Riemann’s Hypothesis — First formulated in 1859, this is another so far unsolved math conundrum dealing with the number of zeros in Riemann zeta function.
More information
More information
See also:
Millennium Prize Problems — Seven math problems that the Clay Mathematics Institute considers ‘important classic questions that have resisted solution over the years’ (P vs NP is also listed)
Mathworld
Facsimile of Riemann’s 1859 paper — in German and English as .pdf files