Covariant forms of Lax one-field operators

Polynomials in differentiation operators are considered. The Darboux transformations covariance determines non-Abelian entries to form the coefficients of the polynomials. Joint covariance of a pair of such polynomials (Lax pair) as a function of one-field is studied. Methodically, the transforms of the coefficients are equalized to Frechet derivatives (first term of the Taylor series on prolonged space) to establish the operator forms. In the commutative (Abelian) case that results in binary Bell (Faa de Bruno) differential polynomials having natural bilinear representation. The example of generalized Boussinesq equation is studied, the chain equations for the case are derived. A set of integrable non-commutative potentials and hence nonlinear equations is constructed altogether with explicit dressing formulas.