the checking is part of the proof, not external code.
4. Automation and Scalability
- Coq wasn't built for massive computations, so they had to optimize:
- Used reflection, a Coq technique where computation can aid in proving.
- Developed SSReflect ("Small Scale Reflection") to scale the proof and make it manageable.
- SSReflect is now widely used for other large formalizations.
Result
The result was a machine-verified formal proof of the Four Color Theorem, including:
- The theoretical reductions.
- The checking of configurations.
- The correctness of the code.
- The soundness of the logic.
This removed any need for human trust in unverified code — everything from logic to computation was covered within the formal system.
Significance
- Validation of controversial proofs: It showed how even a computer-assisted proof could be brought within the rigorous fold of traditional math — and even strengthened.
- Change in philosophy: More mathematicians are now open to formal verification, especially for complex or computation-heavy proofs.
- Legacy: Gonthier’s work led to improvements in Coq itself and helped spur similar formalization efforts in algebra, number theory, and beyond.
Summary
Aspect Original 1976 Proof 2017 Coq Formalization Proof idea Verified 1,936 configurations with a program Same idea, but formalized with verified code and logic Machine code trust Not verified independently Fully verified within Coq Reproducibility Hard to check by hand Fully machine-checkable Philosophical acceptance Controversial Widely accepted in formal methods
Would you like me to show a simplified example of how Coq encodes graph coloring logic, or explain the SSReflect tactic language used in the project?
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