Sporadic group

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In the mathematical classification of finite simple groups, there are a number of 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^[a] 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]

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]

• 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)-member ²F₄(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.

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][b]

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]

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).

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 S₄ ײF₄(2)′ normalising a 2C² subgroup of B, giving rise to a subgroup 2·S₄ ײF₄(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.

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

Group	Discoverer	[16] Year	Generation	[1][4][17] Order	[18] Degree of minimal faithful Brauer character	[19][20] ${\displaystyle (a,b,ab)}$ Generators	[20][c] ${\displaystyle \langle \langle a,b\mid o(z)\rangle \rangle }$ Semi-presentation
M or F1	Fischer, Griess	1973	3rd	808,017,424,794,512,875,886,459,904,961,710,757,005,754,368,000,000,000 = 246·320·59·76·112·133·17·19·23·29·31·41·47·59·71 ≈ 8×1053	196883	2A, 3B, 29	${\displaystyle o{\bigl (}(ab)^{4}(ab^{2})^{2}{\bigr )}=50}$
B or F2	Fischer	1973	3rd	4,154,781,481,226,426,191,177,580,544,000,000 = 241·313·56·72·11·13·17·19·23·31·47 ≈ 4×1033	4371	2C, 3A, 55	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=23}$
Fi24 or F3+	Fischer	1971	3rd	1,255,205,709,190,661,721,292,800 = 221·316·52·73·11·13·17·23·29 ≈ 1×1024	8671	2A, 3E, 29	${\displaystyle o{\bigl (}(ab)^{3}b{\bigr )}=33}$
Fi23	Fischer	1971	3rd	4,089,470,473,293,004,800 = 218·313·52·7·11·13·17·23 ≈ 4×1018	782	2B, 3D, 28	${\displaystyle o{\bigl (}a^{bb}(ab)^{14}{\bigr )}=5}$
Fi22	Fischer	1971	3rd	64,561,751,654,400 = 217·39·52·7·11·13 ≈ 6×1013	78	2A, 13, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=12}$
Th or F3	Thompson	1976	3rd	90,745,943,887,872,000 = 215·310·53·72·13·19·31 ≈ 9×1016	248	2, 3A, 19	${\displaystyle o{\bigl (}(ab)^{3}b{\bigr )}=21}$
Ly	Lyons	1972	Pariah	51,765,179,004,000,000 = 28·37·56·7·11·31·37·67 ≈ 5×1016	2480	2, 5A, 14	${\displaystyle o{\bigl (}ababab^{2}{\bigr )}=67}$
HN or F5	Harada, Norton	1976	3rd	273,030,912,000,000 = 214·36·56·7·11·19 ≈ 3×1014	133	2A, 3B, 22	${\displaystyle o{\bigl (}[a,b]{\bigr )}=5}$
Co1	Conway	1969	2nd	4,157,776,806,543,360,000 = 221·39·54·72·11·13·23 ≈ 4×1018	276	2B, 3C, 40	${\displaystyle o{\bigl (}ab(abab^{2})^{2}{\bigr )}=42}$
Co2	Conway	1969	2nd	42,305,421,312,000 = 218·36·53·7·11·23 ≈ 4×1013	23	2A, 5A, 28	${\displaystyle o{\bigl (}[a,b]{\bigr )}=4}$
Co3	Conway	1969	2nd	495,766,656,000 = 210·37·53·7·11·23 ≈ 5×1011	23	2A, 7C, 17	${\displaystyle o{\bigl (}(uvv)^{3}(uv)^{6}{\bigr )}=5}$[d]
ON or O'N	O'Nan	1976	Pariah	460,815,505,920 = 29·34·5·73·11·19·31 ≈ 5×1011	10944	2A, 4A, 11	${\displaystyle o{\bigl (}abab(b^{2}(b^{2})^{abab})^{5}{\bigr )}=5}$
Suz	Suzuki	1969	2nd	448,345,497,600 = 213·37·52·7·11·13 ≈ 4×1011	143	2B, 3B, 13	${\displaystyle o{\bigl (}[a,b]{\bigr )}=15}$
Ru	Rudvalis	1972	Pariah	145,926,144,000 = 214·33·53·7·13·29 ≈ 1×1011	378	2B, 4A, 13	${\displaystyle o(abab^{2})=29}$
He or F7	Held	1969	3rd	4,030,387,200 = 210·33·52·73·17 ≈ 4×109	51	2A, 7C, 17	${\displaystyle o{\bigl (}ab^{2}abab^{2}ab^{2}{\bigr )}=10}$
McL	McLaughlin	1969	2nd	898,128,000 = 27·36·53·7·11 ≈ 9×108	22	2A, 5A, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=7}$
HS	Higman, Sims	1967	2nd	44,352,000 = 29·32·53·7·11 ≈ 4×107	22	2A, 5A, 11	${\displaystyle o(abab^{2})=15}$
J4	Janko	1976	Pariah	86,775,571,046,077,562,880 = 221·33·5·7·113·23·29·31·37·43 ≈ 9×1019	1333	2A, 4A, 37	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=10}$
J3 or HJM	Janko	1968	Pariah	50,232,960 = 27·35·5·17·19 ≈ 5×107	85	2A, 3A, 19	${\displaystyle o{\bigl (}[a,b]{\bigr )}=9}$
J2 or HJ	Janko	1968	2nd	604,800 = 27·33·52·7 ≈ 6×105	14	2B, 3B, 7	${\displaystyle o{\bigl (}[a,b]{\bigr )}=12}$
J1	Janko	1965	Pariah	175,560 = 23·3·5·7·11·19 ≈ 2×105	56	2, 3, 7	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=19}$
M24	Mathieu	1861	1st	244,823,040 = 210·33·5·7·11·23 ≈ 2×108	23	2B, 3A, 23	${\displaystyle o{\bigl (}ab(abab^{2})^{2}ab^{2}{\bigr )}=4}$
M23	Mathieu	1861	1st	10,200,960 = 27·32·5·7·11·23 ≈ 1×107	22	2, 4, 23	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=8}$
M22	Mathieu	1861	1st	443,520 = 27·32·5·7·11 ≈ 4×105	21	2A, 4A, 11	${\displaystyle o{\bigl (}abab^{2}{\bigr )}=11}$
M12	Mathieu	1861	1st	95,040 = 26·33·5·11 ≈ 1×105	11	2B, 3B, 11	${\displaystyle o{\bigl (}[a,b]{\bigr )}=o{\bigl (}ababab^{2}{\bigr )}=6}$
M11	Mathieu	1861	1st	7,920 = 24·32·5·11 ≈ 8×103	10	2, 4, 11	${\displaystyle o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=4}$
T or 2F4(2)′	Tits	1964	3rd	17,971,200 = 211·33·52·13 ≈ 2×107	104[21]	2A, 3, 13	${\displaystyle o{\bigl (}[a,b]{\bigr )}=5}$

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