1. Engel, K.-J.: Operator matrices and systems of evolution equations, book manuscript (unpublished)
2. Jin, G., Chen, A.: Some basic properties of block operator matrices. arXiv:1403.7732
3. Koshmanenko, V.D., Ôta, S.: On the characteristic properties of singular operators. (Ukrainian); translated from Ukraïn. Mat. Zh. 48(11), 1484–1493 (1996). Ukrainian Math. J. 48(11), 1677–1687 (1997)
4. Möller, M., Szafraniec, F.H.: Adjoints and formal adjoints of matrices of unbounded operators. Proc. Am. Math. Soc. 136(6), 2165–2176 (2008)
5. Mortad, M.H.: Introductory Topology: Exercises and Solutions, vol. xvii, 2nd edn. (English). World Scientific, Hackensack, NJ (ISBN 978-981-3146-93-8/hbk; 978-981-3148-02-4/pbk), 356 p. (2017)