# Good cover (algebraic topology)

The cover on the left is not a good cover, since while all open sets in the cover are contractible, their intersection is disconnected. The cover on the right is a good cover, since the intersection of the two sets is contractible.

In mathematics, an open cover of a topological space ${\displaystyle X}$ is a family of open subsets such that ${\displaystyle X}$ is the union of all of the open sets. In algebraic topology, an open cover is called a good cover if all open sets in the cover and all intersections of finitely many open sets, ${\displaystyle U_{\alpha _{1}\ldots \alpha _{n}}=U_{\alpha _{1}\ldots \alpha _{n-1}}\cap U_{\alpha _{n}}}$, are contractible (Petersen 2006).

The concept was introduced by André Weil in 1952 for differential manifolds, demanding the ${\displaystyle U_{\alpha _{1}\ldots \alpha _{n}}}$ to be differentiably contractible. A modern version of this definition appears in Bott & Tu (1982).

## Application

A major reason for the notion of a good cover is that the Leray spectral sequence of a fiber bundle degenerates for a good cover, and so the Čech cohomology associated with a good cover is the same as the Čech cohomology of the space. (Such a cover is known as a Leray cover.)

## Example

The two-dimensional surface of a sphere ${\displaystyle S^{2}}$ has an open cover by two contractible sets, open neighborhoods of opposite hemispheres. However these two sets have an intersection that forms a non-contractible equatorial band. To form a good cover for this surface, one needs at least four open sets. A good cover can be formed by projecting the faces of a tetrahedron onto a sphere in which it is inscribed, and taking an open neighborhood of each face.