Principal functions of non-selfadjoint matrix Sturm–Liouville equations

In this paper we investigate the principal functions corresponding to the eigenvalues and the spectral singularities of the boundary value problem −y′′+Q(x)y=λ2y,x∈R+=[0,∞),y(0)=0,y(0)=0, where QQ is a non-selfadjoint matrix-valued function.