Sometimes I see something like “a mapping preserves the structures
of its domain and of its codomain”. From Wiki about morphisms in category theory:
a morphism is an abstraction derived from structure-preserving
mappings between two mathematical structures. The notion of morphism
recurs in much of contemporary mathematics. In set theory, morphisms
are functions; in linear algebra, linear transformations; in group
theory, group homomorphisms; in topology, continuous functions, and so
I was wondering why the structure-preserving mappings between two
topological/measurable spaces are defined by the “inverse” of the
mapping, while the structure-preserving mappings between two
groups/vector spaces are not?
Why are the structure-preserving mappings between two topological
spaces chosen to be continuous mappings instead of open mappings?
I also see that “a mapping preserves some property of subsets,
points or whatever”. Such as
Continuous linear mappings between topological vector spaces preserve
According to Brian’s reply to my earlier question, this quote should
be understood as “under a continuous linear mapping, the image of
any bounded domain subset is also a bounded codomain subset”, not as
“under a continuous linear mapping, the inverse image of any bounded
codomain subset is also a bounded domain subset”.
I wonder why? It seems at first to me like how continuous mappings
preserve topologies, but it is actually in the same way as how group
homomorphisms preserve group structures.
Thanks and regards!
Top is the category of topological spaces and continuous maps simply by definition; topology typically deals with continuous maps, making this category the most important one, and thus by convention it’s the one meant when saying “the category of topological spaces”.
(aside: other conventions on what Top or “the category of topological spaces” stands for are far more likely to disagree on what the objects are, rather than the morphisms. e.g. to make the objects be merely the compactly generated Hausdorff spaces)
You can, of course, make all sorts of other categories. The category of topological spaces and open maps is a perfectly reasonable category to make; it’s just less useful.
It takes a bit to get used to, but category theory rejects the mindset that mathematics is about objects, with the mappings between objects being a derived notion. Instead, you need to consider objects and mappings as equals — or even to consider the objects superfluous.
On that last point, my favorite example of a category whose emphasis is on the morphisms is matrix algebra. The set of all matrices, with composition defined by multiplication, form a category. (with addition, you get an Abelian category) The objects of this category really play no role beyond bookkeeping to say which matrix products are defined.
(This category is, of course, equivalent to the category of fintie-dimensional vector spaces and linear maps)
The question actually boils down to:
“Why do we define continuous maps as the way we do?!?”
Most answers go like:
If you consider continuity between metric spaces you can abstractize! it to the algebra of open sets. Not very satisfying!!!
Another approach goes about neighborhoods:
Consider what neighborhoods should be like intuitively. What you get is Felix Hausdorffs definition of neighborhood systems. In this context, one would guess! what a function should be like when talking about continuity, that is for every neighborhood one can find another one who’s image fits inside. That is what we always pictured in our mind when we were checking continuity in first year analysis course. Nice, but it still doesn’t say why this should be continuity in abstract spaces.
Better approach might be a closeness relation:
Consider a relation obeying some sort of axioms that seem intuitive for what we understand by being close. This again needs some sort of intuition, that is in first place not very satisfying since, well, math is about facts not intuition. However, it has at least the advantage that continuity pops up! almost evident in this context: If sth is closeby it stays closeby under continuous maps. Moreover, it is only in this context where the image is involved rather than the preimage!!!
Despite not being a complete answer, I hope this gives at least some insight.
Addendum: One could consider the preimage of closed sets which is super useless or open mappings which do not permit an inverse. See the post on open mappings.