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In Computer Science and Mathematics , a directed acyclic graph, also called a '''dag''' or '''DAG''', is a with no ; that is, for any vertex ''v'', there is no nonempty Directed Path that starts and ends on ''v''. DAGs appear in models where it doesn't make sense for a vertex to have a path to itself; for example, if an edge ''u''→''v'' indicates that ''v'' is a part of ''u'', such a path would indicate that ''u'' is a part of itself, which is impossible. Every directed acyclic graph corresponds to a Partial Order on its vertices, in which ''u'' ≤ ''v'' is in the partial order exactly when there exists a directed path from ''u'' to ''v'' in the graph. However, many different directed acyclic graphs may represent the same partial order in this way. Among these graphs, the one with the fewest edges is the Transitive Reduction and the one with the most edges is the Transitive Closure . TERMINOLOGY A ''source'' is a vertex with no incoming edges, while a ''sink'' is a vertex with no outgoing edges. A finite DAG has at least one source and at least one sink. The ''depth'' of a vertex in a finite DAG is the length of the longest path from a source to that vertex, while its ''height'' is the length of the longest path from that vertex to a sink. The ''length'' of a finite DAG is the length (number of edges) of a longest directed path. It is equal to the maximum height of all sources and equal to the maximum depth of all sinks. PROPERTIES Every directed acyclic graph has a Topological Sort , an ordering of the vertices such that each vertex comes before all vertices it has edges to. In general, this ordering is not unique. Any two graphs representing the same partial order have the same set of topological sort orders. DAGs can be considered to be a generalization of Tree s in which certain subtrees can be shared by different parts of the tree. In a tree with many identical subtrees, this can lead to a drastic decrease in space requirements to store the structure. Conversely, a DAG can be expanded to a forest of rooted trees using this simple algorithm:
If we explore the graph without modifying it or comparing nodes for equality, this forest will appear identical to the original DAG. Some algorithms become simpler when used on DAGs instead of general graphs. For example, search algorithms like Depth-first Search without Iterative Deepening normally must mark vertices they have already visited and not visit them again. If they fail to do this, they may never terminate because they follow a cycle of edges forever. Such cycles do not exist in DAGs. A Polytree is a specifically efficient kind of DAG, with many tree-like properties. Its efficiency is exploited, for example, in the Belief Propagation algorithm for Bayesian Network s. The number of Non-Isomorphic DAGs is obtained by Weisstein's conjecture: the number of DAGs on ''n'' vertices is equal to the number of ''n''x''n'' matrices with entries from {0,1} and only positive real eigenvalues, proved by McKay et al. McKay, B. D.; Royle, G. F.; Wanless, I. M.; Oggier, F. E.; Sloane, N. J. A.; and Wilf, H. "Acyclic Digraphs and Eigenvalues of (0,1)-Matrices." J. Integer Sequences 7, Article 04.3.3, 1-5, 2004. http://www.cs.uwaterloo.ca/journals/JIS/VOL7/Sloane/sloane15.pdf or http://www.cs.uwaterloo.ca/journals/JIS/VOL7/Sloane/sloane15.html . APPLICATIONS Directed acyclic graphs have many important applications in Computer Science , including:
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