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In geometry, a digon, bigon, biangle or 2-gon is a polygon with two sides (edges) and two vertices. Its construction is degenerate in a Euclidean plane because the two sides would either coincide or one or both would have to be curved. A regular digon is represented by Schläfli symbol and may be constructed on a sphere as a pair of 180 degree arcs connecting antipodal points, when it forms a lune.〔Coxeter, ''Regular polytopes'', Chapter 1, ''Polygons and Polyhedra'', p.4 ''digon'', p.12 ''digon'' or ''lunes'', pp. 66-67 ''improper tessellations for p=2''.〕 The digon is the simplest abstract polytope of rank 2. == In Euclidean geometry == A straight-sided ''digon'' is regular even though it is degenerate, because its two edges are the same length and its two angles are equal (both being zero degrees). As such, the regular digon is a constructible polygon.〔http://www.math.iastate.edu/thesisarchive/MSM/EekhoffMSMSS07.pdf〕 Some definitions of a polygon do not consider the digon to be a proper polygon because of its degeneracy in the Euclidean case.〔Coxeter, ''Regular polytopes'', Chapter 1, ''Polygons and Polyhedra'', p.4 〕 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「Digon」の詳細全文を読む スポンサード リンク
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