Sporadic group

In the mathematical classification of finite simple groups, there are 26 or 27 groups which do not fit into any infinite family. These are called the sporadic simple groups, or the sporadic finite groups, or just the sporadic groups.

A simple group is a group G that does not have any normal subgroups except for the trivial group and G itself. The mentioned classification theorem states that the list of finite simple groups consists of 18 countably infinite families ^{[lower-alpha 1]} plus 26 exceptions that do not follow such a systematic pattern. These 26 exceptions are the sporadic groups. The Tits group is sometimes regarded as a sporadic group because it is not strictly a group of Lie type, ^[1] in which case there would be 27 sporadic groups.

The monster group, or friendly giant, is the largest of the sporadic groups, and all but six of the other sporadic groups are subquotients of it. ^[2]

Names

Five of the sporadic groups were discovered by Émile Mathieu in the 1860s and the other twenty-one were found between 1965 and 1975. Several of these groups were predicted to exist before they were constructed. Most of the groups are named after the mathematician(s) who first predicted their existence. The full list is: ^{[1] [3] [4]}

The diagram shows the subquotient relations between the sporadic groups. A connecting line means the lower group is a subquotient of the upper, with no sporadic subquotient in between.
1st generation, 2nd generation, 3rd generation, Pariah

• Mathieu groups M₁₁, M₁₂, M₂₂, M₂₃, M₂₄
• Janko groups J₁, J₂ or HJ, J₃ or HJM, J₄
• Conway groups Co₁ , Co₂ , Co₃
• Fischer groups Fi₂₂, Fi₂₃, Fi₂₄′ or F₃₊
• Higman-Sims group HS
• McLaughlin group McL
• Held group He or F₇₊ or F₇
• Rudvalis group Ru
• Suzuki group Suz or F₃₋
• O'Nan group O'N (ON)
• Harada-Norton group HN or F₅₊ or F₅
• Lyons group Ly
• Thompson group Th or F_3|3 or F₃
• Baby Monster group B or F₂₊ or F₂
• Fischer-Griess Monster group M or F₁

Various constructions for these groups were first compiled in Conway et al. (1985), including character tables, individual conjugacy classes and lists of maximal subgroup, as well as Schur multipliers and orders of their outer automorphisms. These are also listed online at Wilson et al. (1999), updated with their group presentations and semi-presentations. The degrees of minimal faithful representation or Brauer characters over fields of characteristic p ≥ 0 for all sporadic groups have also been calculated, and for some of their covering groups. These are detailed in Jansen (2005).

A further exception in the classification of finite simple groups is the Tits group T, which is sometimes considered of Lie type ^[5] or sporadic — it is almost but not strictly a group of Lie type ^[6] — which is why in some sources the number of sporadic groups is given as 27, instead of 26. ^{[7] [8]} In some other sources, the Tits group is regarded as neither sporadic nor of Lie type, or both. ^{[9] [ citation needed ]} The Tits group is the (n = 0)-member²F₄(2)′ of the infinite family of commutator groups ²F₄(2²ⁿ⁺¹)′; thus in a strict sense not sporadic, nor of Lie type. For n > 0 these finite simple groups coincide with the groups of Lie type ²F₄(2²ⁿ⁺¹), also known as Ree groups of type ²F₄.

The earliest use of the term sporadic group may be Burnside (1911 , p. 504) where he comments about the Mathieu groups: "These apparently sporadic simple groups would probably repay a closer examination than they have yet received." (At the time, the other sporadic groups had not been discovered.)

The diagram at right is based on Ronan (2006 , p. 247). It does not show the numerous non-sporadic simple subquotients of the sporadic groups.

