Talk:Fiber (mathematics)

One or two topics?

The lead implies that there are two topics here. I tried to split the article but that wasn't appreciated :-( Sorry, but anyway, can the lead be re-written or expanded in a way that explains this as a single topic? Volunteer1234 (talk) 17:20, 26 August 2017 (UTC)

Notation

@Deacon Vorbis: I don't agree with your reason for writing

$$f^{-1}(\{y\})=\{x\in X\mid f(x)=y\}$$

as the definition for the fiber. I think this conflates the concept of "fiber" with "preimage". We are trying to express, in symbols, "the fiber of a function over a value in its codomain is the set of all inputs in the domain that map to that output", and that translates to

$$f^{-1}(y)=\{x\in X\mid f(x)=y\}$$

. The preimage is a separate thing, a generalization of the fiber to whole subsets of the codomain, instead of single elements at a time. We use the same notation for it, but definitions are the place where clarity is most valued. We don't need a notational allusion to a related concept. We need a simple function $f^{-1}:Y\to {\mathcal {P}}(X)$, and then we can add a note beneath it, along the lines of

The inverse image $f^{-1}(A)$ generalizes the concept of a fiber to whole subsets of the codomain $A\subseteq Y$ at a time, instead of one element $y\in Y$. The notation $f^{-1}(y)$ is still used to refer to the fiber, as the fiber over $y$ is the inverse image of the singleton set $\{y\}$, as in $f^{-1}(\{y\})$. That is, the fiber creates a function $f^{-1}:Y\to {\mathcal {P}}(X)$, and the inverse image generalizes this to $f^{-1}:{\mathcal {P}}(Y)\to {\mathcal {P}}(X)$.

Howtonotwin (talk) 19:31, 14 September 2018 (UTC)

I put it back along with a ":=" to make it clear that this statement is defining $f^{-1}(y),$ since I had taken it to mean that it was saying that that it's being defined as $f^{-1}(\{y\}),$ which happens to equal the right-hand side. –Deacon Vorbis (carbon • videos) 20:12, 14 September 2018 (UTC)

'Naive Set Theory'

Is there really any need to say that fibers are a notion of naive set theory and not just set theory? I understand that we are not formally proving that the fiber of an element is a set using the ZFC axioms (and similarly if we want to consider the 'fiber function' mapping from the codomain to the powerset of the domain, to actually prove that this is a function would require some possibly tricky uses of the ZFC axioms), however I would much rather change 'naive set theory' to just 'set theory' because by this article's reasoning we should say 'naive set theory' whenever we aren't rigorously applying ZFC axioms, for example with images, preimages, indexed families, set operations, etc. etc. Joel Brennan (talk) 15:15, 4 April 2019 (UTC)

This article should be deleted

For the first definition, it could redirect to Inverse image and for the second one, to Fiber product of schemes. Theres no point in having a dedicated Wikipedia article. — Preceding unsigned comment added by Kkmilo (talk • contribs) 17:05, 4 March 2021 (UTC)