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

The icosian group, consisting of the 120 unit icosians, comprises the distinct even permutations of In this case, the vector (a, b, c, d) refers to the quaternion a + bi + cj + dk, and φ represents the golden ratio (√5 + 1)/2. These 120 vectors form the vertices of a 600-cell, whose symmetry group is the Coxeter group H4 of order 14400. In addition, 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

The icosians lie in the golden field, (a + b√5) + (c + d√5)i + (e + f√5)j + (g + h√5)k, where the eight variables are rational numbers. This quaternion is only an icosian if the vector (a, b, c, d, e, f, g, h) 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 + b√5)2 + (c + d√5)2 + (e + f√5)2 + (g + h√5)2. Its Euclidean norm is defined as u + v if the quaternion norm is u + v√5. 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 H_4 embeds as a subgroup of E_8. Indeed, a linear isomorphism that preserves the quaternion norm also preserves the Euclidean norm.