Pólya conjecture
en.wikipedia.org
en.wikipedia.org
I've never heard this before, but I'll never forget it now. There are some more great Pólya quotes on his Wikipedia page: http://en.wikipedia.org/wiki/George_Pólya
Its probably the most expensive intellectual endeavor known to man (assuming you place a premium value on your time and life.)
Praptak's conjecture: every honest, non-tricky conjecture about natural numbers (that is false) has a counterexample not greater than 10^9.
Given a set of consecutive natural numbers, must one be coprime to all the others?
Consider the set: {3, 4, 5}. 3 is coprime to both 4 and 5. Am I misunderstanding the question?
{2,3} has 2 coprime to 3.
{12,13,14,15} has 13 coprime to 12, 14, and 15.
{24,25,26,27,28} has 25 coprime to 24, 26, 27, and 28.
{48,49,50,51,52} has 49 coprime to 48, 50, 51, and 52.