“…, y n ) that vanishes E-mail Addresses: maghabali@gmail.com, mhbien@hcmus.edu.vn The first author's current institute: School of Mathematics, Statistics and Computer Science, University of Tehran, Tehran, Iran whenever an algebraic element of degree ≤ n is substituted into x. One takes n < degD, substitutes a relevant rational expression into x and proves that the resulting expression cannot vanish on D ⊗ F L, where L is some splitting field of D. In [1] and [6], the expressions substituted into x are single additive, resp. multiplicative, commutators on two variables, whereas here, iterated commutators are considered.…”