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I have come across to a statement in many Functional Analysis books saying that

"Hahn Banach theorem, Uniform Boundedness Principle, Open mapping theorem and Closed graph theorem are the four pillars of Functional Analysis"

I don't exactly know why they are so important, maybe these are used in many parts of Functional Analysis further. can anyone help me, please? thanks and regards in advance.

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    $\begingroup$ What's the question? $\endgroup$ Apr 1 '18 at 9:19
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    $\begingroup$ That they are important will be obvious if you continue to read the book. $\endgroup$ Apr 1 '18 at 9:19
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    $\begingroup$ These theorems are used again and again in functional analysis. $\endgroup$ Apr 1 '18 at 9:27
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    $\begingroup$ Just like ordinary linear algebra has its own toolkit of three-four theorems, linear algebra in Banach spaces has these four pivotal results that capture essential properties of of how continuity of linear functionals works. $\endgroup$
    – user228113
    Apr 1 '18 at 9:29
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    $\begingroup$ The question is to give a big picture overview of how these theorems are useful in functional analysis. I'm interested to hear the answers as well. $\endgroup$
    – littleO
    Apr 1 '18 at 9:39
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Based on my study of the subject I think I have enough information that I can answer my own question.

Hahn-Banach Theorem: It is so much important because it provides us with the linear functionals to work on various spaces as Functional Analysis is all about the study of functionals.

Open Mapping Theorem: It provides us with the open sets in the topology of the range of the mapping.

Uniform Boundedness Principle: An application of Baire Category theorem. It is further used many times as the uniformity is an important property.

Closed Graph Theorem: Closeness of the graph of a map is enough to prove its boundedness or continuity. This fact is further used many times.

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This article might be of help.

https://www.imsc.res.in/~kesh/trinity.pdf

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