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A closed map between topological spaces maps closed sets to closed sets, while a closed linear operator between Banach spaces has a closed graph in the product topology.

Is this just a co-incidental use of the same "closed" terminology or is there some relationship between the two ?

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As Eric Wofsey pointed out, the terms are just coincidence. A closed mapping and a closed linear operator are not equivalent when considering a linear operator between two Banach spaces.

To show that they are unrelated, you might want to do the following exercises, which is Exercise 1.74 in Megginson's An Introduction to Banach Space Theory:

  • Let $X,Y$ be normed spaces and let $T : X \to Y$ be a linear operator that is neither injective nor the zero operator. Find a closed subset $F \subset X$ such that $T(F)$ is not closed in $Y$.
  • Find a linear operator $T : X \to Y$ that satisfies the hypotheses of the closed graph theorem even though there is a closed subset $F \subset X$ such that $T(F)$ is not closed in the range of $T$.
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  • $\begingroup$ I'm thinking about 1.74. Haven't got far yet though. $\endgroup$ – Tom Collinge Aug 15 '16 at 14:39
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It's just a coincidence. They are both derived from "closed" in the sense of closed sets, but they are not directly related to each other.

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  • $\begingroup$ Thanks Eric. I have spent a couple of hours futilely trying to prove that a closed linear operator is a closed map, alternatively to find additional conditions in which a linear operator mapping closed sets to closed sets is a "closed linear operator". Now I can proceed with something more productive. $\endgroup$ – Tom Collinge Aug 14 '16 at 18:57

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