Group of Lie type

In mathematics, a group of Lie type G(k) is a (not necessarily finite) group of rational points of a reductive linear algebraic group G with values in the field k. Finite groups of Lie type form the bulk of nonabelian finite simple groups. Special cases include the classical groups, the Chevalley groups, the Steinberg groups, and the Suzuki–Ree groups.

Dieudonné (1971) and Carter (1989) are standard references for groups of Lie type.

Classical groups

Main article: Classical group

An initial approach to this question was the definition and detailed study of the so-called classical groups over finite and other fields by Jordan (1870). These groups were studied by L. E. Dickson and Jean Dieudonné. Emil Artin investigated the orders of such groups, with a view to classifying cases of coincidence.

A classical group is, roughly speaking, a special linear, orthogonal, symplectic, or unitary group. There are several minor variations of these, given by taking derived subgroups or central quotients, the latter yielding projective linear groups. They can be constructed over finite fields (or any other field) in much the same way that they are constructed over the real numbers. They correspond to the series A_n, B_n, C_n, D_n, ²A_n, ²D_n of Chevalley and Steinberg groups.

Chevalley groups

The theory was clarified by the theory of algebraic groups, and the work of Chevalley (1955) on Lie algebras, by means of which the Chevalley group concept was isolated. Chevalley constructed a Chevalley basis (a sort of integral form) for all the complex simple Lie algebras (or rather of their universal enveloping algebras), which can be used to define the corresponding algebraic groups over the integers. In particular, he could take their points with values in any finite field. For the Lie algebras A_n, B_n, C_n, D_n this gave well known classical groups, but his construction also gave groups associated to the exceptional Lie algebras E₆, E₇, E₈, F₄, and G₂. The ones of type G₂ had already been constructed by Dickson (1905), and the ones of type E₆ by Dickson (1901).

Steinberg groups

Chevalley's construction did not give all of the known classical groups: it omitted the unitary groups and the non-split orthogonal groups. Steinberg (1959) found a modification of Chevalley's construction that gave these groups and two new families ³D₄, ²E₆, the second of which was discovered at about the same time from a different point of view by Tits (1958). This construction generalizes the usual construction of the unitary group from the general linear group.

The unitary group arises as follows: the general linear group over the complex numbers has a diagram automorphism given by reversing the Dynkin diagram A_n (which corresponds to taking the transpose inverse), and a field automorphism given by taking complex conjugation, which commute. The unitary group is the group of fixed points of the product of these two automorphisms.

In the same way, many Chevalley groups have diagram automorphisms induced by automorphisms of their Dynkin diagrams, and field automorphisms induced by automorphisms of a finite field. Analogously to the unitary case, Steinberg constructed families of groups by taking fixed points of a product of a diagram and a field automorphism.

These gave:

• the unitary groups ²A_n, from the order 2 automorphism of A_n;
• further orthogonal groups ²D_n, from the order 2 automorphism of D_n;
• the new series ²E₆, from the order 2 automorphism of E₆;
• the new series ³D₄, from the order 3 automorphism of D₄.

The groups of type ³D₄ have no analogue over the reals, as the complex numbers have no automorphism of order 3. The symmetries of the D₄ diagram also give rise to triality.

Suzuki–Ree groups

Suzuki (1960) found a new infinite series of groups that at first sight seemed unrelated to the known algebraic groups. Ree (1960, 1961) knew that the algebraic group B₂ had an "extra" automorphism in characteristic 2 whose square was the Frobenius automorphism. He found that if a finite field of characteristic 2 also has an automorphism whose square was the Frobenius map, then an analogue of Steinberg's construction gave the Suzuki groups. The fields with such an automorphism are those of order 2²ⁿ⁺¹, and the corresponding groups are the Suzuki groups

²B₂(2²ⁿ⁺¹) = Suz(2²ⁿ⁺¹).

