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In mathematics, the icosians are a specific set of Hamiltonian quaternions with the same symmetry as the 600-cell. The term can be used to refer to two related, but distinct, concepts: * The icosian group: a multiplicative group of 120 quaternions, positioned at the vertices of a 600-cell of unit radius. This group is isomorphic to the binary icosahedral group of order 120. * The icosian ring: all finite sums of the 120 unit icosians. ==Unit icosians== The 120 unit icosians, which form the icosian group, are all even permutations of: * 8 icosians of the form ½(±2, 0, 0, 0) * 16 icosians of the form ½(±1, ±1, ±1, ±1) * 96 icosians of the form ½(0, ±1, ±''ψ'', ±''φ'') In this case, the vector (''a'', ''b'', ''c'', ''d'') refers to the quaternion ''a'' + ''b''i + ''c''j + ''d''k, and ψ,φ represent the numbers (1 ± √5)/2. These 120 vectors form the H4 root system, with a Weyl group of order 14400. In addition to the 120 unit icosians forming the vertices of a 600-cell, the 600 icosians of norm 2 form the vertices of a 120-cell. Other subgroups of icosians correspond to the tesseract, 16-cell and 24-cell. 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「icosian」の詳細全文を読む スポンサード リンク
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