In this category are included any equation or set of equations where the unknowns must be integer numbers. It includes Fermat Last Theorem as a special case.
Fermat's Last Theorem stated, in his words, "It is impossible to separate a cube into two cubes, or a fourth power into two fourth powers, or in general, any power higher than the second into two like powers." This category is for history, proof, and conjectures related to the theorem.