Sporadic group

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]

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 26 **sporadic groups**. A connecting line means the lower group is subquotient of the upper – and no sporadic subquotient in between.
The generations of Robert Griess: 1st, 2nd, 3rd, 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)-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.

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]^[b]

First generation (5 groups): the 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

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 S₄ ×²F₄(2)′ normalising a 2C² subgroup of B, giving rise to a subgroup 2·S₄ ×²F₄(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

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	^[1]^[4]^[17] Order	^[18] Degree of minimal faithful Brauer character	^[19]^[20] $(a,b,ab)$ Generators	^[20]^[c] $\langle \langle a,b\mid o(z)\rangle \rangle$ 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	$o{\bigl (}(ab)^{4}(ab^{2})^{2}{\bigr )}=50$
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	$o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=23$
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	$o{\bigl (}(ab)^{3}b{\bigr )}=33$
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	$o{\bigl (}a^{bb}(ab)^{14}{\bigr )}=5$
Fi₂₂	Fischer	1971	3rd	64,561,751,654,400 = 2¹⁷·3⁹·5²·7·11·13 ≈ 6×10¹³	78	2A, 13, 11	$o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=12$
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	$o{\bigl (}(ab)^{3}b{\bigr )}=21$
Ly	Lyons	1972	Pariah	51,765,179,004,000,000 = 2⁸·3⁷·5⁶·7·11·31·37·67 ≈ 5×10¹⁶	2480	2, 5A, 14	$o{\bigl (}ababab^{2}{\bigr )}=67$
HN or F₅	Harada, Norton	1976	3rd	273,030,912,000,000 = 2¹⁴·3⁶·5⁶·7·11·19 ≈ 3×10¹⁴	133	2A, 3B, 22	$o{\bigl (}[a,b]{\bigr )}=5$
Co₁	Conway	1969	2nd	4,157,776,806,543,360,000 = 2²¹·3⁹·5⁴·7²·11·13·23 ≈ 4×10¹⁸	276	2B, 3C, 40	$o{\bigl (}ab(abab^{2})^{2}{\bigr )}=42$
Co₂	Conway	1969	2nd	42,305,421,312,000 = 2¹⁸·3⁶·5³·7·11·23 ≈ 4×10¹³	23	2A, 5A, 28	$o{\bigl (}[a,b]{\bigr )}=4$
Co₃	Conway	1969	2nd	495,766,656,000 = 2¹⁰·3⁷·5³·7·11·23 ≈ 5×10¹¹	23	2A, 7C, 17	$o{\bigl (}(uvv)^{3}(uv)^{6}{\bigr )}=5$ ^[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	$o{\bigl (}abab(b^{2}(b^{2})^{abab})^{5}{\bigr )}=5$
Suz	Suzuki	1969	2nd	448,345,497,600 = 2¹³·3⁷·5²·7·11·13 ≈ 4×10¹¹	143	2B, 3B, 13	$o{\bigl (}[a,b]{\bigr )}=15$
Ru	Rudvalis	1972	Pariah	145,926,144,000 = 2¹⁴·3³·5³·7·13·29 ≈ 1×10¹¹	378	2B, 4A, 13	$o(abab^{2})=29$
He or F₇	Held	1969	3rd	4,030,387,200 = 2¹⁰·3³·5²·7³·17 ≈ 4×10⁹	51	2A, 7C, 17	$o{\bigl (}ab^{2}abab^{2}ab^{2}{\bigr )}=10$
McL	McLaughlin	1969	2nd	898,128,000 = 2⁷·3⁶·5³·7·11 ≈ 9×10⁸	22	2A, 5A, 11	$o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=7$
HS	Higman, Sims	1967	2nd	44,352,000 = 2⁹·3²·5³·7·11 ≈ 4×10⁷	22	2A, 5A, 11	$o(abab^{2})=15$
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	$o{\bigl (}abab^{2}{\bigr )}=10$
J₃ or HJM	Janko	1968	Pariah	50,232,960 = 2⁷·3⁵·5·17·19 ≈ 5×10⁷	85	2A, 3A, 19	$o{\bigl (}[a,b]{\bigr )}=9$
J₂ or HJ	Janko	1968	2nd	604,800 = 2⁷·3³·5²·7 ≈ 6×10⁵	14	2B, 3B, 7	$o{\bigl (}[a,b]{\bigr )}=12$
J₁	Janko	1965	Pariah	175,560 = 2³·3·5·7·11·19 ≈ 2×10⁵	56	2, 3, 7	$o{\bigl (}abab^{2}{\bigr )}=19$
M₂₄	Mathieu	1861	1st	244,823,040 = 2¹⁰·3³·5·7·11·23 ≈ 2×10⁸	23	2B, 3A, 23	$o{\bigl (}ab(abab^{2})^{2}ab^{2}{\bigr )}=4$
M₂₃	Mathieu	1861	1st	10,200,960 = 2⁷·3²·5·7·11·23 ≈ 1×10⁷	22	2, 4, 23	$o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=8$
M₂₂	Mathieu	1861	1st	443,520 = 2⁷·3²·5·7·11 ≈ 4×10⁵	21	2A, 4A, 11	$o{\bigl (}abab^{2}{\bigr )}=11$
M₁₂	Mathieu	1861	1st	95,040 = 2⁶·3³·5·11 ≈ 1×10⁵	11	2B, 3B, 11	$o{\bigl (}[a,b]{\bigr )}=o{\bigl (}ababab^{2}{\bigr )}=6$
M₁₁	Mathieu	1861	1st	7,920 = 2⁴·3²·5·11 ≈ 8×10³	10	2, 4, 11	$o{\bigl (}(ab)^{2}(abab^{2})^{2}ab^{2}{\bigr )}=4$
T or $2 F 4 (2)'$	Tits	1964	3rd	17,971,200 = 2¹¹·3³·5²·13 ≈ 2×10⁷	104^[21]	2A, 3, 13	$o{\bigl (}[a,b]{\bigr )}=5$

