A zero forcing set of a graph 
is a subset 
of the graph vertices of 
such that, if the vertices in 
are initially colored black and all remaining vertices are
colored white, repeated application of the following color-change rule eventually
colors every vertex black. The color-change rule states that a black vertex with
exactly one white neighboring vertex (i.e., one adjacent
vertex that is white) forces that white vertex to become black (AIM 2008).
The minimum number of vertices in a zero forcing set of 
is called the zero forcing
number 
.
