Pečiulytė, Sigita, and Artūras Štikonas. “On the Generalized Eigenfunctions System of the Sturm–Liouville Problem”. Lietuvos Matematikos Rinkinys, vol. 48, no. proc. LMS, Dec. 2008, pp. 327-32, https://doi.org/10.15388/LMR.2008.14655.