# Ramsey Theory: The Surprising Intersection of Logic and Combinatorics

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## Chapter 1: Understanding Ramsey’s Theorem

Ramsey's Theorem is primarily framed today in terms of graph theory. It states that within a sufficiently large complete graph, where edges are colored either red or blue, there will always be a subset of vertices connected by edges of a single color (a clique). Specifically, there exists a minimum positive integer R(r, s) such that any coloring of the complete graph with R(r, s) vertices must contain a blue clique of size r or a red clique of size s. —source: Wikipedia

To illustrate this concept, one can think of a social network: red edges symbolize friendships, while blue edges denote strangers (in this scenario, each pair of individuals is either friends or strangers, with no in-between). The Ramsey number R(3, 3) signifies the least number of people needed to ensure that either three individuals are mutual friends or three are complete strangers.

It turns out that R(3, 3) = 6, implying that in a gathering of six people, there must either be three who are friends or three who are strangers.

In a complete graph of six vertices, it is impossible to avoid forming either a red or blue clique of three vertices. While the specific case of R(3, 3) is manageable to prove by hand, determining Ramsey numbers R(m, n) in general poses significant challenges. For instance, we know R(4, 4) = 18, but R(5, 5) remains elusive, lying somewhere between 43 and 48.

Consider the sheer magnitude of what these Ramsey numbers represent. If one were to exhaustively check all possible red-blue edge colorings of a complete graph with 20 vertices (which has a total of 190 edges), the number of graphs to evaluate would be 2¹⁹⁰, translating to an astronomical figure exceeding 10¹⁹⁰.

### Paul Erdős and Ramsey Theory

The complexity of calculating Ramsey numbers is captured in a famous anecdote involving Paul Erdős. He suggests imagining an alien civilization far superior to humanity arriving on Earth, demanding the value of R(5, 5) under the threat of destruction. In such a scenario, he argues, we should gather all our mathematicians and computational resources to find the answer. However, if they were to ask for R(6, 6), he humorously proposes that we should instead consider fighting back.

Erdős was instrumental in laying the groundwork for Ramsey Theory, establishing exponential lower and upper bounds for R(k, k) that remained unchallenged for many years.

In this video, titled "What is Ramsey Theory?", you can delve deeper into the foundational aspects and implications of this intriguing mathematical domain.

### Recent Advances in Ramsey Theory

A recent breakthrough was made by a team of four mathematicians who improved Erdős' exponential upper bound for R(k, k), a significant milestone for the combinatorial community.

The author Leila Sloman noted in Quanta that the team felt a renewed sense of optimism: “For a long time, it felt like every door you pushed on was either bolted shut, or at least pretty difficult to get through. And after that change, you just felt like every door was open. Somehow, everything just seemed to work.”

### The Origin of Ramsey Theory

Interestingly, Frank Ramsey originally articulated his well-known theorem as a lemma in a paper discussing another logical result. In fact, his initial statement did not mention graphs but focused on members of logical classes.

While the specific definitions may be complex, the relationship to contemporary graph theory and social network interpretations is clear. The "mutually exclusive classes" correspond to the colors used for graph coloring, while the "subclasses of ? which have exactly r members" relate to the edges within the colored clique.

Ramsey also presented a Theorem B, addressing finite classes, which likely corresponds to what we now denote as the Ramsey number R(n, n). This foundational work remains relevant within the context of what is now known as the Bernays–Schönfinkel–Ramsey class of first-order logic.

### The Genius of Frank Ramsey

Frank Ramsey's brilliance was evident from a young age. At just 19, he was chosen to translate Ludwig Wittgenstein's Tractatus Logico-Philosophicus from German to English, establishing a profound intellectual bond between the two. Despite their language barriers—Wittgenstein's limited English and Ramsey's rudimentary German—they connected through the medium of mathematical logic.

When Wittgenstein arrived at Cambridge to submit his thesis, Ramsey, though significantly younger, served as his supervisor. Over years of discussion, Ramsey significantly influenced Wittgenstein's later thoughts, which would be reflected in his posthumous work, Philosophical Investigations.

Cheryl Misak, in her biography of Ramsey, notes, "Initially, the influence flowed from Wittgenstein to Ramsey, but by the end, it had reversed, with Ramsey imparting his pragmatic insights to Wittgenstein."

Tragically, Ramsey passed away at the age of 26 due to a liver condition in 1930, just two years after publishing "On a Problem of Formal Logic." We can only imagine the contributions he might have made had he lived longer. Nevertheless, the legacy of Frank Ramsey endures through the works of Wittgenstein, Erdős, Turing, and countless other mathematicians who have built upon his foundational ideas.

### A Challenging Combinatorial Puzzle from the 2023 USA Math Olympiad

Would you like to take a crack at solving it?

### A Compelling Graph Theory Problem from the 2022 China IMO Team Selection Test

Are you searching for Eulerian cycles?