Abstract
Let $X$ be a Banach space, and $M,N$ be two closed subspaces of $X$. We collect several necessary and sufficient conditions for the closedness of $M+N$ ($M+N$ is not necessarily direct sum), and show that a necessary condition in a classical textbook is also sufficient.
Citation
Zhe-Ming Zheng. Hui-Sheng Ding. "A Note on Closedness of the Sum of Two Closed Subspaces in a Banach Space." Commun. Math. Anal. 19 (2) 62 - 67, 2016.
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