Organization

Happy Family

Of the 26 sporadic groups, 20 can be seen inside the monster group as subgroups or quotients of subgroups (sections). These twenty have been called the happy family by Robert Griess, and can be organized into three generations. ^{[10] [lower-alpha 2]}

First generation (5 groups): the Mathieu groups

Main article: Mathieu groups

M_n for n = 11, 12, 22, 23 and 24 are multiply transitive permutation groups on n points. They are all subgroups of M₂₄, which is a permutation group on 24 points. ^[11]

Second generation (7 groups): the Leech lattice

See also: Leech lattice and Conway groups

All the subquotients of the automorphism group of a lattice in 24 dimensions called the Leech lattice: ^[12]

• Co₁ is the quotient of the automorphism group by its center {±1}
• Co₂ is the stabilizer of a type 2 (i.e., length 2) vector
• Co₃ is the stabilizer of a type 3 (i.e., length √6) vector
• Suz is the group of automorphisms preserving a complex structure (modulo its center)
• McL is the stabilizer of a type 2-2-3 triangle
• HS is the stabilizer of a type 2-3-3 triangle
• J₂ is the group of automorphisms preserving a quaternionic structure (modulo its center).

Third generation (8 groups): other subgroups of the Monster

Consists of subgroups which are closely related to the Monster group M: ^[13]

• B or F₂ has a double cover which is the centralizer of an element of order 2 in M
• Fi₂₄′ has a triple cover which is the centralizer of an element of order 3 in M (in conjugacy class "3A")
• Fi₂₃ is a subgroup of Fi₂₄′
• Fi₂₂ has a double cover which is a subgroup of Fi₂₃
• The product of Th = F₃ and a group of order 3 is the centralizer of an element of order 3 in M (in conjugacy class "3C")
• The product of HN = F₅ and a group of order 5 is the centralizer of an element of order 5 in M
• The product of He = F₇ and a group of order 7 is the centralizer of an element of order 7 in M.
• Finally, the Monster group itself is considered to be in this generation.

(This series continues further: the product of M₁₂ and a group of order 11 is the centralizer of an element of order 11 in M.)

The Tits group, if regarded as a sporadic group, would belong in this generation: there is a subgroup S₄ײF₄(2)′ normalising a 2C² subgroup of B, giving rise to a subgroup 2·S₄ײF₄(2)′ normalising a certain Q₈ subgroup of the Monster. ²F₄(2)′ is also a subquotient of the Fischer group Fi₂₂, and thus also of Fi₂₃ and Fi₂₄′, and of the Baby Monster B. ²F₄(2)′ is also a subquotient of the (pariah) Rudvalis group Ru, and has no involvements in sporadic simple groups except the ones already mentioned.

Pariahs

Main article: Pariah group

The six exceptions are J₁, J₃, J₄, O'N, Ru and Ly, sometimes known as the pariahs. ^{[14] [15]}

Table of the sporadic group orders (with Tits group)

