Pečiulyte, S., O. Štikoniene, and A. Štikonas. “Sturm‐Liouville Problem for Stationary Differential Operator With Nonlocal Integral Boundary Condition”. Mathematical Modelling and Analysis, vol. 10, no. 4, Dec. 2005, pp. 377-92, doi:10.3846/13926292.2005.9637295.