Vorob’evI. V. “Expansion of a Self-Adjoint Absolutely Continuous Singular Integral Operator in Generalized Eigenvectors and Its Diagonalization”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 1, Jan. 2003, pp. 138-45, https://umj.imath.kiev.ua/index.php/umj/article/view/3896.