Group	Discoverer	[16] Year	Generation	[4] [17] Order		[1] [4] Factorized order	[18] Minimal faithful Brauer character	[19] [20] ${\displaystyle (a,b,ab)}$ Generators	[20] [lower-alpha 3] ${\displaystyle \langle \langle a,b\mid o(z)\rangle \rangle }$ Semi-presentation
M or F1	Fischer, Griess	1973	3rd	808017424794512875886459904961710757005754368000000000	≈ 8×1053	246 × 320 × 59 × 76 × 112 ×  133 × 17 × 19 × 23 × 29 × 31 × 41 × 47 × 59 × 71	196883	2A, 3B, 29	${\displaystyle o{\bigl (}(ab)^{4}(ab^{2})^{2}{\bigr )}=50}$
B or F2	Fischer	1973	3rd	4154781481226426191177580544000000	≈ 4×1033	241 × 313 × 56 × 72 × 11 × 13 × 17 × 19 × 23 × 31 × 47	4371	2C, 3A, 55	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=23}$
Fi24 or F3+	Fischer	1971	3rd	1255205709190661721292800	≈ 1×1024	221 × 316 × 52 × 73 × 11 × 13 × 17 × 23 × 29	8671	2A, 3E, 29	${\displaystyle o{\bigl (}(ab)^{3}b{\bigr )}=33}$
Fi23	Fischer	1971	3rd	4089470473293004800	≈ 4×1018	218 × 313 × 52 × 7 × 11 × 13 × 17 × 23	782	2B, 3D, 28	${\displaystyle o{\bigl (}a^{bb}(ab)^{14}{\bigr )}=5}$
Fi22	Fischer	1971	3rd	64561751654400	≈ 6×1013	217 × 39 × 52 × 7 × 11 × 13	78	2A, 13, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=12}$
Th or F3	Thompson	1976	3rd	90745943887872000	≈ 9×1016	215 × 310 × 53 × 72 × 13 × 19 × 31	248	2, 3A, 19	${\displaystyle o{\bigl (}(ab)^{3}b{\bigr )}=21}$
Ly	Lyons	1972	Pariah	51765179004000000	≈ 5×1016	28 × 37 × 56 × 7 × 11 × 31 × 37 × 67	2480	2, 5A, 14	${\displaystyle o{\bigl (}ababab^{2}{\bigr )}=67}$
HN or F5	Harada, Norton	1976	3rd	273030912000000	≈ 3×1014	214 × 36 × 56 × 7 × 11 × 19	133	2A, 3B, 22	${\displaystyle o{\bigl (}[a,b]{\bigr )}=5}$
Co1	Conway	1969	2nd	4157776806543360000	≈ 4×1018	221 × 39 × 54 × 72 × 11 × 13 × 23	276	2B, 3C, 40	${\displaystyle o{\bigl (}ab(abab^{2})^{2}{\bigr )}=42}$
Co2	Conway	1969	2nd	42305421312000	≈ 4×1013	218 × 36 × 53 × 7 × 11 × 23	23	2A, 5A, 28	${\displaystyle o{\bigl (}[a,b]{\bigr )}=4}$
Co3	Conway	1969	2nd	495766656000	≈ 5×1011	210 × 37 × 53 × 7 × 11 × 23	23	2A, 7C, 17	${\displaystyle o{\bigl (}(uvv)^{3}(uv)^{6}{\bigr )}=5}$ [lower-alpha 4]
ON or O'N	O'Nan	1976	Pariah	460815505920	≈ 5×1011	29 × 34 × 5 × 73 × 11 × 19 × 31	10944	2A, 4A, 11	${\displaystyle o{\bigl (}abab(b^{2}(b^{2})^{abab})^{5}{\bigr )}=5}$
Suz	Suzuki	1969	2nd	448345497600	≈ 4×1011	213 × 37 × 52 × 7 × 11 × 13	143	2B, 3B, 13	${\displaystyle o{\bigl (}[a,b]{\bigr )}=15}$
Ru	Rudvalis	1972	Pariah	145926144000	≈ 1×1011	214 × 33 × 53 × 7 × 13 × 29	378	2B, 4A, 13	${\displaystyle o(abab^{2})=29}$
He or F7	Held	1969	3rd	4030387200	≈ 4×109	210 × 33 × 52 × 73 × 17	51	2A, 7C, 17	${\displaystyle o{\bigl (}ab^{2}abab^{2}ab^{2}{\bigr )}=10}$
McL	McLaughlin	1969	2nd	898128000	≈ 9×108	27 × 36 × 53 × 7 × 11	22	2A, 5A, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=7}$
HS	Higman, Sims	1967	2nd	44352000	≈ 4×107	29 × 32 × 53 × 7 × 11	22	2A, 5A, 11	${\displaystyle o(abab^{2})=15}$
J4	Janko	1976	Pariah	86775571046077562880	≈ 9×1019	221 × 33 × 5 × 7 × 113 × 23 × 29 × 31 × 37 × 43	1333	2A, 4A, 37	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=10}$
J3 or HJM	Janko	1968	Pariah	50232960	≈ 5×107	27 × 35 × 5 × 17 × 19	85	2A, 3A, 19	${\displaystyle o{\bigl (}[a,b]{\bigr )}=9}$
J2 or HJ	Janko	1968	2nd	604800	≈ 6×105	27 × 33 × 52 × 7	14	2B, 3B, 7	${\displaystyle o{\bigl (}[a,b]{\bigr )}=12}$
J1	Janko	1965	Pariah	175560	≈ 2×105	23 × 3 × 5 × 7 × 11 × 19	56	2, 3, 7	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=19}$
T or 2F4(2)′	Tits	1964	3rd	17971200	≈ 2×107	211 × 33 × 52 × 13	104 [21]	2A, 3, 13	${\displaystyle o{\bigl (}[a,b]{\bigr )}=5}$
M24	Mathieu	1861	1st	244823040	≈ 2×108	210 × 33 × 5 × 7 × 11 × 23	23	2B, 3A, 23	${\displaystyle o{\bigl (}ab(abab^{2})^{2}ab^{2}{\bigr )}=4}$
M23	Mathieu	1861	1st	10200960	≈ 1×107	27 × 32 × 5 × 7 × 11 × 23	22	2, 4, 23	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=8}$
M22	Mathieu	1861	1st	443520	≈ 4×105	27 × 32 × 5 × 7 × 11	21	2A, 4A, 11	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=11}$
M12	Mathieu	1861	1st	95040	≈ 1×105	26 × 33 × 5 × 11	11	2B, 3B, 11	${\displaystyle o{\bigl (}[a,b]{\bigr )}=o{\bigl (}ababab^{2}{\bigr )}=6}$
M11	Mathieu	1861	1st	7920	≈ 8×103	24 × 32 × 5 × 11	10	2, 4, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=4}$