Notes

^ 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.
^ 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."
^ 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.
^ Where $u=(b^{2}(b^{2})abb)^{3}$ and $v=t(b^{2}(b^{2})t)^{2}$ with $t=abab^{3}a^{2}$ .

References

^ ^a ^b ^c Conway et al. (1985, p. viii)
^ Griess, Jr. (1998, p. 146)
^ Gorenstein, Lyons & Solomon (1998, pp. 262–302)
^ ^a ^b Ronan (2006, pp. 244–246)
^ 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)."
^ Gorenstein (1979, p.111)
^ Conway et al. (1985, p.viii)
^ 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."
^ Wilson et al. (1999, Sporadic groups & Exceptional groups of Lie type)
^ Griess, Jr. (1982, p. 91)
^ Griess, Jr. (1998, pp. 54–79)
^ Griess, Jr. (1998, pp. 104–145)
^ Griess, Jr. (1998, pp. 146−150)
^ Griess, Jr. (1982, pp. 91−96)
^ Griess, Jr. (1998, pp. 146, 150−152)
^ Hiss (2003, p. 172)
Tabelle 2. Die Entdeckung der sporadischen Gruppen (Table 2. The discovery of the sporadic groups)
^ Sloane (1996)
^ Jansen (2005, pp. 122–123)
^ Nickerson & Wilson (2011, p. 365)
^ ^a ^b Wilson et al. (1999)
^ Lubeck (2001, p. 2151)

Works cited

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Conway, J. H. (1968). "A perfect group of order 8,315,553,613,086,720,000 and the sporadic simple groups". Proc. Natl. Acad. Sci. U.S.A. 61 (2): 398–400. Bibcode:1968PNAS...61..398C. doi:10.1073/pnas.61.2.398. MR 0237634. PMC 225171. PMID 16591697. S2CID 29358882. Zbl 0186.32401.

