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A Euclidean graph (a graph embedded in some Euclidean space) is periodic if there exists a basis of that Euclidean space whose corresponding translations induce symmetries of that graph (i.e., application of any such translation to the graph embedded in the Euclidean space leaves the graph unchanged). Equivalently, a periodic Euclidean graph is a periodic realization of an abelian covering graph over a finite graph . A Euclidean graph is uniformly discrete if there is a minimal distance between any two vertices. Periodic graphs are closely related to tessellations of space (or honeycombs) and the geometry of their symmetry groups, hence to geometric group theory, as well as to discrete geometry and the theory of polytopes, and similar areas. Much of the effort in periodic graphs is motivated by applications to natural science and engineering, particularly of three-dimensional crystal nets to crystal engineering, crystal prediction (design), and modeling crystal behavior. Periodic graphs have also been studied in modeling very-large-scale integration (VLSI) circuits. ==Basic formulation== A Euclidean graph is a pair (''V'', ''E''), where ''V'' is a set of points (sometimes called vertices or nodes) and ''E'' is a set of edges (sometimes called bonds), where each edge joins two vertices. While an edge connecting two vertices ''u'' and ''v'' is usually interpreted as the set , an edge is sometimes interpreted as the line segment connecting u and v so that the resulting structure is a CW complex. There is a tendency in the polyhedral and chemical literature to refer to geometric graphs as nets (contrast with polyhedral nets), and the nomenclature in the chemical literature differs from that of graph theory. Most of the literature focuses on periodic graphs that are uniformly discrete in that there exists ''e'' > 0 such that for any two distinct vertices, their distance apart is |''u'' – ''v''| > ''e''. From the mathematical view, a Euclidean periodic graph is a realization of an infinite-fold abelian covering graph over a finite graph. 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「Periodic graph (geometry)」の詳細全文を読む スポンサード リンク
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