Approximation in algebraic groups

(Redirected from Strong approximation theorem)

In algebraic group theory, approximation theorems are an extension of the Chinese remainder theorem to algebraic groups G over global fields k.

History

Eichler (1938) proved strong approximation for some classical groups. Strong approximation was established in the 1960s and 1970s, for semisimple simply-connected algebraic groups over global fields. The results for number fields are due to Kneser (1966) and Platonov (1969); the function field case, over finite fields, is due to Margulis (1977) and Prasad (1977). In the number field case Platonov also proved a related result over local fields called the Kneser–Tits conjecture.

Formal definitions and properties

Let G be a linear algebraic group over a global field k, and A the adele ring of k. If S is a non-empty finite set of places of k, then we write A for the ring of S-adeles and A_S for the product of the completions k_s, for s in the finite set S. For any choice of S, G(k) embeds in G(A_S) and G(A).

The question asked in weak approximation is whether the embedding of G(k) in G(A_S) has dense image. If the group G is connected and k-rational, then it satisfies weak approximation with respect to any set S (Platonov & Rapinchuk 1994, p.402). More generally, for any connected group G, there is a finite set T of finite places of k such that G satisfies weak approximation with respect to any set S that is disjoint with T (Platonov & Rapinchuk 1994, p.415). In particular, if k is an algebraic number field then any connected group G satisfies weak approximation with respect to the set S = S_∞ of infinite places.

The question asked in strong approximation is whether the embedding of G(k) in G(A) has dense image, or equivalently whether the set

G(k)G(A_S)

is a dense subset in G(A). The main theorem of strong approximation (Kneser 1966, p.188) states that a non-solvable linear algebraic group G over a global field k has strong approximation for the finite set S if and only if its radical N is unipotent, G/N is simply connected, and each almost simple component H of G/N has a non-compact component H_s for some s in S (depending on H).

The proofs of strong approximation depended on the Hasse principle for algebraic groups, which for groups of type E₈ was only proved several years later.

Weak approximation holds for a broader class of groups, including adjoint groups and inner forms of Chevalley groups, showing that the strong approximation property is restrictive.

References

Eichler, Martin (1938), "Allgemeine Kongruenzklasseneinteilungen der Ideale einfacher Algebren über algebraischen Zahlkörpern und ihre L-Reihen.", Journal für die Reine und Angewandte Mathematik (in German), 179: 227–251, doi:10.1515/crll.1938.179.227, ISSN 0075-4102
Kneser, Martin (1966), "Strong approximation", Algebraic Groups and Discontinuous Subgroups (Proc. Sympos. Pure Math., Boulder, Colo., 1965), Providence, R.I.: American Mathematical Society, pp. 187–196, MR 0213361
Margulis, G. A. (1977), "Cobounded subgroups in algebraic groups over local fields", Akademija Nauk SSSR. Funkcional'nyi Analiz i ego Priloženija, 11 (2): 45–57, 95, ISSN 0374-1990, MR 0442107
Platonov, V. P. (1969), "The problem of strong approximation and the Kneser–Tits hypothesis for algebraic groups", Izvestiya Akademii Nauk SSSR. Seriya Matematicheskaya, 33: 1211–1219, ISSN 0373-2436, MR 0258839
Platonov, Vladimir; Rapinchuk, Andrei (1994), Algebraic groups and number theory. (Translated from the 1991 Russian original by Rachel Rowen.), Pure and Applied Mathematics, vol. 139, Boston, MA: Academic Press, Inc., ISBN 0-12-558180-7, MR 1278263
Prasad, Gopal (1977), "Strong approximation for semi-simple groups over function fields", Annals of Mathematics, Second Series, 105 (3): 553–572, doi:10.2307/1970924, ISSN 0003-486X, JSTOR 1970924, MR 0444571

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