Create R3.sage

R3.sage is an implementation of the Euclidean 3-dimensional space as a differentiable manifold. I demonstrate some calculations with exterior derivatives and Hodge star by doing Exercise 8 (Problem 8) in Section 11.3.2 of Sh. Sternberg's Semi-Riemannian Geometry and General Relativity. Sagemanifolds, the package, along with Sage Math makes calculations involving differentiable forms easy.