G. Freiling and V. Rykhlov, “Pointwise convergence of eigenfunction expansions for a general class of regular eigenvalue problem”, MFAT, vol. 3, no. 2, pp. 27–45, Jun. 1997, Accessed: Dec. 19, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/35