Euclid's Parallel Postulate
Through a point, not on a line, there exists exactly 1 line parallel to the given line.
sum (k=1..inf) 1/kn = ?
Although others have found that this expression equals PI2 / 6 when n=2, PI4 / 90 when n = 4 and simular solutions for all possible even values of n, no one has discovered an exact value when n is an odd integer (3, 5, 7, ...) (note: when n=1, the sum does not converge, but it does has relations to the gamma constant).
Sorry I copied and pasted:
http://www.geocities.com/RainForest/Vines/2977/gauss/euclidpp.html