General Relativity/Differentiable manifolds

<General Relativity

A smooth -dimensional manifold is a set together with a collection of subsets with the following properties:

  1. Each lies in at least one , that is .
  2. For each , there is a bijection , where is an open subset of
  3. If is non-empty, then the map is smooth.

The bijections are called charts or coordinate systems. The collection of charts is called an atlas. The atlas induces a topology on M such that the charts are continuous. The domains of the charts are called coordinate regions.

ExamplesEdit

  • Euclidean space,   with a single chart (  identity map) is a trivial example of a manifold.
  • 2-sphere  .
Notice that   is not an open subset of  . The identity map on   restricted to   does not satisfy the requirements of a chart since its range is not open in  
The usual spherical coordinates map   to a region in  , but again the range is not open in   Instead, one can define two charts each defined on a subset of   that omits a half-circle. If these two half-circles do not intersect, the union of the domains of the two charts is all of  . With these two charts,   becomes a 2-dimensional manifold. It can be shown that no single chart can possibly cover all of   if the topology of   is to be the usual one.