(Strictly speaking, the group Suz(2) is not counted as a Suzuki group as it is not simple: it is the Frobenius group of order 20.) Ree was able to find two new similar families

²F₄(2²ⁿ⁺¹)

and

²G₂(3²ⁿ⁺¹)

of simple groups by using the fact that F₄ and G₂ have extra automorphisms in characteristic 2 and 3. (Roughly speaking, in characteristic p one is allowed to ignore the arrow on bonds of multiplicity p in the Dynkin diagram when taking diagram automorphisms.) The smallest group ²F₄(2) of type ²F₄ is not simple, but it has a simple subgroup of index 2, called the Tits group (named after the mathematician Jacques Tits). The smallest group ²G₂(3) of type ²G₂ is not simple, but it has a simple normal subgroup of index 3, isomorphic to SL₂(8). In the classification of finite simple groups, the Ree groups

²G₂(3²ⁿ⁺¹)

are the ones whose structure is hardest to pin down explicitly. These groups also played a role in the discovery of the first modern sporadic group. They have involution centralizers of the form Z/2Z × PSL₂(q) for q = 3ⁿ, and by investigating groups with an involution centralizer of the similar form Z/2Z × PSL₂(5) Janko found the sporadic group J₁.

The Suzuki groups are of great interest as the only finite non-abelian simple groups with order not divisible by 3. They have order (2^2(2n+1))(2^2(2n+1) + 1)(2⁽²ⁿ⁺¹⁾ − 1).

Relations with finite simple groups

Finite groups of Lie type were among the first groups to be considered in mathematics, after cyclic, symmetric and alternating groups, with the projective special linear groups over prime finite fields, PSL(2,p) being constructed by Évariste Galois in the 1830s. The systematic exploration of finite groups of Lie type started with Camille Jordan's theorem that the projective special linear group PSL₂(q) is simple for q ≠ 2, 3. This theorem generalizes to projective groups of higher dimensions and gives an important infinite family PSL_n(q) of finite simple groups. Other classical groups were studied by Leonard Dickson in the beginning of 20th century. In the 1950s Claude Chevalley realized that after an appropriate reformulation, many theorems about semisimple Lie groups admit analogues for algebraic groups over an arbitrary field k, leading to construction of what are now called Chevalley groups. Moreover, as in the case of compact simple Lie groups, the corresponding groups turned out to be almost simple as abstract groups (Tits simplicity theorem). Although it was known since 19th century that other finite simple groups exist (for example, Mathieu groups), gradually a belief formed that nearly all finite simple groups can be accounted for by appropriate extensions of Chevalley's construction, together with cyclic and alternating groups. Moreover, the exceptions, the sporadic groups, share many properties with the finite groups of Lie type, and in particular, can be constructed and characterized based on their geometry in the sense of Tits.

The belief has now become a theorem – the classification of finite simple groups. Inspection of the list of finite simple groups shows that groups of Lie type over a finite field include all the finite simple groups other than the cyclic groups, the alternating groups, the Tits group, and the 26 sporadic simple groups.

Small groups of Lie type

Many of the smallest groups in the families above have special properties not shared by most members of the family.

• Sometimes the smallest groups are solvable rather than simple; for example the groups SL₂(2) and SL₂(3) are solvable.

• There is a bewildering number of "accidental" isomorphisms between various small groups of Lie type (and alternating groups). For example, the groups SL₂(4), PSL₂(5), and the alternating group on 5 points are all isomorphic.

• Some of the small groups have a Schur multiplier that is larger than expected. For example, the groups A_n(q) usually have a Schur multiplier of order (n + 1, q − 1), but the group A₂(4) has a Schur multiplier of order 48, instead of the expected value of 3.

For a complete list of these exceptions see the list of finite simple groups. Many of these special properties are related to certain sporadic simple groups.

Alternating groups sometimes behave as if they were groups of Lie type over the field with one element. Some of the small alternating groups also have exceptional properties. The alternating groups usually have an outer automorphism group of order 2, but the alternating group on 6 points has an outer automorphism group of order 4. Alternating groups usually have a Schur multiplier of order 2, but the ones on 6 or 7 points have a Schur multiplier of order 6.

Notation issues

Unfortunately there is no standard notation for the finite groups of Lie type, and the literature contains dozens of incompatible and confusing systems of notation for them.

• The groups of type A_n−1 are sometimes denoted by PSL_n(q) (the projective special linear group) or by L_n(q).

• The groups of type C_n are sometimes denoted by Sp_2n(q) (the symplectic group) or (confusingly) by Sp_n(q).

