Phylogeny graph

Phylogeny graph --- филогенный граф.

Given an acyclic digraph [math]\displaystyle{ D = (V,A) }[/math], its phylogeny graph [math]\displaystyle{ P(D) }[/math] is the undirected graph [math]\displaystyle{ (V,E) }[/math] with the same vertex set as [math]\displaystyle{ D }[/math] and with the following properties for [math]\displaystyle{ x \neq y }[/math]:

[math]\displaystyle{ xy \in E \Leftrightarrow (\exists a \in V)[(x,a) \in A \& (y,a) \in A] }[/math]

[math]\displaystyle{ or \; [(x,y) \in A] \; or [(y,x) \in A]. }[/math]