Rationality conjecture for Jacobian points and twisted Eisenstein series

Let X0(l)X_0(l) be the modular curve with its natural Q{\mathbb Q}-structure and Jacobian, and let hh determine the degree-zero twist sheaf F0(h){\mathcal F}_0(h) on X0(l)X_0(l). For each weight kk, write Ek,i;hE_{k,i;h} for the associated twisted Eisenstein series, indexed by ii, and let Rk(q)R_k(q) be a formal power series.

Rationality conjecture. The sheaf F0(h){\mathcal F}_0(h) gives a rational point on the Jacobian if and only if, for each kk, there exists a nonzero formal power series Rk(q)R_k(q) such that all coefficients of Ek,i;hRk(q)E_{k,i;h}R_k(q) are rational for every ii. Equivalently, the ratios

Ek,i;hEk,j;h\frac{E_{k,i;h}}{E_{k,j;h}}

have rational Fourier coefficients for all kk, ii, and jj whenever Ek,j;h≢0E_{k,j;h}\not\equiv 0.

This conjecture proposes that rationality of the Jacobian point represented by the twisted sheaf is exactly detected by rational Fourier-coefficient data from the associated twisted Eisenstein series. The abstract gives the equivalent formulation in terms of ratios of twisted Eisenstein series, while the supplied text provides no evidence that the conjecture has been resolved.

Sources & referencesView supporting material

Primary source

Lev A. Borisov, “Holomorphic Eisenstein series with Jacobian twists”, arXiv:math/0410333 (2004).

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