Can we exactly calculate [Ramsey numbers][1]? Erdős once famously remarked:

> "Suppose aliens invade the earth and threaten to obliterate it in a year's time unless human beings can find the Ramsey number for red five and blue five. We could marshal the world's best minds and fastest computers, and within a year we could probably calculate the value. If the aliens demanded the Ramsey number for red six and blue six, however, we would have no choice but to launch a preemptive attack."




  [1]: http://en.wikipedia.org/wiki/Ramsey%2527s_theorem#Ramsey_numbers