Closed simple geodesics on a polyhedron 

It is well-known that every isosceles tetrahedron (disphenoid) admits infinitely many simple closed geodesics on its surface. They can be naturally enumerated by pairs of coprime integers n > m > 1 with two additional cases (1, 0) and (1, 1). The (n, m)- geodesic is a broken line with 4(n+ m) vertices, its length tends to infinity as m → ∞. Are there other polyhedra possessing this property? The answer depends on convexity. We give an elementary proof that among convex polyhedra only disphenoids admit arbitrarily long closed simple geodesics. For non-convex polyhedra, this is not true. We present a counterexample with the corresponding polyhedron being a union of seven equal cubes. Several open problems are formulated.