Mathieu group M23

In the area of modern algebra known as group theory, the Mathieu group M₂₃ is a sporadic simple group of order

   10,200,960 = 2⁷ · 3² · 5 · 7 · 11 · 23

≈ 1 × 10⁷.

History and properties

M₂₃ is one of the 26 sporadic groups and was introduced by Mathieu (1861, 1873). It is a 4-fold transitive permutation group on 23 objects. The Schur multiplier and the outer automorphism group are both trivial.

calculated the integral cohomology, and showed in particular that M₂₃ has the unusual property that the first 4 integral homology groups all vanish.

The inverse Galois problem seems to be unsolved for M₂₃. In other words, no polynomial in Z[x] seems to be known to have M₂₃ as its Galois group. The inverse Galois problem is solved for all other sporadic simple groups.

Construction using finite fields

Let F_2¹¹ be the finite field with 2¹¹ elements. Its group of units has order 2¹¹ − 1 = 2047 = 23 · 89, so it has a cyclic subgroup C of order 23.

The Mathieu group M₂₃ can be identified with the group of F₂-linear automorphisms of F_2¹¹ that stabilize C. More precisely, the action of this automorphism group on C can be identified with the 4-fold transitive action of M₂₃ on 23 objects.

Representations

M₂₃ is the point stabilizer of the action of the Mathieu group M24 on 24 points, giving it a 4-transitive permutation representation on 23 points with point stabilizer the Mathieu group M22.

M₂₃ has 2 different rank 3 actions on 253 points. One is the action on unordered pairs with orbit sizes 1+42+210 and point stabilizer M₂₁.2, and the other is the action on heptads with orbit sizes 1+112+140 and point stabilizer 2⁴.A₇.

The integral representation corresponding to the permutation action on 23 points decomposes into the trivial representation and a 22-dimensional representation. The 22-dimensional representation is irreducible over any field of characteristic not 2 or 23.

Over the field of order 2, it has two 11-dimensional representations, the restrictions of the corresponding representations of the Mathieu group M24.

Maximal subgroups

There are 7 conjugacy classes of maximal subgroups of M₂₃ as follows:

Maximal subgroups of M₂₃

No.	Structure	Order	Index	Comments
1	M22	443,520 = 27·32·5·7·11	23	point stabilizer
2	L3(4):2	40,320 = 27·32·5·7	253 = 11·23	has orbits of sizes 21 and 2
3	24:A7	40,320 = 27·32·5·7	253 = 11·23	has orbits of sizes 7 and 16; stabilizer of W23 block
4	A8	20,160 = 26·32·5·7	506 = 2·11·23	has orbits of sizes 8 and 15
5	M11	7,920 = 24·32·5·11	1,288 = 23·7·23	has orbits of sizes 11 and 12
6	(24:A5):S3 ≅ M20:S3	5,760 = 27·32·5	1,771 = 7·11·23	has orbits of sizes 3 and 20 (5 blocks of 4); one-point stabilizer of the sextet group
7	23:11	253 = 11·23	40,320 = 27·32·5·7	simply transitive

Conjugacy classes

Order	No. elements	Cycle structure
1 = 1	1	123
2 = 2	3795 = 3 · 5 · 11 · 23	1728
3 = 3	56672 = 25 · 7 · 11 · 23	1536
4 = 22	318780 = 22 · 32 · 5 · 7 · 11 · 23	132244
5 = 5	680064 = 27 · 3 · 7 · 11 · 23	1354
6 = 2 · 3	850080 = 25 · 3 · 5 · 7 · 11 · 23	1·223262
7 = 7	728640 = 26 · 32 · 5 · 11 · 23	1273	power equivalent
	728640 = 26 · 32 · 5 · 11 · 23	1273
8 = 23	1275120 = 24 · 32 · 5 · 7 · 11 · 23	1·2·4·82
11 = 11	927360= 27 · 32 · 5 · 7 · 23	1·112	power equivalent
	927360= 27 · 32 · 5 · 7 · 23	1·112
14 = 2 · 7	728640= 26 · 32 · 5 · 11 · 23	2·7·14	power equivalent
	728640= 26 · 32 · 5 · 11 · 23	2·7·14
15 = 3 · 5	680064= 27 · 3 · 7 · 11 · 23	3·5·15	power equivalent
	680064= 27 · 3 · 7 · 11 · 23	3·5·15
23 = 23	443520= 27 · 32 · 5 · 7 · 11	23	power equivalent
	443520= 27 · 32 · 5 · 7 · 11	23

References

• Reprinted in

External links

• MathWorld: Mathieu Groups
• Atlas of Finite Group Representations: M₂₃