Hexagonal tortoise problem
The hexagonal tortoise problem was invented by Korean aristocrat and mathematician Choi Seok-jeong. It is a mathematical problem that involves a hexagonal lattice, like the hexagonal pattern on some tortoises' shells, to the vertices of which must be assigned integers in such a way that the sum of all integers at the vertices of each hexagon is the same. The problem has apparent similarities to a magic square although it is a vertex-magic format rather than an edge-magic form or the more typical rows-of-cells form.
His book, Gusuryak, contains many mathematical discoveries.