Bidirectional arcs

Bidirectional arcs --- бинаправленные дуги.

That is a pair of arcs [math]\displaystyle{ (x,y), \; (y,x) }[/math]. If a directed graph [math]\displaystyle{ D }[/math] has no bidirectional arcs, then [math]\displaystyle{ D }[/math] is called an orientation of underlying graph [math]\displaystyle{ UG(D) }[/math].