why is the Ramsey number $R(3,4)$ not equal to $R(3,3)$?

357 Views Asked by At

I understand why R(3,3) is equal to 6 since we have a clique of 3 in a graph of 6 vertices. What I don't understand is that by definition of Ramsay number it says that for a R(r,s) we have to find a clique of either 3 or 4 in size. In this case, we can find a clique of 3 in a graph of 6 nodes unconditionally. Could someone explain to this hopeless student why R(3,4) = 9?

1

There are 1 best solutions below

4
On BEST ANSWER

For $R(3,4)$ you must find either a clique of size $3$ or an independent set of size $4$. The graph

   *    *    *
   |    |    |
   *    *    *

on $6$ vertices has neither a clique of size $3$ nor an independent set of size $4$. Every set of $3$ vertices contains two that are not adjacent, and every set of $4$ vertices contains two that are adjacent.