Directed acyclic graph - Wikipedia
https://en.wikipedia.org/wiki/Directed_acyclic_graph
In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG / ˈ d æ ɡ / ()), is a finite directed graph with no directed cycles.That is, it consists of finitely many vertices and edges (also called arcs), with each edge directed from one vertex to another, such that there is no way to start at any vertex v and follow a consistently-directed sequence of ...
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