Quadratic pencil of difference equations: Jost solutions, spectrum, and   principal vectors
  In this paper, a quadratic pencil of Schr\"odinger type difference operator $L_{\lambda}$ is taken under investigation to give a general perspective on the spectral analysis of non-selfadjoint difference equations of second order. Introducing Jost-type solutions, structural and quantitative properties of spectrum of the operator $L_{\lambda}$ are analyzed and hence, a discrete analog of the theory in Degasperis, (\emph{J.Math.Phys}. 11: 551--567, 1970) and Bairamov et. al, (\emph{Quaest. Math.} 26: 15--30, 2003) is developed. In addition, several analogies are established between difference and $q$-difference cases. Finally, the principal vectors of $L_{\lambda}$ are introduced to lay a groundwork for the spectral expansion.   Mathematics Subject Classification (2000): 39A10, 39A12, 39A13 
Murat Adivar