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In graph theory, a peripheral cycle (or peripheral circuit) in an undirected graph is, intuitively, a cycle that does not separate any part of the graph from any other part. Peripheral cycles (or, as they were initially called, peripheral polygons) were first studied by , and play important roles in the characterization of planar graphs and in generating the cycle spaces of nonplanar graphs.〔.〕 ==Definitions== A peripheral cycle in a graph can be defined formally in one of several equivalent ways: * is peripheral if it is a simple cycle in a connected graph with the property that, for every two edges and in , there exists a path in that starts with , ends with , and has no interior vertices belonging to .〔.〕 * is peripheral if it is an induced cycle with the property that the subgraph formed by deleting the edges and vertices of is connected.〔This is, essentially, the definition used by . However, Bruhn distinguishes the case that has isolated vertices from disconnections caused by the removal of .〕 *If is any subgraph of , a ''bridge''〔Not to be confused with bridge (graph theory), a different concept.〕 of is a minimal subgraph of that is edge-disjoint from and that has the property that all of its points of attachments (vertices adjacent to edges in both and ) belong to .〔.〕 A simple cycle is peripheral if it has exactly one bridge.〔This is the definition of peripheral cycles originally used by . use the same definition of a peripheral cycle, but with a different definition of a bridge that more closely resembles the induced-cycle definition for peripheral cycles.〕 The equivalence of these definitions is not hard to see: a connected subgraph of (together with the edges linking it to ), or a chord of a cycle that causes it to fail to be induced, must in either case be a bridge, and must also be an equivalence class of the binary relation on edges in which two edges are related if they are the ends of a path with no interior vertices in .〔See e.g. Theorem 2.4 of , showing that the vertex sets of bridges are path-connected, see for a definition of bridges using chords and connected components, and also see for a definition of bridges using equivalence classes of the binary relation on edges.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「peripheral cycle」の詳細全文を読む スポンサード リンク
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