Second kind representations of Sobolev space solutions to a first order general elliptic linear system in a simply connected plane domain

We consider a second kind representation for solutions to a first order general uniformly elliptic linear system in a simply connected plane domain $G$ with the $W^{k-\frac{1}{p}}_p$-boundary. We prove that the operator of the system is an isomorphism of Sobolev's space $W^k_p(\overline G)$, $k\geq 1$, $p>2$, under appropriate assumptions about coefficients and the boundary. These results are new even for solutions to the canonical first order elliptic system (generalized analytic functions in the sense of Vekua).