In mathematics, particularly topology, an atlas describes how a manifold is equipped with a differential structure. Each piece is given by a chart (also known as coordinate chart or local coordinate system).
Before giving the formal definition of an atlas, we recall that a chart on a manifold M is defined to be a homeomorphism φ from an open subset U of M to an open subset V of . If and are two charts on M such that is nonempty, then define the transition map
Note that since and are both homeomorphisms, the transition maps are also homeomorphisms. So, the transition maps are already endowed with a kind of compatibility in the sense that changing from the coordinate system on one chart to the coordinate system on another chart is continuous.
Then an atlas on a manifold M is a collection of charts on M whose domains cover M.
Now, we say that two overlapping charts and are smoothly compatible if the transition map between them is infinitely differentiable as a map from Euclidean space to itself.
Having defined these notions, a smooth atlas on M is an atlas where we make the additional requirement that, for any two overlapping charts on M, the transition maps between them are smoothly compatible.
Two atlases and on M are smoothly compatible if all charts in which overlap charts in are smoothly compatible. If this is the case then is also a smooth atlas on M. This gives a natural equivalence relation, from which we can consider an equivalence class of smoothly compatible atlases, which we call the maximal atlas. A manifold M together with a maximal atlas is said to have a smooth structure. There are, in higher dimensions, examples of topological manifolds with multiple different smooth structures. One of the first examples was John Milnor's discovery of an exotic sphere, a 7manifold which is homeomorphic to the 7sphere but not diffeomorphic.
In general, doing computations with the maximal atlas of a manifold is unwieldy and we need only choose one particular smooth atlas to work with. Maximal atlases are needed for the unambiguous definition of smooth maps from one manifold to another.
The differentiability requirements on the transition functions can be weakened, so that we only require the transition maps to be ktimes continuously differentiable; or strengthened, so that we require the transition maps to realanalytic. Accordingly, this gives a C^{k} or analytic structure on the manifold rather than a smooth one. Similarly, we can define a complex manifold by requiring the transition maps to be holomorphic.
