Generalized separants of differential polynomials

Let f ∈ K{y} be an element of the ring of differential polynomials in one differential variable y with one differential operator δ. For any variable y _k , the polynomial g = δ ⁿ (f) can be represented in the form g = A _k y _k + go, where go does not depend on y _k . If y _k is the leader of g, then A _k is a separant of the polynomial f. A formula for A _k is obtained for sufficiently large numbers n and k and some applications of this formula are presented.