• The notation for groups of type D_n ("orthogonal" groups) is particularly confusing. Some symbols used are O_n(q), O⁻_n(q),PSO_n(q), Ω_n(q), but there are so many conventions that it is not possible to say exactly what groups these correspond to without it being specified explicitly. The source of the problem is that the simple group is not the orthogonal group O, nor the projective special orthogonal group PSO, but rather a subgroup of PSO,^[1] which accordingly does not have a classical notation. A particularly nasty trap is that some authors, such as the ATLAS, use O_n(q) for a group that is not the orthogonal group, but the corresponding simple group. The notation Ω,PΩ was introduced by Jean Dieudonné, though his definition is not simple for $n \leq 4$ and thus the same notation may be used for a slightly different group, which agrees in $n \geq 5$ but not in lower dimension.^[1]

• For the Steinberg groups, some authors write ²A_n(q²) (and so on) for the group that other authors denote by ²A_n(q). The problem is that there are two fields involved, one of order q², and its fixed field of order q, and people have different ideas on which should be included in the notation. The "²A_n(q²)" convention is more logical and consistent, but the "²A_n(q)" convention is far more common and is closer to the convention for algebraic groups.

• Authors differ on whether groups such as A_n(q) are the groups of points with values in the simple or the simply connected algebraic group. For example, A_n(q) may mean either the special linear group SL_n+1(q) or the projective special linear group PSL_n+1(q). So ²A₂(2²) may be any one of 4 different groups, depending on the author.

See also

• Deligne–Lusztig theory
• Modular Lie algebra

Notes

1. ^ ^{a b} ATLAS, p. xi

References

• Carter, Roger W. (1989) [1972], Simple groups of Lie type, Wiley Classics Library, New York: John Wiley & Sons, ISBN 978-0-471-50683-6, MR0407163 
• Chevalley, Claude (1955), "Sur certains groupes simples", The Tohoku Mathematical Journal. Second Series 7: 14–66, doi:10.2748/tmj/1178245104, ISSN 0040-8735, MR0073602, http://projecteuclid.org/euclid.tmj/1178245104 
• Dickson, Leonard Eugene (1901b), "Theory of Linear Groups in An Arbitrary Field" (in English), Transactions of the American Mathematical Society (Providence, R.I.: American Mathematical Society) 2 (4): 363–394, ISSN 0002-9947, Reprinted in volume II of his collected papers, http://www.jstor.org/stable/1986251 
• Dickson, Leonard Eugene (1901), "A class of groups in an arbitrary realm connected with the configuration of the 27 lines on a cubic surface", The quarterly journal of pure and applied mathematics 33: 145–173, Reprinted in volume 5 of his collected works, http://books.google.com/books?id=I_SWAAAAMAAJ&pg=PA145 
• Dickson, L. E. (1905), "A new system of simple groups", Math. Ann. 60: 137–150, doi:10.1007/BF01447497  Leonard E. Dickson reported groups of type G₂
• Dieudonné, Jean A. (1971) [1955], La géométrie des groupes classiques (3rd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-05391-2, MR0310083 
• Jordan, Camille (1870), Traité des substitutions et des équations algébriques, Paris: Gauthier-Villars, http://gallica.bnf.fr/notice?N=FRBNF35001297 
• Ree, Rimhak (1960), "A family of simple groups associated with the simple Lie algebra of type (G₂)", Bulletin of the American Mathematical Society 66: 508–510, doi:10.1090/S0002-9904-1960-10523-X, ISSN 0002-9904, MR0125155, http://www.ams.org/journals/bull/1960-66-06/S0002-9904-1960-10523-X/home.html 
• Ree, Rimhak (1961), "A family of simple groups associated with the simple Lie algebra of type (F₄)", Bulletin of the American Mathematical Society 67: 115–116, doi:10.1090/S0002-9904-1961-10527-2, ISSN 0002-9904, MR0125155, http://www.ams.org/journals/bull/1961-67-01/S0002-9904-1961-10527-2/home.html 
• Steinberg, Robert (1959), "Variations on a theme of Chevalley", Pacific Journal of Mathematics 9: 875–891, ISSN 0030-8730, MR0109191, http://projecteuclid.org/euclid.pjm/1103039126 
• Steinberg, Robert (1968), Lectures on Chevalley groups, Yale University, New Haven, Conn., MR0466335, http://www.math.ucla.edu/~rst/ 

• Suzuki, Michio (1960), "A new type of simple groups of finite order", Proceedings of the National Academy of Sciences of the United States of America 46: 868–870, ISSN 0027-8424, JSTOR 70960, MR0120283 
• Tits, Jacques (1958), Les "formes réelles" des groupes de type E₆, Séminaire Bourbaki; 10e année: 1957/1958. Textes des conférences; Exposés 152 à 168; 2e èd. corrigée, Exposé 162, 15, Paris: Secrétariat math'ematique, MR0106247, http://www.numdam.org/item?id=SB_1956-1958__4__351_0