Pierre de Fermat (1605-1665) wrote in the margin of a book, next to the equation xⁿ + yⁿ = zⁿ, “I have discovered a truly remarkable proof of this theorem which this margin is too small to contain.” He then died, leaving no indication of what this proof might be.

It took 357 years before his proof was rediscovered by Andrew Wiles in 1994. It can be seen here.

The theorem was noted by Guinness Book of World Records as “The Most Difficult Mathematical Problem.”

source