It's commonly known that in general topology, a continuous map f from a topological space (X,τ) to another topological space (Y,τ′) will send every compact set to another compact set. Moreover, I'm aware that the converse does not hold, as so long as #Y≥2, we can pick some set S⊆X and define
f(x)={y1x∈Sy2x∈S∁.
This would send not only compact sets to compact sets, but all sets to compact sets because f(X) is finite, and thus compact (as are all its subsets). But this map could easily not be continuous.
NOTE: Understand for the rest of this post that f:X→Y is surjective.
My question is if there are more interesting counter-examples. Say X=R with the Euclidean metric; then what would be an example of such a discontinuous map f:X→Y where Y=N (with the usual metric), or Y=Q (again with the usual topology). What about R, or R∖Q? Is there perhaps a limit on how large in cardinality Y can be (with respect to X)? I'm curious to see less trivial counterexamples to the converse. Thanks!
EDIT: Many of the examples I'm getting still rely on compact sets having finite image. So a problem I might find more interesting: Can I construct a discontinuous counterexample where f is also injective?
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