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In mathematics, the Weil conjecture on Tamagawa numbers is the statement that the Tamagawa number τ(G) of a simply connected simple algebraic group defined over a number field is 1. Weil (1959) did not explicitly conjecture this, but calculated the Tamagawa number in many cases and observed that in the cases he calculated it was an integer, and equal to 1 when the group is simply connected. The first observation does not hold for all groups: Ono (1963) found some examples whose Tamagawa numbers are not integers. The second observation, that the Tamagawa numbers of simply connected semisimple groups seem to be 1, became known as the Weil conjecture. Several authors checked this in many cases, and finally Kottwitz proved it for all groups in 1988.

Ono (1965) used the Weil conjecture to calculate the Tamagawa numbers of all semisimple algebraic groups.

Tamagawa numbers were introduced by Tamagawa (1966), and named after him by Weil (1959).

Here simply connected is in the algebraic group theory sense of not having a proper algebraic covering, which is not always the topologists' meaning.

Tamagawa measure and Tamagawa numbers

Let k be a global field, A its ring of adeles, and G an algebraic group defined over k.

The Tamagawa measure on the adelic algebraic group G(A) is defined as follows. Take a left-invariant n-form ω on G(k) defined over k, where n is the dimension of G. This induces Haar measures on G(ks) for all places of s, and hence a Haar measure on G(A), if the product over all places converges. This Haar measure on G(A) does not depend on the choice of ω, because multiplying ω by an element of k* multiplies the Haar measure on G(A) by 1, using the product formula for valuations.

The Tamagawa number τ(G) is the Tamagawa measure of G(A)/G(k).
History

Weil checked this in enough classical group cases to propose the conjecture. In particular for spin groups it implies the known Smith–Minkowski–Siegel mass formula.

Robert Langlands (1966) introduced harmonic analysis methods to show it for Chevalley groups. J. G. M. Mars gave further results during the 1960s.

K. F. Lai (1980) extended the class of known cases to quasisplit reductive groups. Kottwitz (1988) proved it for all groups satisfying the Hasse principle, which at the time was known for all groups without E8 factors. V. I. Chernousov (1989) removed this restriction, by proving the Hasse principle for the resistant E8 case (see strong approximation in algebraic groups), thus completing the proof of Weil's conjecture.

In 2011, Jacob Lurie and Dennis Gaitsgory announced a proof of the conjecture for algebraic groups over function fields over finite fields.Lurie (2011) Lurie (2014)

Ran space
Draft:Gaitsgory-Lurie proof of Weil conjecture on Tamagawa numbers

References

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Lai, K. F. (1980), "Tamagawa number of reductive algebraic groups", Compositio Mathematica 41 (2): 153–188, MR 581580
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Tamagawa, Tsuneo (1966), "Adèles", Algebraic Groups and Discontinuous Subgroups, Proc. Sympos. Pure Math. IX, Providence, R.I.: American Mathematical Society, pp. 113–121, MR 0212025
Voskresenskii, V. E. (1991), Algebraic Groups and their Birational Invariants, AMS translation
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Lurie, Jacob (2011), Tamagawa Numbers via Nonabelian Poincaré Duality

Lurie, Jacob (2014), Tamagawa Numbers via Nonabelian Poincaré Duality