Weil's conjecture on Tamagawa numbers
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 measure and Tamagawa numbers
Let k be a global field, A its ring of adeles, and G a semisimple algebraic group defined over k.
Choose Haar measures on the completions kv of k such that Ov has volume 1 for all but finitely many places v. These then induce a Haar measure on A, which we further assume is normalised so that A/k has volume 1 with respect to the induced quotient measure.
The Tamagawa measure on the adelic algebraic group G(A) is now defined as follows. Take a left-invariant n-form ω on G(k) defined over k, where n is the dimension of G. This, together with the above choices of Haar measure on the kv , induces Haar measures on G(kv) for all places of v. As G is semisimple, the product of these measures yields a Haar measure on G(A), called the Tamagawa measure. The Tamagawa measure does not depend on the choice of ω, nor on our choice of measures on the kv, 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 defined to be the Tamagawa measure of G(A)/G(k).
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.
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