Skučaitė A., and Štikonas A. “Zeroes and Poles of a Characteristic Function for Sturm–Liouville Problem With Nonlocal Integral Condition”. Lietuvos Matematikos Rinkinys, Vol. 56, no. A, Dec. 2015, pp. 95-100, doi:10.15388/LMR.A.2015.17.