Essential independent set

Essential independent set --- существенное независимое множество.

An independent set [math]\displaystyle{ Y }[/math] in [math]\displaystyle{ G }[/math] is called an essential independent set (or essential set for simplicity), if there is [math]\displaystyle{ \{y_{1}, y_{2}\} \subseteq Y }[/math] such that [math]\displaystyle{ dist(y_{1}, y_{2}) = 2 }[/math].