PIEKARSKI, Sławomir. “The Method of the Deformation Operator in Quantum Acoustics – a Formulation of Perturbation Calculus”. Archives of Acoustics 8, no. 2 (March 7, 2016): 155–168. Accessed April 25, 2025. https://acoustics3.ippt.pan.pl/index.php/aa/article/view/3252.