EN
We describe the polynomials P ∈ ℂ[x,y] such that $P(1/vⁿ,A₁vⁿ + A₂v^{2n} + ... + A_{m-1}v^{n(m-1)} + v^{nm-k}w) ∈ ℂ[v,w]$. As applications we give new examples of bad field generators and examples of families of polynomials with smooth and irreducible fibers.