Braid group

The n-th braid group consist of all different ways n strings can be braided. The generators are all the over-crossings and under-crossings of neighbouring strings. They can be also imagined as world lines of n particles in 2 dimensions which changes its order.

Examples

The generators \sigma_i^{+1} and  \sigma_i^{-1}:

Image:braid.jpg

And the transformation like:

Image:knot.jpg

is not a braid.

Applications

The braid group plays key-role in the topological computing.