Finding [Ramsey numbers](http://en.wikipedia.org/wiki/Ramsey%27s_theorem http://en.wikipedia.org/wiki/Ramsey%27s_theorem) is one. They are hard to calculate eve

Finding [Ramsey numbers](http://en.wikipedia.org/wiki/Ramsey%27s_theorem http://en.wikipedia.org/wiki/Ramsey%27s_theorem) is one. They are hard to calculate even for small values. Most open problems are either to prove something because we can make a good guess to what the answer should be.