Imagine an audience of readers who know rather little about functional analysis but whose interest in that subject might be piqued by an exposition of its uses in differential and integral equations. Does such an expository article exist?

Reference request: Expository paper on the use of functional analysis in differential and integral equations Nobody's said a word about this question, but somebody down-voted it. They didn't explain what was objectionable about it. Can someone?

Let us define a group $G$ to be a hyperlinear group if it satisfies the conclusion of Theorem 3.6. in these notes by Vladimir Pestov. It is well-known that one can use the so-called amplification argument to replace $1/4$ by $ \sqrt{ 2}- \epsilon$ for every $\epsilon >0$ in this definition. A sim...