Let (X, L) be a principally polarized abelian variety (ppav) of dimension g such that L is a symmetric line bundle, i.e. i L -L where i is the inversion map i(x) --x. We shall denote by X[2] the two torsion points of X which are fixed by i. For any x in X [2] we have an isomorphism(1) t*(LΫ^L 2 .Here t x is the translation map.Let ί be a point of the Siegel upper half space H^ and X be the abelian varie-As symmetric line bundle L we take C* x C/(τZ* + Z*)Here e(t) stands for exp(2τπ£). Sometime, if it will be necessary, we shall writeIt is a well known fact that a basis of H (X, L ) is given by the 2 8 theta function $ n (2r, 2z) and from (1) we have theta relation We know that all these maps are injective, cf [6] and [8].Here we shall show that the injectivity of these maps extends to the Satake compactifications and we shall characterize the points of these compactifications in terms of vanishing of Thetanullwerte. Moreover we shall show that the characterization of the reducible points, obtained in this way, is ideal-theoretic. Under θ, /^(4,8)\H^ is biholomorphic onto its image, cf [6]; Θ and Θ have the same property when g is less or equal to 2, cf [5]. We shall show that Θ and Θ are not immersions for g > 4 and that Θ is an immersion when g is 3. This result still holds when we extend Θ to the Satake compactification of 7^3(2,4)\H 3 .We are grateful to G. Lupacciolu and A. Silva for helpful discussions.
Satake compactificationLet R be an associative ring with the unity, then we shall denote by