Secant sheaves on abelian n-folds with real multiplication and Weil classes on abelian 2n-folds with complex multiplication

Abstract

Let K be a CM-field, i.e., a totally complex quadratic extension of a totally real field F. Let X be a g-dimensional abelian variety admitting an algebra embedding of F into the rational endomorphisms End_Q(X) of X. Let A be the product of X and Pic^0(X). We construct an embedding e of K into End_Q(A) associated to a choice of an F-bilinear polarization h on X and a purely imaginary element q in K. We get the [K:Q]-dimensional subspace HW(A,e) of Hodge Weil classes in the d-th cohomology of A, where d:=4g/[K:Q]. Let V be the first cohomology of A. The even cohomology S^+ of X is the half-spin representation of the group Spin(V) and so the projectivization P(S^+) contains the even spinorial variety. The latter is a component of the Grassmannian of maximal isotropic subspaces of V. We associate to (h,q) a rational 2^[F:Q]-dimensional subspace B of S^+ such that P(B) is secant to the spinorial variety. Associated to two coherent sheaves G and G' on X with Chern characters in B we obtain the object E in D^b(A) by applying Orlov's equivalence between D^b(XxX) and D^b(A) to the outer tensor product of G and G'. The flat deformations of a normalized Chern class k(E) of E remain of Hodge type under every deformation of (A,e) as an abelian variety (A',e') of Weil type. We provide a criterion for the tensor product of ch(G) and ch(G') to belong to the open subset in the tensor square of B for which the algebraicity of the flat deformation of k(E) implies the algebraicity of all classes in HW(A',e'). The algebraicity would thus follow if E is semiregular in the appropriate sense. Examples of such secant sheaves G and G' are provided for X the Jacobian with real multiplication by a real quadratic number field F of a genus 4 curve. The semiregularity of E has not been addressed yet.