Pipa, H.M. and Storozh, O.G. (2005) “On some perturbations changing the domain of proper extension of positively definite operator”, Methods of Functional Analysis and Topology, 11(3), pp. 257–269. Available at: https://zen.imath.kiev.ua/index.php/mfat/article/view/284 (Accessed: 16 December 2025).