WHATIS... ? a Linear Algebraic ? Skip Garibaldi

From a marketing perspective, algebraic groups group over F, a , or a is are poorly named.They are not the groups you met a “” in the of affine varieties as a student in abstract , which I will call over F, smooth , or topological , concrete groups for clarity. Rather, an algebraic respectively. For algebraic groups, this implies group is the analogue in algebra of a topological that the G(K) is a concrete group for each field group (fromtopology) or a Liegroup (from K containing F. and ). Algebraic groups provide a unifying language Examples for apparently different results in algebra and The basic example is the general . This unification can not only sim- GLn for which GLn(K) is the concrete group of plify proofs, it can also suggest generalizations invertible n-by-n matrices with entries in K. It is the and bring new tools to bear, such as Galois co- collection of solutions (t, X)—with t ∈ K and X an , Steenrod operations in Chow theory, n-by-n over K—to the equation etc. t ·det X = 1. Similar reasoning shows that familiar matrix groups such as SLn, orthogonal groups, Definitions and symplectic groups can be viewed as linear A linear over a field F is a smooth algebraic groups. The main difference here is that affine variety over F that is also a group, much instead of viewing them as collections of matrices like a topological group is a topological over F, we view them as collections of matrices that is also a group and a Lie group is a smooth over every K of F. that is also a group. (For nonexperts: Roughly, the theory of linear algebraic groups it is useful to think of an affine variety G as a generalizes that of linear Lie groups over the real natural assignment—i.e., a —that takes any or complex numbers to give something that makes field extension K of F and gives the set G(K) of sense over an arbitrary field. The category of linear common solutions over K of some fixed family of algebraic groups over R contains a full subcategory with coefficients in F.) equivalent to the compact Lie groups; see [3, Properly speaking, for each of the three types §5]. And the parameterization of the irreducible of groups in the previous paragraph, one needs to finite-dimensional representations of a complex require that the group operations are reductive Lie group in terms of dominant weights in the appropriate category, so, e.g., for a topo- holds more generally for so-called “split reductive” logical group G, the G × G → G and groups over any field. inversion G → G are required to be continuous. Algebraic groups allow one to deal system- In more categorical language, a linear algebraic atically with familiar matrix groups and their generalizations in a way that works over arbitrary Skip Garibaldi is Winship Professor of Mathematics at fields, whether they are the rationals for number Emory University. His email address is [email protected] theory, finite fields for finite , or the ams.org. real or complex numbers for geometry. This is not The author thanks B. Conrad, G. McNinch, R. Parimala, just a language. There is enough theory available G. Prasad, and J.-P. Serre for their valuable suggestions. that one can often avoid computing with actual

October 2010 Notices of the AMS 1125 matrices, or at least with matrices larger than Group Theory 2-by-2! In the list of finite simple (concrete) groups, most are of Lie type.That is, take a linear algebraicgroup Local-Global Principles G that is absolutely simple, simply connected, and Quadratic forms and division over a defined over a finite field F that is not very small. global field1 F are determined by their proper- Then the concrete group G(F) its is finite and simple. This is helpful because one ties at the completions Fv of F; this is roughly the content of the Hasse-Minkowski theorem and can use the general framework of algebraic groups the Albert-Brauer-Hasse-Noether theorem, respec- to prove theorems about these finite groups. One tively. These theorems can be viewed as saying example of this is Deligne-Lusztig theory, which that the natural map is the most effective approach to the complex representations of the finite groups of Lie type. 1 1 (∗) H (F, G) → Y H (Fv ,G) The construction of simple concrete groups in places v of F the previous paragraph works for many algebraic groups G and many fields F, not just for finite in Galois is injective, where G is an fields. But for precisely which G does it work? or PGL . This reformulation of n The Kneser-Tits problem (1964) asks: Let G be a the problem in terms of algebraic groups has two that is simply connected, is advantages. First, we see that the two theorems absolutely simple, and contains GL1. Is G(F) mod- are two faces of the same phenomenon, and ulo its center simple? Much like the Hasse principle second, we get a natural general question: For discussed above, Kneser-Tits is a generalization ∗ which linear algebraic groups G is ( ) injective for in terms of algebraic groups of earlier problems— every global field F? The Hasse principle (proved such as the Tannaka-Artin problem—regarding by Kneser,Harder,and Chernousov) saysthat (∗) is more classical algebraic structures. injective if G is connected and absolutely simple. (A The answer to Kneser-Tits seems to depend linear algebraic group is absolutely simple if, when on the complexity of the field F. The viewed as an algebraic group over an algebraic answer can be “no” for fields of at least of F, it is not commutative and does 4 (Platonov, 1975). not contain any proper, connected, and normal In contrast, the answer is “yes” for global fields, algebraic besides the identity.) This which “are 2-dimensional”. This was an open broadly generalizes the two results mentioned at question for some , until Philippe Gille settled the start of the paragraph. the last remaining case in 2007 by discovering If we weaken our request that (∗) be injective, the following interesting criterion that works over we arrive at a substantial result due to Borel and every field F: for a group G as in Kneser-Tits, the Serre (early 1960s) that (∗) has finite for concrete group G(F) modulo its center is simple (a every linear algebraic group G in the case in which purely algebraic criterion) if and only if the variety F is a number field. This result has recently been G is, roughly speaking, “ connected” (a purely extended to global fields of prime geometric criterion). See [1] for details. by Brian Conrad. Because these latter fields are We still don’t know the answer to Kneser-Tits not perfect, this extension relies on the classifica- for other fields of dimension 2 and don’t even tion of “pseudo-reductive” linear algebraic groups have strong indications of what the answer should recently completed by Conrad, Ofer Gabber, and be for dimension 3. . In closing, I urge you: Please do not be misled Another way to generalize the local-global ques- by the short list of topics in this article. There are many other areas of mathematics in which tion is to weakenthe hypotheses on F, for example, algebraic groups play an essential role, such as to assume that F has at the , geometric theory most 2, which holds for totally imaginary number and Schubert varieties in , Tits’s fields and C(x, y). For such fields and G simply theory of buildings... Algebraic groups deserve connected, Serre’s “Conjecture II” (1962) asserts more attention. that H1(F, G) is zero. This is known to hold if F is the field of a complex (de Jong, References He, and Starr, 2008) or G is a (Bayer [1] P. Gille, Le problème de Kneser-Tits, Astérisque 326 and Parimala, 1995). There are many other results (2009), 39–82. on this conjecture and generalizations such as the [2] V. P. Platonov and A. Rapinchuk, Algebraic Groups Hasse principle conjecture II; Google can provide and Number Theory, Academic Press, Boston, 1994. details. [3] J.-P. Serre, Gèbres, L’Enseignement Math. 39 (1993), 33–85. (Reprinted in Oeuvres, vol. IV, pp. 272–324.)

1A global field is a finite extension of Q or k(t) for k a finite field.

1126 Notices of the AMS Volume 57, Number 9