Forcing number

Forcing number --- форсированное число паросочетаний. Let [math]\displaystyle{ G }[/math] be a graph that admits a perfect matching. A forcing set for a perfect matching [math]\displaystyle{ M }[/math] of [math]\displaystyle{ G }[/math] is a subset [math]\displaystyle{ S }[/math] of [math]\displaystyle{ M }[/math], such that [math]\displaystyle{ S }[/math] is contained in no other perfect matching of [math]\displaystyle{ G }[/math]. The cardinality of a forcing set of [math]\displaystyle{ M }[/math] with the smallest size is called the forcing number of [math]\displaystyle{ M }[/math] and is denoted by [math]\displaystyle{ f(G,M) }[/math].