Mwanzia, J. M., M. Kavila, and J. Khalagai. “On Fuglede – Putnam Theorems for Moore-Penrose Invertible Operators on Hilbert Spaces”. African Journal of Pure and Applied Sciences 4, no. 1 (June 29, 2023): 42–49. Accessed February 12, 2025. https://journal.ku.ac.ke/index.php/AJPAS/article/view/372.