In
graph theory,
reachability refers to the ability to get from one
vertex to another within a graph. We say that a vertex
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=4132)
can reach a vertex
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=3854)
(or that
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=3854)
is reachable from
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=4132)
) if there exists a sequence of
adjacent vertices (i.e. a
path) which starts with
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=4132)
and ends with
![](http://bis.babylon.com/?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=3854)
.