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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:

Unit icosians[edit]

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, ±1, ±φ)

In this case, the vector (abcd) refers to the quaternion a + bi + cj + dk, and φ represents the golden ratio (5 + 1)/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.

Icosian ring[edit]

The icosians lie in the golden field, (a + b5) + (c + d5)i + (e + f5)j + (g + h5)k, where the eight variables are rational numbers. This quaternion is only an icosian if the vector (abcdefgh) is a point on a lattice L, which is isomorphic to an E8 lattice.

More precisely, the quaternion norm of the above element is (a + b5)2 + (c + d5)2 + (e + f5)2 + (g + h5)2. Its Euclidean norm is defined as u + v if the quaternion norm is u + v5. This Euclidean norm defines a quadratic form on L, under which the lattice is isomorphic to the E8 lattice.

This construction shows that the Coxeter group embeds as a subgroup of . Indeed, a linear isomorphism that preserves the quaternion norm also preserves the Euclidean norm.