Thamban Nair, M. “Approximation of Spectral Sets and Spectral Subspaces in Banach Spaces”. The Journal of the Indian Mathematical Society 54, no. 1-4 (December 1, 1989): 187–200. Accessed May 20, 2024. http://www.informaticsjournals.com/index.php/jims/article/view/21890.