Notes

1. The groups of prime order, the alternating groups of degree at least 5, the infinite family of commutator groups ²F₄(2²ⁿ⁺¹)′ of groups of Lie type (containing the Tits group), and 15 families of groups of Lie type.
2. Conway et al. (1985, p. viii) organizes the 26 sporadic groups in likeness:

   "The sporadic simple groups may be roughly sorted as the Mathieu groups, the Leech lattice groups, Fischer's 3-transposition groups, the further Monster centralizers, and the half-dozen oddments."

3. Here listed are semi-presentations from standard generators of each sporadic group. Most sporadic groups have multiple presentations & semi-presentations; the more prominent examples are listed.
4. Where ${\displaystyle u=(b^{2}(b^{2})abb)^{3}}$ and ${\displaystyle v=t(b^{2}(b^{2})t)^{2}}$ with ${\displaystyle t=abab^{3}a^{2}}$.

References

1. Conway et al. (1985 , p. viii)
2. Griess, Jr. (1998 , p. 146)
3. Gorenstein, Lyons & Solomon (1998 , pp. 262–302)
4. Ronan (2006 , pp. 244–246)
5. Howlett, Rylands & Taylor (2001 , p.429)

   "This completes the determination of matrix generators for all groups of Lie type, including the twisted groups of Steinberg, Suzuki and Ree (and the Tits group)."

6. Gorenstein (1979 , p.111)
7. Conway et al. (1985 , p.viii)
8. Hartley & Hulpke (2010 , p.106)

   "The finite simple groups are the building blocks of finite group theory. Most fall into a few infinite families of groups, but there are 26 (or 27 if the Tits group ²F₄(2)′ is counted also) which these infinite families do not include."

9. Wilson et al. (1999 , Sporadic groups & Exceptional groups of Lie type)
10. Griess, Jr. (1982 , p. 91)
11. Griess, Jr. (1998 , pp. 54–79)
12. Griess, Jr. (1998 , pp. 104–145)
13. Griess, Jr. (1998 , pp. 146−150)
14. Griess, Jr. (1982 , pp. 91−96)
15. Griess, Jr. (1998 , pp. 146, 150−152)
16. Hiss (2003 , p. 172)

    Tabelle 2. Die Entdeckung der sporadischen Gruppen (Table 2. The discovery of the sporadic groups)

17. Sloane (1996)
18. Jansen (2005 , pp. 122–123)
19. Nickerson & Wilson (2011 , p. 365)
20. Wilson et al. (1999)
21. Lubeck (2001 , p. 2151)

Works cited

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• Ronan, Mark (2006). Symmetry and the Monster: One of the Greatest Quests of Mathematics . New York: Oxford University Press. pp. vii, 1–255. ISBN   978-0-19-280722-9. MR   2215662. OCLC   180766312. Zbl   1113.00002.

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• Wilson, R.A.; Parker, R.A.; Nickerson, S.J.; Bray, J.N. (1999). "ATLAS of Group Representations". ATLAS of Finite Group Representations. Queen Mary University of London.

External links

• Atlas of Finite Group Representations: Sporadic groups
• Weisstein, Eric W. "Sporadic Group". MathWorld .