On the Noether theory of two-dimensional singular operators and applications to boundary-value problems for systems of fourth-order elliptic equations

It is known that the general theory of multidimensional singular integral operators over the entire space $E_m$ was constructed by S. G. Mikhlin. It is shown that in the two-dimensional case, if the operator symbol does not turn into zero, then the Fredholm theory holds. As for operators over a bounded domain, in this case the boundary of the domain significantly affects the solvability of such operator equations. In this paper we consider two-dimensional singular operators with continuous coefficients over a bounded domain. Such operators are widely used in many problems of the theory of partial differential equations. In this regard, it would be interesting to find criteria of Noetherity of such operators as explicit conditions for its coefficients. Depending on the $2m + 1$ connected components, necessary and sufficient conditions of Noetherity for such operators are obtained and a formula for the evaluation of the index is given. The results are applied to the Dirichlet problem for general fourth-order elliptic systems.