• Source: 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.




Groups






= Finite simple groups

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

PSL2(4) ≅ PSL2(5) ≅ A5, the smallest non-abelian simple group (order 60);
PSL2(7) ≅ PSL3(2), the second-smallest non-abelian simple group (order 168) – PSL(2,7);
PSL2(9) ≅ A6;
PSL4(2) ≅ A8;
PSU4(2) ≅ PSp4(3), between a projective special unitary group and a projective symplectic group.




= Alternating groups and symmetric groups

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There are coincidences between symmetric/alternating groups and small groups of Lie type/polyhedral groups:

S3 ≅ PSL2(2) ≅ dihedral group of order 6,
A4 ≅ PSL2(3),
S4 ≅ PGL2(3) ≅ PSL2(Z / 4),
A5 ≅ PSL2(4) ≅ PSL2(5),
S5 ≅ PΓL2(4) ≅ PGL2(5),
A6 ≅ PSL2(9) ≅ Sp4(2)′,
S6 ≅ Sp4(2),
A8 ≅ PSL4(2) ≅ O+6(2)′,
S8 ≅ O+6(2).
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

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

C1, the cyclic group of order 1;
A0 ≅ A1 ≅ A2, the alternating group on 0, 1, or 2 letters;
S0 ≅ S1, 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

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

S0 ≅ Spin(1) ≅ O(1) ≅ (Z / 2Z)+ ≅ Z×, the last being the group of units of the integers;
S1 ≅ Spin(2) ≅ SO(2) ≅ U(1) ≅ R / Z ≅ circle group;
S3 ≅ Spin(3) ≅ SU(2) ≅ Sp(1) ≅ unit quaternions.




= Spin groups

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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.




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


Exceptional object
Mathematical coincidence, for numerical coincidences




Notes






References

Kata Kunci Pencarian:

• Daftar masalah matematika yang belum terpecahkan
• Exceptional isomorphism
• Projective linear group
• Alternating group
• Exception
• Exceptional object
• Spin group
• Tits group
• Covering groups of the alternating and symmetric groups
• Tutte–Coxeter graph
• Cross-ratio