Sobhy El-Sayed IBRAHIM. Studies on the Deficiency Indices of Product Differential Operators in Direct Sum Spaces . Current Topics on Mathematics and Computer Science Vol. 5, [S. l.], p. 130–147, 2021. DOI: 10.9734/bpi/ctmcs/v5/1817C. Disponível em: https://stm.bookpi.org/CTMCS-V5/article/view/2438. Acesso em: 21 jul. 2026.