Divbyzero's profile

I am an associate professor of mathematics at Dickinson College.

I love math, music, coffee, and spending time with my family (not in that order!).

My favorite piece of mathematics is Euler's polyhedron formula, which states that every convex polyhedron with V vertices, E edges, and F faces satisfies V-E+F=2.

Recently I discovered another cool theorem called the Japanese Theorem. Begin with a polygon, all of whose vertices lie on a circle. Divide the polygon into n-2 triangles by drawing nonintersecting diagonals. (Interesting aside: there are many ways to do this—2 ways for a rectangle, 5 ways for a hexagon, 14 ways for a hexagon, and in general c_n ways to do this for an n-gon where c_n is the (n-1)st Catalan number.) Inscribe a circle in each triangle and let r_i be the radius of the ith circle. Then the sum

(1)
\sum_{i=1}^{n-2}r_{i}

does not depend on the choice of triangulation!

For more information see Mathworld.