Analysis of two-operator boundary-domain integral equations for variable-coefficient mixed BVP

Applying the two-operator approach, the mixed (Dirichlet–Neumann) boundary value problem for a second-order scalar elliptic differential equation with variable coefficients is reduced to several systems of Boundary Domain Integral Equations, briefly BDIEs. The two-operator BDIE system equivalence to the boundary value problem, BDIE solvability and the invertibility of the boundary-domain integral operators are proved in the appropriate Sobolev spaces.