Exceptional isomorphism

The exceptional isomorphisms between the series of finite simple groups mostly involve projective special linear groups and alternating groups, and are: • PSL(4) ≅ PSL(5) ≅ A, the smallest non-abelian simple group (order 60); • PSL(7) ≅ PSL(2), the second-smallest non-abelian simple group (order 168) – PSL(2,7); • PSL(9) ≅ A; • PSL(2) ≅ A; • PSU(2) ≅ PSp(3), between a projective special unitary group and a projective symplectic group. Alternating groups and symmetric groups expresses the exceptional isomorphism between the chiral icosahedral group and the alternating group on five letters. There are coincidences between symmetric/alternating groups and small groups of Lie type/polyhedral groups: • S ≅ PSL(2) ≅ dihedral group of order 6, • A ≅ PSL(3), • S ≅ PGL(3) ≅ PSL(Z/4), • A ≅ PSL(4) ≅ PSL(5), • S ≅ PΓL(4) ≅ PGL(5), • A ≅ PSL(9) ≅ Sp(2)′, • S ≅ Sp(2), • A ≅ PSL(2) ≅ O(2)′, • S ≅ O(2). These can all be explained in a systematic way by using linear algebra (and the action of S on affine nspace) to define the isomorphism going from the right side to the left side. (The above isomorphisms for A and S are linked via the exceptional isomorphism .) 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: • C, the cyclic group of order 1; • A ≅ A ≅ A, the alternating group on 0, 1, or 2 letters; • S ≅ S, the symmetric group on 0 or 1 letters; • GL(0, K) ≅ SL(0, K) ≅ PGL(0, K) ≅ PSL(0, K), linear groups of a 0-dimensional vector space; • SL(1, K) ≅ PGL(1, K) ≅ PSL(1, K), linear groups of a 1-dimensional vector space • and many others. == Spheres ==

The spheres S0, S1, and S3 admit group structures, which can be described in many ways: • S ≅ Spin(1) ≅ O(1) ≅ (Z/2Z) ≅ Z, the last being the group of units of the integers; • S ≅ Spin(2) ≅ SO(2) ≅ U(1) ≅ R/Z ≅ circle group; • S ≅ Spin(3) ≅ SU(2) ≅ Sp(1) ≅ unit quaternions. ==Classical groups==

In addition to Spin(1), Spin(2) and Spin(3) above, there are isomorphisms for higher dimensional spin groups: • Spin(4) ≅ Sp(1) × Sp(1) ≅ SU(2) × SU(2) • Spin(5) ≅ Sp(2) • Spin(6) ≅ SU(4) Also, Spin(8) has an exceptional order 3 triality automorphism. In non-Euclidean signature, there are also the isomorphisms • \mathrm{Spin}^+(1,2) \cong \mathrm{SL}(2,\mathbb R) • \mathrm{Spin}^+(1,3) \cong \mathrm{SL}(2,\mathbb C) • \mathrm{Spin}^+(2,2) \cong \mathrm{SL}(2,\mathbb R)\times\mathrm{SL}(2,\mathbb R) • \mathrm{Spin}^+(1,4) \cong \mathrm{Sp}(1,1) • \mathrm{Spin}^+(2,3) \cong \mathrm{Sp}(4,\mathbb R) • \mathrm{Spin}^+(2,4) \cong \mathrm{SU}(2,2) • \mathrm{Spin}^+(3,3) \cong \mathrm{SL}(4,\mathbb R) • \mathrm{Spin}^+(1,5) \cong \mathrm{SL}(2,\mathbb H) (i.e., \mathrm{SU}^*(4)) where \mathrm{Spin}^+ refers to the connected component of the identity. (These group isomorphisms are sometimes presented as central isogenies or isomorphisms of the associated Lie algebras instead.) == 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: == See also ==