Weil Operators and Remainders of Anderson Generating Functions over a Punctured Projective Line

Abstract

Let $A_ρ$ be the coordinate ring of the projective line over $\Fq$ with a closed point of degree $N$ removed. We give finite formulas for higher Weil operators in a basis of Riemann--Roch spaces, which recover the formulas of Hu--Ou--Yang when $N=1$. We identifythese operators with representatives of the residue Casimir tensors and obtain a determinant formula for the Weil pairing, relative to a chosen determinant realization. For a normalized rank-one Drinfeld module, we compute the remainder of its Anderson generating function modulo any nonconstant $p\in A_ρ$. The formula includes multiple zeros and a zero at the ordinary point at infinity. Its last coefficient is a nonzero scalar multiple of an exponential division value. When $(p)$ is a finite principal prime, a value of the associated special function at one zero of $p$ generates the torsion field after adjoining that zero. We give an explicit $q$-linear formula recovering a torsion generator from this value. We also specialize the evaluation theorem of Gazda--Maurischat and compute the different as the principal ideal $(\partial_xp)$.