Cocârlan, P. “Intégrales ultrasingulières: Ultrasingular Integrals”. Anal. Numér. Théor. Approx., vol. 11, no. 1, Aug. 1982, pp. 35-40, https://ictp.acad.ro/jnaat/journal/article/view/1982-vol11-nos1-2-art4.