Exceptional isomorphism

In mathematics, an exceptional isomorphism, also called an accidental isomorphism, is an isomorphism between members ai and bj of two families, usually infinite, of mathematical objects, which is incidental, in that it is not an instance of a general pattern of such isomorphisms. These coincidences are at times considered a matter of trivia, but in other respects they can give rise to consequential phenomena, such as exceptional objects. In the following, coincidences are organized according to the structures where they occur. The exceptional isomorphisms between the series of finite simple groups mostly involve projective special linear groups and alternating groups, and are: the smallest non-abelian simple group (order 60) – icosahedral symmetry; the second-smallest non-abelian simple group (order 168) – PSL(2,7); between a projective special orthogonal group and a projective symplectic group. The exceptional outer automorphism of S6 There are coincidences between symmetric/alternating groups and small groups of Lie type/polyhedral groups: Dihedral group of order 6, tetrahedral group, full tetrahedral group octahedral group, icosahedral group, These can all be explained in a systematic way by using linear algebra (and the action of on affine -space) to define the isomorphism going from the right side to the left side. (The above isomorphisms for and are linked via the exceptional isomorphism .) There are also some coincidences with symmetries of regular polyhedra: the alternating group A5 agrees with the icosahedral group (itself an exceptional object), and the double cover of the alternating group A5 is the binary icosahedral group. The trivial group arises in numerous ways. The trivial group is often omitted from the beginning of a classical family. For instance: the cyclic group of order 1; the alternating group on 0, 1, or 2 letters; the symmetric group on 0 or 1 letters; linear groups of a 0-dimensional vector space; linear groups of a 1-dimensional vector space and many others.