The problem of finding in how many ways a Plane convex Polygon of sides can be divided into Triangles by diagonals. Euler first proposed it to Christian Goldbach in 1751, and the solution is the Catalan Number .
See also Catalan Number, Catalan's Problem
Guy, R. K. ``Dissecting a Polygon Into Triangles.'' Bull. Malayan Math. Soc. 5, 57-60, 1958.