Well...there are many problems that have remained unsolved......so any one of them might be the "hardest."
But....one of the most difficult to solve was a problem known as "Fermat's Last Theorem"...it states that no known unique integers a,b,c exist such that a^n + b^n = c^n for n≥ 3.
This problem took more than 350 years to solve. {At least one hefty sum of money was offered for its solution). The solution was finally found by an English mathematician, Andrew Wiles, in the mid-1990s. Oddly enough, he found it so frustrating that he almost gave up.....!!!
BTW.....Wiles' proof is over 150 pages in length.....!!! Definitely not "light reading"
