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.

Groups

Finite simple groups

The exceptional isomorphisms between the series of finite simple groups mostly involve projective special linear groups and alternating groups, and are:

Alternating groups and symmetric groups

There are coincidences between symmetric/alternating groups and small groups of Lie type/polyhedral groups: These can all be explained in a systematic way by using linear algebra (and the action of Sn on affine nspace) to define the isomorphism going from the right side to the left side. (The above isomorphisms for A8 and S8 are linked via the exceptional isomorphism SL4/μ2 ≅ SO6.) There are also some coincidences with symmetries of regular polyhedra: the alternating group A5 agrees with the chiral icosahedral group (itself an exceptional object), and the double cover of the alternating group A5 is the binary icosahedral group.

Trivial group

The trivial group arises in numerous ways. The trivial group is often omitted from the beginning of a classical family. For instance:

Spheres

The spheres S0, S1, and S3 admit group structures, which can be described in many ways:

Spin groups

In addition to Spin(1), Spin(2) and Spin(3) above, there are isomorphisms for higher dimensional spin groups: Also, Spin(8) has an exceptional order 3 triality automorphism.

Coxeter–Dynkin diagrams

There are some exceptional isomorphisms of Dynkin diagrams, yielding isomorphisms of the corresponding Coxeter groups and of polytopes realizing the symmetries, as well as isomorphisms of Lie algebras whose root systems are described by the same diagrams. These are: