We primarily investigate congruences modulo p for finite sums of the form ₖrkkxᵏ/k over the ranges 0<k<p and 0<k<p/r, where p is a prime larger than the positive integer r. Here x is an indeterminate, thus allowing specialization to numerical congruences where x takes certain algebraic numbers as values. We employ two different approaches that have complementary strengths. In particular, we obtain congruences modulo p² for the sum ₀<₊<rkkxᵏ, expressed in terms of finite polylogarithms of certain quantities related to x.
Mattarei et al. (Wed,) studied this question.