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Root systems and elliptic curves. (English) Zbl 0358.17016
The Macdonald-Weyl formula
$\prod_{w\in W_{\text{aff}}} \det(w)e^{2\pi is(w)(z)\tau}=C \prod_{a\in R^{+}_{\text{aff}}}(1-e^{2\pi ia(z)\tau}) \tag{*}$
where $$C = \prod\limits_{n=1}^\infty (1-e^{2\pi in \tau})$$ is reproved here [cf. I. G. Macdonald, Invent. Math. 15, 91–143 (1972; Zbl 0244.17005) and also M. Demazure, Sémin. Bourbaki 1975/76, Lect. Notes Math. 567, Exp. No. 483 (1977; Zb1 0345.17003)].
The method consists in comparing two theta functions over the family of abelian varieties $$A_\tau = Q^\vee \otimes E_\tau$$ where $$R$$ is a root system, $$Q^\vee$$ the lattice generated by the coroots and $$E_\tau$$ is the elliptic curve $$\mathbb C/\mathbb Z \oplus \tau\mathbb Z$$ $$(\operatorname{Im}(\tau)>0)$$. As the Weyl group $$W$$ acts on $$Q^\vee$$, it also acts on $$A_\tau$$ and there is a basic $$W$$-antiinvariant theta function $$\theta_A$$, relative to $$A_\tau$$ with divisor $$\Delta = \sum\limits_{\alpha \in R_+} \text{Ker}^\alpha$$ (each root $$\alpha$$ determines a homomorphism $$A_\tau \to E_\tau$$ hence $$|\Delta|$$ is a union of reflection hypertori of $$A_\tau$$). The modular behavior $$(\tau \mapsto \tau + 1, \tau \mapsto -1/\tau)$$ of $$\theta_A$$ leads to a uniqueness result and and eventually to the identity (*).
The construction of this paper seems quite new and interesting: it throws some new light on the subject (for another point of view connected with group representation, cf. B. Kostant [Adv. Math. 20, 179–212 (1976; Zbl 0339.10019)]).
Reviewer: Alain Robert

##### MSC:
 17B22 Root systems 11F12 Automorphic forms, one variable 14K25 Theta functions and abelian varieties 14H45 Special algebraic curves and curves of low genus
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##### References:
 [1] Bourbaki, N.: Groupes et algèbres de Lie, Ch. 4, 5 et 6. Paris: Hermann 1969 · Zbl 0205.06001 [2] Freudenthal, H., de Vries, H.: Linear Lie groups. New York: Academic Press 1969 · Zbl 0377.22001 [3] Gunning, R. C.: Lectures on modular forms. Princeton, University Press 1962 · Zbl 0178.42901 [4] Lang, S.: Elliptic functions. Reading (Mass.): Addison Wesley 1973 · Zbl 0316.14001 [5] Macdonald, I. G.: Affine root systems and Dedekind’s ?-function. Inventiones math.15, 91-143 (1972) · Zbl 0244.17005 [6] Mumford, D.: Abelian varieties. Oxford: University Press 1970 · Zbl 0223.14022
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