Crăciun, Maria. “On Compound Operators Depending on S Parameters”. Rev. Anal. Numér. Théor. Approx. 33, no. 1 (February 1, 2004): 51–60. Accessed March 1, 2024. https://ictp.acad.ro/jnaat/journal/article/view/2004-vol33-no1-art6.