MATH 175 A Theorem of Ramsey 2022
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JUL 01 2022 MATH 175
3 .3 A Theorem of Ramsey
Of six (or more) people, either there are three, each pair of whom are acquainted, or there are three, each pair of whom are unacquainted. We formulate it as
K6 → K3 , K3 .
Theorem. If m ≥ 2 and n ≥ 2 are integers, then there is a positive integer p such that Kp → Km , Kn .
Definition (Ramsey number). The Ramsey number r(m, n) is the smallest integer p such that Kp → Km , Kn . The Ramsey’s theorem asserts the existence of the number r(m, n)
Remark.
r(3, 3) = 6; r(3, 4) = 9; r(3, 5) = 14; r(3, 6) = 18; r(3, 7) = 23.
Example (Page 82 # 20). Prove that r(3, 3, 3) ≤ 17 .
Solution. So, we are trying to show that
K17 → K3 , K3 , K3 .
We prove it by contradiction, suppose this is not possible. Say we have 17 dots, and we assign three
colors, RED , GREEN , BLUE , to the line segments between them . We assume that we can not find three
dots such that the line segments between them are the same color . Fix a dot, then there are 16 line
segments connecting this dot to other 16 dots . By the Pigeonhole principle, at least 6 line segments will
share the same color, say RED for convenience . For 6 dots at the other side of these 6 line segments,
we may assume that the line segments between these 6 points cannot be RED . Otherwise you find a
RED K3, which implies that K17 → K3 , K3 , K3 is possible . So we assume the line segments between
these 6 points are either GREEN or BLUE . Then in fact, this is another Ramsey Theory, we know that
r(3, 3) = 6, which means that it is possible for 6 dots to have either there are 3 dots such that the line
segments between them are GREEN , or there are 3 dots such that the line segments between them are
BLUE . Thus we get a contradiction, for 17 dots, you can either find 3 dots such that the line segments
between them are RED, or 3 dots such that the line segments between them are Green, or 3 dots such
that the line segments between them are Blue . This implies that K17 → K3 , K3 , K3 is possible, which
gives r(3, 3, 3) ≤ 17. □
2022-07-07