# Mathieu group

In group theory, a topic in abstract algebra, the **Mathieu groups** are the five sporadic simple groups *M*_{11}, *M*_{12}, *M*_{22}, *M*_{23} and *M*_{24} introduced by Mathieu (1861, 1873). They are multiply transitive permutation groups on 11, 12, 22, 23 or 24 objects. They were the first sporadic groups to be discovered.

Sometimes the notation *M*_{9}, *M*_{10}, *M*_{20} and *M*_{21} is used for related groups (which act on sets of 9, 10, 20, and 21 points, respectively), namely the stabilizers of points in the larger groups. While these are not sporadic simple groups, they are subgroups of the larger groups and can be used to construct the larger ones. John Conway has shown that one can also extend this sequence up, obtaining the Mathieu groupoid *M*_{13} acting on 13 points. *M*_{21} is simple, but is not a sporadic group, being isomorphic to PSL(3,4).

Mathieu (1861, p.271) introduced the group *M*_{12} as part of an investigation of multiply transitive permutation groups, and briefly mentioned (on page 274) the group *M*_{24}, giving its order. In Mathieu (1873) he gave further details, including explicit generating sets for his groups, but it was not easy to see from his arguments that the groups generated are not just alternating groups, and for several years the existence of his groups was controversial. Miller (1898) even published a paper mistakenly claiming to prove that *M*_{24} does not exist, though shortly afterwards in (Miller 1900) he pointed out that his proof was wrong, and gave a proof that the Mathieu groups are simple. Witt (1938a, 1938b) finally removed the doubts about the existence of these groups, by constructing them as successive transitive extensions of permutation groups, as well as automorphism groups of Steiner systems.

After the Mathieu groups no new sporadic groups were found until 1965, when the group J_{1} was discovered.

Mathieu was interested in finding **multiply transitive** permutation groups, which will now be defined. For a natural number *k*, a permutation group *G* acting on *n* points is ** k-transitive** if, given two sets of points

*a*

_{1}, ...

*a*

_{k}and

*b*

_{1}, ...

*b*

_{k}with the property that all the

*a*

_{i}are distinct and all the

*b*

_{i}are distinct, there is a group element

*g*in

*G*which maps

*a*

_{i}to

*b*

_{i}for each

*i*between 1 and

*k*. Such a group is called

**sharply**if the element

*k*-transitive*g*is unique (i.e. the action on

*k*-tuples is regular, rather than just transitive).

*M*_{24} is 5-transitive, and *M*_{12} is sharply 5-transitive, with the other Mathieu groups (simple or not) being the subgroups corresponding to stabilizers of *m* points, and accordingly of lower transitivity (*M*_{23} is 4-transitive, etc.).

The only 4-transitive groups are the symmetric groups *S*_{k} for *k* at least 4, the alternating groups *A*_{k} for *k* at least 6, and the Mathieu groups *M*_{24}, *M*_{23}, *M*_{12} and *M*_{11}. (Cameron 1999, p. 110) The full proof requires the classification of finite simple groups, but some special cases have been known for much longer.

It is a classical result of Jordan that the symmetric and alternating groups (of degree *k* and *k* + 2 respectively), and *M*_{12} and *M*_{11} are the only *sharply* *k*-transitive permutation groups for *k* at least 4.

*M*_{12} has a simple subgroup of order 660, a maximal subgroup. That subgroup is isomorphic to the projective special linear group PSL_{2}(**F**_{11}) over the field of 11 elements. With −1 written as **a** and infinity as **b**, two standard generators are (0123456789a) and (0b)(1a)(25)(37)(48)(69). A third generator giving *M*_{12} sends an element *x* of **F**_{11} to 4*x*^{2} − 3*x*^{7}; as a permutation that is (26a7)(3945).

This group turns out not to be isomorphic to any member of the infinite families of finite simple groups and is called sporadic. *M*_{11} is the stabilizer of a point in *M*_{12}, and turns out also to be a sporadic simple group. *M*_{10}, the stabilizer of two points, is not sporadic, but is an almost simple group whose commutator subgroup is the alternating group A_{6}. It is thus related to the exceptional outer automorphism of A_{6}. The stabilizer of 3 points is the projective special unitary group PSU(3,2^{2}), which is solvable. The stabilizer of 4 points is the quaternion group.

The stabilizers of 1 and 2 points, *M*_{23} and *M*_{22} also turn out to be sporadic simple groups. The stabilizer of 3 points is simple and isomorphic to the projective special linear group PSL_{3}(4).

These constructions were cited by Carmichael (1956, pp. 151, 164, 263). Dixon & Mortimer (1996, p.209) ascribe the permutations to Mathieu.

There exists up to equivalence a unique *S*(5,8,24) Steiner system **W _{24}** (the Witt design). The group

*M*

_{24}is the automorphism group of this Steiner system; that is, the set of permutations which map every block to some other block. The subgroups

*M*

_{23}and

*M*

_{22}are defined to be the stabilizers of a single point and two points respectively.

Similarly, there exists up to equivalence a unique S(5,6,12) Steiner system **W _{12}**, and the group

*M*

_{12}is its automorphism group. The subgroup

*M*

_{11}is the stabilizer of a point.

*W*_{12} can be constructed from the affine geometry on the vector space *F*_{3}×*F*_{3}, an *S*(2,3,9) system.

An introduction to a construction of *W*_{24} via the Miracle Octad Generator of R. T. Curtis and Conway's analog for *W*_{12}, the miniMOG, can be found in the book by Conway and Sloane.

The group *M*_{24} is the permutation automorphism group of the extended binary Golay code *W*, i.e., the group of permutations on the 24 coordinates that map *W* to itself. All the Mathieu groups can be constructed as groups of permutations on the binary Golay code.

*M*_{12} has index 2 in its automorphism group, and *M*_{12}:2 happens to be isomorphic to a subgroup of *M*_{24}. *M*_{12} is the stabilizer of a **dodecad**, a codeword of 12 1's; *M*_{12}:2 stabilizes a partition into 2 complementary dodecads.

There is a natural connection between the Mathieu groups and the larger Conway groups, because the Leech lattice was constructed on the binary Golay code and in fact both lie in spaces of dimension 24. The Conway groups in turn are found in the Monster group. Robert Griess refers to the 20 sporadic groups found in the Monster as the **Happy Family**, and to the Mathieu groups as the **first generation**.

The Mathieu groups can be constructed via dessins d'enfants, with the dessin associated to *M*_{12} suggestively called "Monsieur Mathieu" by le Bruyn (2007).