The hexagonal tortoise problem (Korean: 지수귀문도; Hanja: 地數龜文圖; RR: jisugwimundo) was invented by Korean aristocrat and mathematician Choi Seok-jeong (1646–1715). It is a mathematical problem that involves a hexagonal lattice, like the hexagonal pattern on some tortoises' shells, to the (N) vertices of which must be assigned integers (from 1 to N) 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.
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