Double edge dominating set

Double edge dominating set --- двойное рёберное доминирующее множество.

A set [math]\displaystyle{ F \subseteq E }[/math] is a double edge dominating set of [math]\displaystyle{ G }[/math] if each edge in [math]\displaystyle{ E }[/math] is adjacent to at least two edges in [math]\displaystyle{ F }[/math]. The double edge domination number [math]\displaystyle{ dd_{e}(G) }[/math] of [math]\displaystyle{ G }[/math] is the minimum cardinality of a double edge dominating set of [math]\displaystyle{ G }[/math].