Talk:Essential range

The last example is wrong

"If $X\subseteq \mathbb {R} ^{n}$ is open, $f:X\to \mathbb {C}$ and $\mu$ the Lebesgue measure, then $\operatorname {ess.im} (f)={\overline {\operatorname {im} (f)}}$ holds. This holds more generally for all Borel measures that assign non-zero measure to every non-empty open set."

This does not hold. Just take any characteristic function over a non-empty set of zero measure, say the rationals. Then its image is {0,1} but the essential range is just {0}. One needs some kind of continuity.

188.174.190.122 (talk) 14:45, 26 December 2016 (UTC)[reply]

What's the point?

Nothing in this article explains why this concept is important, useful or illuminating.
It is not clear why restrict to complex valued functions and why restrict to $L^{\infty }$ functions.
Theorem 1 is trivial and not stated correctly.
Theorem 2 is also not worthy of the name "theorem", but if it needs mention, the appropriate way to state this is to say that the essential range is always a closed set.

Proof: It is by definition the complement of the union of all open sets that have zero measure preimage. Oded (talk) 05:35, 11 July 2008 (UTC)[reply]

Oded (talk) 04:03, 11 July 2008 (UTC)[reply]

Note that if you restrict to L^infinity and complex valued functions, the essential range satisfies more interesting properties. I have found out these properties through my own research and therefore I am reluctant to put in on Wikipedia.

Topology Expert (talk) 03:25, 14 July 2008 (UTC)[reply]

Not so pointless

As User:Topology Expert has mentioned, the essential range is related to the spectrum of a multiplication operator: see e.g. Conway's A Course in Operator Theory, pp. 9-10. IMO there are many mathematical articles on topics which are an order of magnitude more pointless than this: see e.g. the article Maris-McGwire-Sosa pairs, which not only exists but is linked to from fundamental theorem of arithmetic.

I think this article could be useful if it had a tighter exposition (the lengthy proofs are not necessary) and a bit more about the applications -- and, of course, citations! Plclark (talk) 07:38, 12 July 2008 (UTC)Plclark[reply]

Theorem

Dear Oded,

I can certainly understand your proof and I also think that theorem 2 is obvious. Please remember though that I am still 'somewhat' new to Wikipedia and I am not sure how long proofs should be. For example, your proof basically says that the complement of the essential range is the union of open sets that have a preimage of 0 measure. But you haven't really proved set equality (if you know what I mean) even though proving this may take 2-3 lines. I follow the practice that all proofs that I put in Wikipedia should be reasonably detailed so that the reader can follow it without too much difficulty. If you notice, my proof has the maximum possible detail. Could you please let me know how much detail should be put into proofs on Wikipedia?

Thanks

Topology Expert (talk) 03:30, 14 July 2008 (UTC)[reply]