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 = λ 2 y , x ∈ R + = [ 0 , ∞ ) , y ( 0 ) = 0 , where Q is a non-selfadjoint matrix-valued function.