Conway, J. H.; Curtis, R. T.; Norton, S. P.; Parker, R. A.; Wilson, R. A. (1985). ATLAS of Finite Groups: Maximal Subgroups and Ordinary Characters for Simple Groups. Oxford: Clarendon Press. pp. xxxiii, 1–252. ISBN 978-0-19-853199-9. MR 0827219. OCLC 12106933. S2CID 117473588. Zbl 0568.20001.

Gorenstein, D. (1979). "The classification of finite simple groups. I. Simple groups and local analysis". Bulletin of the American Mathematical Society. New Series. 1 (1). American Mathematical Society: 43–199. doi:10.1090/S0273-0979-1979-14551-8. MR 0513750. S2CID 121953006. Zbl 0414.20009.

Gorenstein, D.; Lyons, Richard; Solomon, Ronald (1998). The classification of the finite simple groups, Number 3. Mathematical Surveys and Monographs. Vol. 40. Providence, R.I.: American Mathematical Society. pp. xiii, 1–362. doi:10.1112/S0024609398255457. ISBN 978-0-8218-0391-2. MR 1490581. OCLC 6907721813. S2CID 209854856.

Griess, Jr., Robert L. (1982). "The Friendly Giant". Inventiones Mathematicae. 69: 1−102. Bibcode:1982InMat..69....1G. doi:10.1007/BF01389186. hdl:2027.42/46608. MR 0671653. S2CID 123597150. Zbl 0498.20013.

Griess, Jr., Robert L. (1998). Twelve Sporadic Groups. Springer Monographs in Mathematics. Berlin: Springer-Verlag. pp. 1−169. ISBN 9783540627784. MR 1707296. OCLC 38910263. Zbl 0908.20007.

Hartley, Michael I.; Hulpke, Alexander (2010), "Polytopes Derived from Sporadic Simple Groups", Contributions to Discrete Mathematics, 5 (2), Alberta, CA: University of Calgary Department of Mathematics and Statistics: 106−118, doi:10.11575/cdm.v5i2.61945, ISSN 1715-0868, MR 2791293, S2CID 40845205, Zbl 1320.51021

Hiss, Gerhard (2003). "Die Sporadischen Gruppen (The Sporadic Groups)" (PDF). Jahresber. Deutsch. Math.-Verein. (Annual Report of the German Mathematicians Association). 105 (4): 169−193. ISSN 0012-0456. MR 2033760. Zbl 1042.20007. (German)

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External links

[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.

[12] 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."

[23] 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.

[24] Where $u=(b^{2}(b^{2})abb)^{3}$ and $v=t(b^{2}(b^{2})t)^{2}$ with $t=abab^{3}a^{2}$ .

[Conway-2] Conway et al. (1985, p. viii)

[3] Griess, Jr. (1998, p. 146)

[4] Gorenstein, Lyons & Solomon (1998, pp. 262–302)

[Ronan-5] Ronan (2006, pp. 244–246)

[6] 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)."

[7] Gorenstein (1979, p.111)

[Conway2-8] Conway et al. (1985, p.viii)

[9] 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."

[10] Wilson et al. (1999, Sporadic groups & Exceptional groups of Lie type)

[11] Griess, Jr. (1982, p. 91)

[13] Griess, Jr. (1998, pp. 54–79)

[14] Griess, Jr. (1998, pp. 104–145)

[15] Griess, Jr. (1998, pp. 146−150)

[16] Griess, Jr. (1982, pp. 91−96)

[17] Griess, Jr. (1998, pp. 146, 150−152)

[18] Hiss (2003, p. 172)
Tabelle 2. Die Entdeckung der sporadischen Gruppen (Table 2. The discovery of the sporadic groups)

[19] Sloane (1996)

[20] Jansen (2005, pp. 122–123)

[21] Nickerson & Wilson (2011, p. 365)

[Wilsonetal-22] Wilson et al. (1999)

[25] Lubeck (2001, p. 2151)

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