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 
.
Euler's Polygon Division Problem
See also
Catalan Number, Catalan's Problem, Polygon Diagonal, Regular Polygon Division by DiagonalsExplore with Wolfram|Alpha
References
Forder, H. G. "Some Problems in Combinatorics." Math. Gaz. 41, 199-201, 1961.Guy, R. K. "Dissecting a Polygon Into Triangles." Bull. Malayan Math. Soc. 5, 57-60, 1958.Referenced on Wolfram|Alpha
Euler's Polygon Division ProblemCite this as:
Weisstein, Eric W. "Euler's Polygon Division Problem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/EulersPolygonDivisionProblem.html