Weil's conjecture on Tamagawa numbers

Short description: Conjecture in algebraic geometry

In mathematics, the Weil conjecture on Tamagawa numbers is the statement that the Tamagawa number [math]\displaystyle{ \tau(G) }[/math] of a simply connected simple algebraic group defined over a number field is 1. In this case, simply connected means "not having a proper algebraic covering" in the algebraic group theory sense, which is not always the topologists' meaning.

History

Weil (1959) calculated the Tamagawa number in many cases of classical groups and observed that it is an integer in all considered cases and that it was equal to 1 in the cases when the group is simply connected. The first observation does not hold for all groups: (Ono 1963) found examples where the 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.

Robert Langlands (1966) introduced harmonic analysis methods to show it for Chevalley groups. 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, formally published in (Gaitsgory Lurie), and a future proof using a version of the Grothendieck-Lefschetz trace formula will be published in a second volume.

Applications

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

For spin groups, the conjecture implies the known Smith–Minkowski–Siegel mass formula.

See also

• Tamagawa number

References

• Hazewinkel, Michiel, ed. (2001), "Tamagawa number", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=T/t092060 
• Chernousov, V. I. (1989), "The Hasse principle for groups of type E8", Soviet Math. Dokl. 39: 592–596 
• Gaitsgory, Dennis; Lurie, Jacob (2019), Weil's Conjecture for Function Fields (Volume I), Annals of Mathematics Studies, 199, Princeton: Princeton University Press, pp. viii, 311, ISBN 978-0-691-18213-1, https://press.princeton.edu/books/paperback/9780691182148/weils-conjecture-for-function-fields 
• Kottwitz, Robert E. (1988), "Tamagawa numbers", Ann. of Math., 2 (Annals of Mathematics) 127 (3): 629–646, doi:10.2307/2007007 .
• Lai, K. F. (1980), "Tamagawa number of reductive algebraic groups", Compositio Mathematica 41 (2): 153–188, http://www.numdam.org/item?id=CM_1980__41_2_153_0 
• Langlands, R. P. (1966), "The volume of the fundamental domain for some arithmetical subgroups of Chevalley groups", Algebraic Groups and Discontinuous Subgroups, Proc. Sympos. Pure Math., Providence, R.I.: Amer. Math. Soc., pp. 143–148 
• Lurie, Jacob (2014), Tamagawa Numbers via Nonabelian Poincaré Duality, http://www.math.harvard.edu/~lurie/282y.html 
• Ono, Takashi (1963), "On the Tamagawa number of algebraic tori", Annals of Mathematics, Second Series 78 (1): 47–73, doi:10.2307/1970502, ISSN 0003-486X 
• Ono, Takashi (1965), "On the relative theory of Tamagawa numbers", Annals of Mathematics, Second Series 82 (1): 88–111, doi:10.2307/1970563, ISSN 0003-486X, http://projecteuclid.org/euclid.bams/1183525960 
• Tamagawa, Tsuneo (1966), "Adèles", Algebraic Groups and Discontinuous Subgroups, Proc. Sympos. Pure Math., IX, Providence, R.I.: American Mathematical Society, pp. 113–121 
• Voskresenskii, V. E. (1991), Algebraic Groups and their Birational Invariants, AMS translation 
• Weil, André (1959), Exp. No. 186, Adèles et groupes algébriques, Séminaire Bourbaki, 5, pp. 249–257, http://www.numdam.org/item?id=SB_1958-1960__5__249_0 
• Weil, André (1982), Adeles and algebraic groups, Progress in Mathematics, 23, Boston, MA: Birkhäuser Boston, ISBN 978-3-7643-3092-7, https://books.google.com/books?id=vQvvAAAAMAAJ 

Further reading

• Aravind Asok, Brent Doran and Frances Kirwan, "Yang-Mills theory and Tamagawa Numbers: the fascination of unexpected links in mathematics", February 22, 2013
• J. Lurie, The Siegel Mass Formula, Tamagawa Numbers, and Nonabelian Poincaré Duality posted June 8, 2012.